TOTAL SET ASTRO MIDTERM --- SRI GUTTIKONDA

A light year measures

1 light-year = 9.46 × 1012 km 1 light-year is about 745 million times the diameter of Earth

Movement through the sky: Moon

1 month--> 12 degrees or 24 times its own width on the sky each day if a star has 15 degreee/hour cycle --> amount of time it takes to go 360 degrees is one full cycle

Three ways energy is transferred

conduction, convection, radiation Conduction and convection are both important in the interiors of planets. In stars, which are much more transparent, radiation and convection are important, whereas conduction can usually be ignored.

fixed stars

those that maintain fixed patterns among themselves through many generations—and the Wandering Stars/Planet

dark rift

which runs lengthwise down a long part of the Milky Way in our sky and appears to split it in two, is produced by a collection of such obscuring clouds.

Parallax

An apparent shift in the position of an object when viewed from different locations Triangulation allows us to measure distances to inaccessible objects. By getting the angle to a tree from two different vantage points, we can calculate the properties of the triangle they make and thus the distance to the tree.

Conduction

atoms or molecules pass on their energy by colliding with others nearby. This happens, for example, when the handle of a metal spoon heats up as you stir a cup of hot coffee. heat transfer by physical contact during which the energetic motion of particles in one region spread to other regions and even to adjacent objects in close contact

charged coupled device (CCDs)

photons of radiation hitting any part of the detector generate a stream of charged particles (electrons) that are stored and counted at the end of the exposure. Each place where the radiation is counted is called a pixel (picture element), and modern detectors can count the photons in millions of pixels

why was the neutrino so hard to find?

so hard to find is that neutrinos interact very weakly with other matter and therefore are very difficult to detect Earth is more transparent to a neutrino than the thinnest and cleanest pane of glass is to a photon of light. In fact, most neutrinos can pass completely through a star or planet without being absorbed.

telescopes tha collect visible radiation

use a lens or mirror to gather the light.

Great Circle

any circle on the surface of a sphere whose center is at the center of the sphere Earth's equator is a great circle on Earth's surface, halfway between the North and South Poles. We can also imagine a series of great circles that pass through both the North and South Poles. Each of these circle is called the meridian= they are each perpendicular to the equator cross it at right angles Any point of the surface of Earth will have a meridian passing through it The meridian specifies the east-west location = longitude ( the number of degrees of arc along the equator between your meridian and the one passing through Greenwich, England

Most stars actually generate more energy each second when they

are fusing hydrogen in the shell surrounding the helium core than they did when hydrogen fusion was confined to the central part of the star; thus, they increase in luminosity. With all the new energy pouring outward, the outer layers of the star begin to expand, and the star eventually grows and grows until it reaches enormous proportions

Atoms in a hot gas ....

are moving at high speeds and continually colliding with one another and with any loose electrons hey can be excited (electrons moving to a higher level) and de-excited (electrons moving to a lower level) by these collisions as well as by absorbing and emitting light.

propagation of light

as the same expanding shell of light covers a larger and larger area, there must be less and less of it in any given place. ****Light (and all other electromagnetic radiation) gets weaker and weaker as it gets farther from its source. The increase in the area that the light must cover is proportional to the square of the distance that the light has traveled-- ****inverse square law for light propagation

which atoms have the greatest binding energy?

atoms with a mass near that of the iron nucleus (with a combined number of protons and neutrons equal to 56) and less for both the lighter and the heavier nuclei. Iron, therefore, is the most stable element: since it gives up the most energy when it forms, it would require the most energy to break it back down into its component particles.

Mean Standard Time

based on the average value of the solar day over the course of the year--> contains exactly 24 hours--> still inconvenient determined by position of the Sun For example, noon occurs when the Sun is highest in the sky on the meridian (but not necessarily at the zenith). But because we live on a round Earth, the exact time of noon is different as you change your longitude by moving east or west. Within each zone, all places keep the same standard time, with the local mean solar time of a standard line of longitude running more or less through the middle of each zone--> six zones Daylight saving time is simply the local standard time of the place plus 1 hour. It has been adopted for spring and summer use in most states in the United States, as well as in many countries, to prolong the sunlight into evening hours

Dusty clouds in space betray their presence in several ways:

by blocking the light from distant stars, by emitting energy in the infrared part of the spectrum, by reflecting the light from nearby stars, and by making distant stars look redder than they really are.

how are radio waves reflected?

by conducting surfaces, just as light is reflected from a shiny metallic surface, and according to the same laws of optics. A radio-reflecting telescope consists of a concave metal reflector (called a dish), analogous to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed.

what happens to electrons that are captured by the hydrogen nuclei

cascade down through the various energy levels of the hydrogen atoms on their way to the lowest level, or ground state. During each transition downward, they give up energy in the form of light. The process of converting ultraviolet radiation into visible light is called fluorescence. Interstellar gas contains other elements besides hydrogen. Many of them are also ionized in the vicinity of hot stars; they then capture electrons and emit light, just as hydrogen does, allowing them to be observed by astronomers. But generally, the red hydrogen line is the strongest, and that is why H II regions look red.

relationship between electric & magnetic phenomena

changing magnetic fields could produce electric currents (and thus changing electric fields), and changing electric currents could in turn produce changing magnetic fields. So once begun, electric and magnetic field changes could continue to trigger each other.

Ancient Observatories

could measure the positions of celestial objects, mostly to keep track of time and date --> religious and ritualistic functions -->only used eye and written records

energy level

each of the permitted electron orbits around a given atom has a certain energy value to move from one orbit to another require's a change in energy If the electron goes to a lower level, the energy difference will be given off; if the electron goes to a higher level, the energy difference must be obtained from somewhere else. Each jump (or transition) to a different level has a fixed and definite energy change associated with it.

conclusion of bohr model

each type of atom has its own unique pattern of electron orbits, and no two sets of orbits are exactly alike. This means that each type of atom shows its own unique set of spectral lines, produced by electrons moving between its unique set of orbits.

Radiation

energetic photons move away from hot material and are absorbed by some material to which they convey some or all of their energy. You can feel this when you put your hand close to the coils of an electric heater, allowing infrared photons to heat up your hand. the transfer of heat energy by electromagnetic radiation.

radiation

for waves (including light waves) that radiate outward from a source..

Maxwell: what if electric changes were oscillating?

found that the resulting pattern of electric and magnetic fields would spread out and travel rapidly through space. --> like a drop striking water and causes a wave atoms and molecules (which consist of charged particles) oscillate back and forth all the time. The resulting electromagnetic disturbances are among the most common phenomena in the universe.

he three basic types of clusters astronomers have discovered are

globular clusters, open clusters, and stellar associations

temp vs luminosity

hotter stars are more luminous than cooler ones.

The hydrogen lines in the visible part of the spectrum are strongest

in stars with intermediate temperatures—not too hot and not too cold. Calculations show that the optimum temperature for producing visible hydrogen lines is about 10,000 K. At this temperature, an appreciable number of hydrogen atoms are excited to the second energy level. They can then absorb additional photons, rise to still-higher levels of excitation, and produce a dark absorption line. *****every other chemical element, in each of its possible stages of ionization, has a characteristic temperature at which it is most effective in producing absorption lines in any particular part of the spectrum.

While density of interstellar matter is very low, the volume of space...

in which such matter is found is huge, and so its total mass is substantial. To see why, we must bear in mind that stars occupy only a tiny fraction of the volume of the Milky Way Galaxy. For example, it takes light only about four seconds to travel a distance equal to the diameter of the Sun, but more than four years to travel from the Sun to the nearest star. Even though the spaces among the stars are sparsely populated, there's a lot of space out there!

flamsteed star naming system:

in which the brighter stars eventually got a number in each constellation in order of their location in the sky or, more precisely, their right ascension. (The system of sky coordinates that includes right ascension was discussed in Earth, Moon, and Sky.) In this system, Betelgeuse is called 58 Orionis and 61 Cygni is the 61st star in the constellation of Cygnus, the swan --> NOW SPECIALIZED STAR CATALOGS

detecting neutral interstellar gas

including hydrogen, at many other wavelengths from the infrared to the ultraviolet.

Nebula

interstellar material is concentrated into giant clouds, each of which is known as a nebula (plural "nebulae," Latin for "clouds"). The best-known nebulae are the ones that we can see glowing or reflecting visible light

Ionosphere

is a layer of charged particles at the top of our atmosphere, produced by interactions with sunlight and charged particles that are ejected from the Sun

constellation

is one of the 88 sections into which astronomers divide the sky, each named after a prominent star pattern within it.

most powerful infrared telescope

is the 0.85-meter Spitzer Space Telescope, which launched in 2003. With infrared observations, astronomers can detect cooler parts of cosmic objects, such as the dust clouds around star nurseries and the remnants of dying stars, that visible-light images don't reveal.

active control

it is possible to measure that sag many times each second and apply forces at 120 different locations to the back of the mirror to correct the sag, a process called active control

what happens when light is reflected over a page and enter the human eye?

its changing electric and magnetic fields stimulate nerve endings, which then transmit the information contained in these changing fields to the brain science of astronomy is primarily about analyzing radiation from distant objects to understand what they are and how they work

larger interferometer seperations

larger interferometer separations can be achieved if the telescopes do not require a physical connection. Astronomers, with the use of current technology and computing power, have learned to time the arrival of electromagnetic waves coming from space very precisely at each telescope and combine the data later. If the telescopes are as far apart as California and Australia, or as West Virginia and Crimea in Ukraine, the resulting resolution far surpasses that of visible-light telescopes.

how do we know how far away stars are ?

light fades with increasing distance. The energy we receive is inversely proportional to the square of the distance. If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. If it is three times farther away, it will look nine (three squared) times dimmer, and so forth.

How is resolution measured?

measured in units of angle on the sky, typically in units of arc seconds One arcsecond is 1/3600 degree, and there are 360 degrees in a full circle.

astronomers call all the elements heavier than helium...

metals, even though most of them do not show metallic properties.

ultraviolet

mostly blocked by the ozone layer of Earth's atmosphere, but a small fraction of ultraviolet rays from our Sun do penetrate to cause sunburn or, in extreme cases of overexposure, skin cancer in human beings. Ultraviolet astronomy is also best done from space.

Picking The Best Observing Sites

mountains, far from the lights and pollution of cities. takes place far away, often on desert mountains or isolated peaks in the Atlantic and Pacific Oceans

where is most of sun's energy generated?

nearly all of the Sun's energy is generated within about 150,000 kilometers of its core, or within less than 10% of its total volume.

RR lyrae star clusters

occurring in any particular cluster all have about the same apparent brightness. Since stars in a cluster are all at approximately the same distance, it follows that RR Lyrae variables must all have nearly the same intrinsic luminosity, which turns out to be about 50 LSun. In this sense, RR Lyrae stars are a little bit like standard light bulbs and can also be used to obtain distances, particularly within our Galaxy

how can these estimates be used to understand lives of stars

on average, 90% of all stars are located on the main sequence of the H-R diagram. If we can identify some activity or life stage with the main sequence, then it follows that stars must spend 90% of their lives in that activity or life stage.

the first meter

one ten-millionth of the distance along Earth's surface from the equator to the pole

nuclear force

only capable of acting over distances about the size of the atomic nucleus. The strong nuclear force is an attractive force, stronger than the electrical force, and it keeps the particles of the nucleus tightly bound together.

Triangulation in humans

our depth perception fails for objects more than a few tens of meters away. In order to see the shift of an object a city block or more from you, your eyes would need to be spread apart a lot farther.

gamma-ray astronomers: CTA

planning the Cherenkov Telescope Array (CTA), two arrays of telescopes, one in each hemisphere, which will indirectly measure gamma rays from the ground. The CTA will measure gamma-ray energies a thousand times as great as the Fermi telescope can detect. astronomers are planning on a bigger and bigger mirror 27.5-39 meters

Hendrik van de Hulst

predicted that hydrogen would produce a strong line at a wavelength of 21 centimeters. That's quite a long wavelength, implying that the wave has such a low frequency and low energy that it cannot come from electrons jumping between energy levels

modern determination of solar system dimensions is

radar, a type of radio wave that can bounce off solid objects by timing how long a radar beam (traveling at the speed of light) takes to reach another world and return, we can measure the distance involved very accurately it is not possible to use radar to measure the distance to the Sun directly because the Sun does not reflect radar very efficiently. But we can measure the distance to many other solar system objects and use Kepler's laws to give us the distance to the Sun.

how to overcome resolution difficulty with radio waves?

radio astronomers have learned to sharpen their images by linking two or more radio telescopes together electronically.

radio astronomy measurement

radio astronomers measure the amount of energy being collected each second by each square meter of a radio telescope and express the brightness of each source in terms of, for example, watts per square meter.

who started dealing with cosmic radio waves

radio astronomy pioneered by Grote Reber & Karl Jasky

The masses of molecular clouds

range from a thousand times the mass of the Sun to about 3 million solar masses.

Gregory Halle

realized that 40 inches was close to the maximum feasible aperture for refracting telescopes. -> larger apertures = reflecting telescopes --> set out to construct 60 in reflector

Resolution

refers to the precision of detail present in an image: that is, the smallest features that can be distinguished. --> astronomers want sharpest images

Scientific laws

rules of the game that nature plays (ie. same laws apply everywhere in the universe)--> gives the ability to understand the behavior of people in those different regions --> STILL SUBJECT TO CHANGE Distances are dealt with in scientific notation --> use SI units to denote measurements

how does the Sun's luminosity of 4 * 10^26 watts get produced?

some 600 million tons of hydrogen must be converted into helium each second, of which about 4 million tons are converted from matter into energy. As large as these numbers are, the store of hydrogen (and thus of nuclear energy) in the Sun is still more enormous, and can last a long time—billions of years, in fact.

Maxwell: what is the speed at which electromagnetic disturbances moves through space?

speed of light--> lead to conclusion that light was one form of a family of possible electromagnetic disturbances called electromagnetic radiation (verified experimentally)

Studies of Orion and other star-forming regions show

star formation is not a very efficient process. In the region of the Orion Nebula, about 1% of the material in the cloud has been turned into stars. That is why we still see a substantial amount of gas and dust near the Trapezium stars. The leftover material is eventually heated, either by the radiation and winds from the hot stars that form or by explosions of the most massive stars. Whether gently or explosively, the material in the neighborhood of the new stars is blown away into interstellar space. Older groups or clusters of stars can now be easily observed in visible light because they are no longer shrouded in dust and gas

The primary reason that stellar spectra look different is because

stars have different temperatures.

why two key pieces lead to discovery of Sun's energy?

structure of the nucleus of the atom and the fact that mass can be converted into energy.

Sir William Huggins and Lady Margaret Huggins

succeeded in identifying some of the lines in stellar spectra as those of known elements on Earth, showing that the same chemical elements found in the Sun and planets exist in the stars

what about the stars 100x more luminous than the sun?

such stars are rare, they are visible to the unaided eye, even when hundreds to thousands of light-years away. A star with a luminosity 10,000 times greater than that of the Sun can be seen without a telescope out to a distance of 5000 light-years. The volume of space included within a distance of 5000 light-years, however, is enormous; so even though highly luminous stars are intrinsically rare, many of them are readily visible to our unaided eye.

The Ultimate Fate of Stars and Substellar Objects with Different Masses

table 23.1

types of electromagnetic radiation

table 5.1

Almagest

textbook or handbook, especially dealing with astronomy Ptolemy + Hipparchus work's --> main source of Greek astronomer knowledge

One of the most difficult things about precisely measuring the tiny angles of parallax shifts from Earth is

that you have to observe the stars through our planet's atmosphere

Solar Pulsations

the Sun pulsates—that is, it alternately expands and contracts—just as your chest expands and contracts as you breathe. This pulsation is very slight, but it can be detected by measuring the radial velocity of the solar surface—the speed with which it moves toward or away from us. The velocities of small regions on the Sun are observed to change in a regular way, first toward Earth, then away, then toward, and so on. It is as if the Sun were "breathing" through thousands of individual lungs, each having a size in the range of 4000 to 15,000 kilometers, each fluctuating back and forth

absolute luminosity

the actual amount of energy radiated from an object L = 4πR^2σT^4

cosmology

the concept, structure and origin of the cosmos Ancients developed cosmologies that combined religion and philosophy Eastern Mediterranean knew Earth was round --> stemmed from Pythagoras that said perfect forms came in shapes of circles and spheres so Earth must be round Greeks = gods likes spheres hence moon is round

Interstellar Medium (ISM)

the gas and dust in the interstellar space between a galaxy's stars the entire collection of interstellar matter

spectrometer

the instrument between telescope and detector may be one of several devices that spread the light out into its full rainbow of colors so that astronomers can measure individual lines in the spectrum. allows astronomers to measure (to meter) the spectrum of a source of radiation.

Law of Conservation of Energy

the law that states that energy cannot be created or destroyed but can be changed from one form to another

class L Brown dwarfs

the lines of titanium oxide, which are strong in M stars, have disappeared. This is because the L dwarfs are so cool that atoms and molecules can gather together into dust particles in their atmospheres; the titanium is locked up in the dust grains rather than being available to form molecules of titanium oxide/.

ground state

the lowest possible energy of an atom described by quantum mechanics

mass-luminosity relation

the more massive a star is, the more luminous it is M^3.9 ~L (deviations from this rule are white dwarf stars) Refer to figure 18.9 --> 90 percent of stars obey this rule L/LSun =⎝M/MSun)^4

what happens after the neutrons that they cannot be held?

the neutrons are squeezed out of the nuclei and can exert a new force. As is true for electrons, it turns out that the neutrons strongly resist being in the same place and moving in the same way. The force that can be exerted by such degenerate neutrons is much greater than that produced by degenerate electrons, so unless the core is too massive, they can ultimately stop the collapse.

Frequency

the number of wave cycles that pass by per second. Heinrich Hertz, the physicist who—inspired by Maxwell's work—discovered radio waves, a cps is also called a hertz (Hz).

Nuclear reactions in the Sun's Interior

the sun taps taps the energy contained in the nuclei of atoms through nuclear fusion

Molecular Clouds

the vibration and rotation of atoms within molecules can leave spectral fingerprints in radio and infrared waves. If we spread out the radiation at such longer wavelengths, we can detect emission or absorption lines in the spectra that are characteristic of specific molecules. Over the years, experiments in our laboratories have shown us the exact wavelengths associated with changes in the rotation and vibration of many common molecules, giving us a template of possible lines against which we can now compare our observations of interstellar matter.

what happens when the source of waves moves towards you

the wavelength decreases a bit. If the waves involved are visible light, then the colors of the light change slightly. As wavelength decreases, they shift toward the blue end of the spectrum: astronomers call this a blueshift (towards the violet end of spectrum)

visible light

the waves that human vision can perceive. band of the electromagnetic spectrum that most readily reaches Earth's surface. human eyes evolved to see the kinds of waves that arrive from the Sun most effectively

what do stronger lines mean..

there are more atoms in the stellar photosphere absorbing light. Therefore, we know immediately that the star with stronger sodium lines contains more sodium

What about the other stars on the H-R diagram—the giants and supergiants, and the white dwarfs?

these are what main-sequence stars turn into as they age: They are the later stages in a star's life. As a star consumes its nuclear fuel, its source of energy changes, as do its chemical composition and interior structure. These changes cause the star to alter its luminosity and surface temperature so that it no longer lies on the main sequence on our diagram. Because stars spend much less time in these later stages of their lives, we see fewer stars in those regions of the H-R diagram.

why are the rays of light from the same star parallel?

they are extremely far away By the time the few rays of light pointed toward us actually arrive at Earth, they are, for all practical purposes, parallel to each other.

important info about neutrinos

three different types of neutrinos discovered to have a tiny amount of mass (electrons are at least 500,000 times more massive.)

In order to specify the exact color of a star, astronomers normally measure a star's apparent brightness....

through filters, each of which transmits only the light from a particular narrow band of wavelengths (colors). A crude example of a filter in everyday life is a green-colored, plastic, soft drink bottle, which, when held in front of your eyes, lets only the green colors of light through.

Measuring three or four evenly spaced transits is normally enough

to "discover" an exoplanet. But in a new field like exoplanet research, we would like to find further independent verification. The strongest confirmation happens when ground-based telescopes are also able to detect a Doppler shift with the same period as the transits. However, this is generally not possible for Earth-size planets. One of the most convincing ways to verify that a dip in brightness is due to a planet is to find more planets orbiting the same star—a planetary system. Multi-planet systems also provide alternative ways to estimate the masses of the planets, as we will discuss in the next section.

Estimating Interstellar Mass

total mass = volume × density of atoms × mass per atom V = πR^2 h = cylindrical Galaxy V = (4/3)πR^3= spherical objects

How can we get nuclei close enough to participate in fusion?

tremendous heat—which speeds the protons up enough to overcome the electrical forces that try to keep protons apart. In the Sun: Two protons can fuse only in regions( like the center where temp is 15 mill K) where the temperature is greater than about 12 million K, and the speed of the protons average around 1000 kilometers per second or more

binary stars

two stars that orbit each other, bound together by gravity. Masses of binary stars can be calculated from measurements of their orbits, just as the mass of the Sun can be derived by measuring the orbits of the planets around it

spectrograph to analyze starlight

use a spectrograph to spread out the light into a spectrum

microwave waves

used in short-wave communication and microwave ovens. (1mm to 1 meter)

radial velocity

velocity along the line of sight toward or away from the observer

first planetary system

was found around the star Upsilon Andromedae in 1999 using the Doppler method, and many others have been found since then (about 2600 as of 2016). If such exoplanetary system are common, let's consider which systems we expect to find in the Kepler transit data.

Since hydrogen is the main constituent of interstellar gas, we often characterize a region of space according to

whether its hydrogen is neutral or ionized.

what happens if you try to cool a telescope within the atmosphere?

would quickly become coated with condensing water vapor and other gases, making it useless. Only in the vacuum of space can optical elements be cooled to hundreds of degrees below freezing and still remain operational.

exoplanets with Known densities

xoplanets with known masses and radii (red circles) are plotted along with solid lines that show the theoretical size of pure iron, rock, water, and hydrogen planets with increasing mass. Masses are given in multiples of Earth's mass. (For comparison, Jupiter contains enough mass to make 320 Earths.) The green triangles indicate planets in our solar system.

In a transit, the planet's circular disk blocks the light of the star's circular disk. The area of a circle is πR2. The amount of light the planet blocks, called the transit depth, is then given by

πR^2planet/πR^2star = R^2 planet/ R^2 star

important functions of a telescope ?

(1) to collect the faint light from an astronomical source and (2) to focus all the light into a point or an image. **more light we collect the better we can study objects

what happens when light atomic nuclei come together to form a heavier one or vice versa?

--What this means is that, in general, when light atomic nuclei come together to form a heavier one (up to iron), mass is lost and energy is released. This joining together of atomic nuclei is called nuclear fusion. -->Energy can also be produced by breaking up heavy atomic nuclei into lighter ones (down to iron); this process is called nuclear fission.

atomic nucleus

-particles held together by nuclear force

how to determine whether a specific object is a brown dwarf or a very low mass star

. An independent measure of mass is required to determine whether a specific object is a brown dwarf or a very low mass star. Since brown dwarfs cool steadily throughout their lifetimes, the spectral type of a given brown dwarf changes with time over a billion years or more from late M through L, T, and Y spectral types.

what happens in the atoms inside as a degenerate star cools?

. Calculations show that as a degenerate star cools, the atoms inside it in essence "solidify" into a giant, highly compact lattice (organized rows of atoms, just like in a crystal). When carbon is compressed and crystallized in this way, it becomes a giant diamond-like star.

distinct characteristics of waves generated by charged particles

1.electromagnetic waves do not require water or air: the fields generate each other and so can move through a vacuum (such as outer space). --->19th century scientists made aehter- something that fils space so em waves have some medium through which they move 2.all electromagnetic waves move at the same speed in empty space--> speed of light 3*10^8 m/s--> no matter what they move at this fastest possible speed --> we perceive differences as color

Property of light

1.light can be reflected from a surface. 2.Light is also bent, or refracted, when it passes from one kind of transparent material into another—say, from the air into a glass lens.

what determines how good resolution is

1.size of the telescope. Larger apertures produce sharper images

Using a Spectrum to Determine Stellar Rotation

A rotating star will show broader spectral lines than a nonrotating star. Figure 17.14

Lyman series

A set of spectral lines that appear in the UV region when a hydrogen atom undergoes a transition from energy levels n>1 to n=1.

Balmer series

A set of spectral lines that appear in the visible light region when a hydrogen atom undergoes a transition from energy levels n>2 to n=2.

spectroscopic binary

A star like Mizar A, which appears as a single star when photographed or observed visually through the telescope, but which spectroscopy shows really to be a double star

The main-sequence lifetimes of stars of different masses

A star of 1 solar mass remains there for roughly 10 billion years, while a star of about 0.4 solar mass has a main-sequence lifetime of some 200 billion years, which is longer than the current age of the universe. (Bear in mind, however, that every star spends most of its total lifetime on the main sequence. Stars devote an average of 90% of their lives to peacefully fusing hydrogen into helium.)

adaptive optics

A technique in which telescope mirrors flex rapidly to compensate for the bending of starlight caused by atmospheric turbulence -->most effective in the infrared region of the spectrum ****use of a small flexible mirror placed in the beam of a telescope. A sensor measures how much the atmosphere has distorted the image, and as often as 500 times per second, it sends instructions to the flexible mirror on how to change shape in order to compensate for distortions produced by the atmosphere. The light is thus brought back to an almost perfectly sharp focus at the detector.

Reflector telescope

A telescope that uses a large mirror to reflect and focus light.

refracting telescope

A telescope that uses convex lenses to gather and focus light Galileo's telescopes were refractors, as are today's binoculars and field glasses. However, there is a limit to the size of a refracting telescope (largest is 49 inches)

Vela Supernova Remnant

About 11,000 years ago, a dying star in the constellation of Vela exploded, becoming as bright as the full moon in Earth's skies. You can see the faint rounded filaments from that explosion in the center of this colorful image. The edges of the remnant are colliding with the interstellar medium, heating the gas they plow through to temperatures of millions of K. Telescopes in space also reveal a glowing sphere of X-ray radiation from the remnant.

H-R Diagrams of Older Clusters

After 4 billion years have passed, many more stars, including stars that are only a few times more massive than the Sun, have left the main sequence (Figure 22.13). This means that no stars are left near the top of the main sequence; only the low-mass stars near the bottom remain. The older the cluster, the lower the point on the main sequence (and the lower the mass of the stars) where stars begin to move toward the red giant region.

As clusters get older, their H-R diagrams begin to change.....

After a short time (less than a million years after they reach the main sequence), the most massive stars use up the hydrogen in their cores and evolve off the main sequence to become red giants and supergiants. As more time passes, stars of lower mass begin to leave the main sequence and make their way to the upper right of the H-R diagram

what happens quickly and then what happens slowly

After the deuterium nucleus is formed, it survives an average of only about 6 seconds before being converted into 3He. About a million years after that (on average), the 3He nucleus will combine with another to form 4He.

what happens to stars with mass 150 Msun and greater?

After the helium in its core is exhausted (see The Evolution of More Massive Stars), the evolution of a massive star takes a significantly different course from that of lower-mass stars. In a massive star, the weight of the outer layers is sufficient to force the carbon core to contract until it becomes hot enough to fuse carbon into oxygen, neon, and magnesium. This cycle of contraction, heating, and the ignition of another nuclear fuel repeats several more times. After each of the possible nuclear fuels is exhausted, the core contracts again until it reaches a new temperature high enough to fuse still-heavier nuclei. The products of carbon fusion can be further converted into silicon, sulfur, calcium, and argon. And these elements, when heated to a still-higher temperature, can combine to produce iron. Massive stars go through these stages very, very quickly. In really massive stars, some fusion stages toward the very end can take only months or even days! This is a far cry from the millions of years they spend in the main-sequence stage.

Herman Vogel

Algol is a spectroscopic binary. The spectral lines of Algol were not observed to be double because the fainter star of the pair gives off too-little light compared with the brighter star for its lines to be conspicuous in the composite spectrum. Nevertheless, the periodic shifting back and forth of the brighter star's lines gave evidence that it was revolving about an unseen companion. (The lines of both components need not be visible for a star to be recognized as a spectroscopic binary.) verified Goodricke's hypothesis.

k-62 system

All but one of the planets in the K-62 system are larger than Earth. These are super-Earths, and one of them (62d) is in the size range of a mini-Neptune, where it is likely to be largely gaseous. The smallest planet in this system is about the size of Mars. The three inner planets orbit very close to their star, and only the outer two have orbits larger than Mercury in our system. The green areas represent each star's "habitable zone," which is the distance from the star where we calculate that surface temperatures would be consistent with liquid water. The Kepler-62 habitable zone is much smaller than that of the Sun because the star is intrinsically fainter

electromagnetic spectrum

All of the frequencies or wavelengths of electromagnetic radiation refer to 5.6

NGC 3293.

All the stars in an open star cluster like NGC 3293 form at about the same time. The most massive stars, however, exhaust their nuclear fuel more rapidly and hence evolve more quickly than stars of low mass. As stars evolve, they become redder. The bright orange star in NGC 3293 is the member of the cluster that has evolved most rapidly. The dense clouds of gas and dust are gone. One massive star has evolved to become a red giant and stands out as an especially bright orange member of the cluster.

More faint stars being discovered?

Along with the L and T brown dwarfs already observed in our neighborhood, astronomers expect to find perhaps hundreds of additional T dwarfs. Many of these are likely to be even cooler than the coolest currently known T dwarf. The reason the lowest-mass dwarfs are so hard to find is that they put out very little light—ten thousand to a million times less light than the Sun. Only recently has our technology progressed to the point that we can detect these dim, cool objects.

Doppler effect

An observed change in the frequency of a wave when the source or observer is moving The greater the motion toward or away from us, the greater the Doppler shift.

Planet Transits

As the planet transits, it blocks out some of the light from the star, causing a temporary dimming in the brightness of the star. The top figure shows three moments during the transit event and the bottom panel shows the corresponding light curve: (1) out of transit, (2) transit ingress, and (3) the full drop in brightness.

astronomy as an observational science

Astronomy is called an observational science--> make tests by observing may samples of the kind of object we want to study and noting carefully how different samples vary --> do more tests and change based on new tech and info Astronomy is also a historical science--> observe what has already happened

focus

At the focus, an image of the light source appears. In the case of parallel light rays, the distance from the lens to the location where the light rays focus, or image, behind the lens is called the focal length of the lens.

Solar Day

Basic unit of time = day Solar day: rotation period of Earth with respect to the Sun--> set our clocks to sun time

why are CCD's better

Because CCDs typically record as much as 60-70% of all the photons that strike them, and the best silicon and infrared CCDs exceed 90% sensitivity, we can detect much fainter objects. CDs also provide more accurate measurements of the brightness of astronomical objects than photography, and their output is digital—in the form of numbers that can go directly into a computer for analysis.

why are there many molecular clouds that form only low mass stars?

Because low-mass stars do not have strong winds and do not die by exploding, triggered star formation cannot occur in these clouds. There are also stars that form in relative isolation in small cores. Therefore, not all star formation is originally triggered by the death of massive stars. However, there are likely to be other possible triggers, such as spiral density waves and other processes we do not yet understand.

Why use a color index if it ultimately implies temperature?

Because the brightness of a star through a filter is what astronomers actually measure, and we are always more comfortable when our statements have to do with measurable quantities.

why do telescopes designed with mirrors avoid the problems of refracting telescopes

Because the light is reflected from the front surface only, flaws and bubbles within the glass do not affect the path of the light. In a telescope designed with mirrors, only the front surface has to be manufactured to a precise shape, and the mirror can be supported from the back. For these reasons, most astronomical telescopes today (both amateur and professional) use a mirror rather than a lens to form an image; this type of telescope is called a reflecting telescope-->newton

Betelgeuse

Betelgeuse is in the constellation Orion, the hunter; in the right image, it is marked with a yellow "X" near the top left. In the left image, we see it in ultraviolet with the Hubble Space Telescope, in the first direct image ever made of the surface of another star. As shown by the scale at the bottom, Betelgeuse has an extended atmosphere so large that, if it were at the center of our solar system, it would stretch past the orbit of Jupiter.

colors and temperature

Blue 25000k White 10000k Yellow 6000k Orange 4000k Red 3000k The hottest stars have temperatures of over 40,000 K, and the coolest stars have temperatures of about 2000 K. Our Sun's surface temperature is about 6000 K; its peak wavelength color is a slightly greenish-yellow. In space, the Sun would look white, shining with about equal amounts of reddish and bluish wavelengths of light. It looks somewhat yellow as seen from Earth's surface because our planet's nitrogen molecules scatter some of the shorter (i.e., blue) wavelengths out of the beams of sunlight that reach us, leaving more long wavelength light behind. This also explains why the sky is blue: the blue sky is sunlight scattered by Earth's atmosphere.

Ernest Rutherford

Bombarded a thin piece of gold foil with a stream of alpha particles Most of these particles passed through the gold foil just as if the atoms in it were nearly empty space 1/8000 bounced back and reversed direction from the foil

best x-ray telescopes

Chandra X-ray Observatory, which was launched in 1999 (It is producing X-ray images with unprecedented resolution and sensitivity.

interferometer

Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes. recent advances make it possible to do interferometry at visible-light and infrared wavelengths.

colors and stars

Color does not depend on the distance to the object. Blue colors dominate the visible light output of very hot stars (with much additional radiation in the ultraviolet). On the other hand, cool stars emit most of their visible light energy at red wavelengths (with more radiation coming off in the infrared)

Helio-Centric Model

Copernicus worked on a general model with sun in the middle and deduced that planets closer to the Sun have greater orbital speeds--> could explain retrograde motions w/o epicycles Copernicus could not prove that Earth revolves around the Sun--> old Ptolemaic system could have accounted for planetary motion but Copernicus said it was clumsy Experiments were frowned upon--> human thought + divine intervention best evidence--> heliocentrism not able to prove as valid Now scientists scurry for tests to find experimental evidence

Equation for estimating

D^3 = (M1 + M2)P^2 where D is in astronomical units, P is measured in years, and M1 + M2 is the sum of the masses of the two stars in units of the Sun's mass. This is a very useful formula for astronomers; it says that if we can observe the size of the orbit and the period of mutual revolution of the stars in a binary system, we can calculate the sum of their masses. Most spectroscopic binaries have periods ranging from a few days to a few months, with separations of usually less than 1 AU between their member stars.

difference between the density of a molecular cloud core and the density of the youngest stars that can be detected

Direct observations of this collapse to higher density are nearly impossible for two reasons. First, the dust-shrouded interiors of molecular clouds where stellar births take place cannot be observed with visible light. Second, the timescale for the initial collapse—thousands of years—is very short, astronomically speaking. Since each star spends such a tiny fraction of its life in this stage, relatively few stars are going through the collapse process at any given time. Nevertheless, through a combination of theoretical calculations and the limited observations available, astronomers have pieced together a picture of what the earliest stages of stellar evolution are likely to be.

radical velocity

Doppler effect is produced only by a motion toward or away from the observer Sideways motion does not produce such an effect.

converting matter into energy

E= mc^2 energy, mass, speed of light (3 * 10^8 m/s) he factor of c2 is just the number that Einstein showed must be used to relate mass and energy. --> conversion of even small mass results in a lot of energy --> this conversion is the source of the Sun's light and heat

calculating the diameter of the sun

EXAMPLE 19.2 pg 670

how do the light look when it reaches the telescope ?

Each light beam path will be slightly different, and each will reach the detector of the telescope at a slightly different place. The result is a blurred image, and because the cells are being blown by the wind, the nature of the blur will change many times each second.

model of the sun

Energy is generated through fusion in the core of the Sun, which extends only about one-quarter of the way to the surface but contains about one-third of the total mass of the Sun. At the center, the temperature reaches a maximum of approximately 15 million K, and the density is nearly 150 times that of water. The energy generated in the core is transported toward the surface by radiation until it reaches a point about 70% of the distance from the center to the surface. At this point, convection begins, and energy is transported the rest of the way, primarily by rising columns of hot gas.

Magnetism

Experiments with electric charges demonstrated that magnetism was the result of moving charged particles. -sometimes motion is clear and sometimes more subtle, - as in the kind of magnet you buy in a hardware store, in which many of the electrons inside the atoms are spinning in roughly the same direction; it is the alignment of their motion that causes the material to become magnetic.

Stefan-Botlzmann Law

F = σT^4 where F stands for the energy flux and σ (Greek letter sigma) is a constant number (5.67 × 10-8).

how the temperature, density, rate of energy generation, and composition vary from the center of the Sun to its surface.

FIGURE 16.16

Light Curve of an Eclipsing Binary

FIGURE 18.10 The light curve of an eclipsing binary star system shows how the combined light from both stars changes due to eclipses over the time span of an orbit. This light curve shows the behavior of a hypothetical eclipsing binary star with total eclipses (one star passes directly in front of and behind the other). The numbers indicate parts of the light curve corresponding to various positions of the smaller star in its orbit. In this diagram, we have assumed that the smaller star is also the hotter one so that it emits more flux (energy per second per square meter) than the larger one. When the smaller, hotter star goes behind the larger one, its light is completely blocked, and so there is a strong dip in the light curve. When the smaller star goes in front of the bigger one, a small amount of light from the bigger star is blocked, so there is a smaller dip in the light curve.

Spectral Lines

Figure 17.9 A giant star with a very-low-pressure photosphere shows very narrow spectral lines (bottom), whereas a smaller star with a higher- pressure photosphere shows much broader spectral lines (top).

Barnard 68 in Infrared.

Figure 20.14 In this image, the red color shows radiation emitted in the infrared at a wavelength of 2.2 microns. Interstellar extinction is much smaller at infrared than at visible wavelengths, so the stars behind the cloud become visible in the infrared channel.

Kepler's Third Law

First 2 laws of planetary motion describe the shape with planets orbit and allows us to calculate the speed of its motion at any point in the orbit Wanted to know why the orbits of the planets were spaced out as they are and tried to find a mathematical pattern of their movements Orbital period: the time it takes a planet to travel once around the sun tell me major axis= average distance from the sun Kepler's third law says that a planet's orbital speed(years) squared is proportional to the semimajor axis(astronomical unit) of its orbit cubed (P^2 ∝ a^3) Astronomical Unit: average distance between earth and the sun (1.5*10^8 km), in these units --> P^2 = a^3 Kepler's third law applies to all objects orbiting the Sun, including Earth, and provides a means for calculating their relative distances from the Sun from the time they take to orbit

advantage for infrared observations from space

First is the elimination of all interference from the atmosphere. Equally important is the opportunity to cool the entire optical system of the instrument in order to nearly eliminate infrared radiation from the telescope itself.

Declination

From the celestial equator toward the north (positive) or south (negative) --> like latitude Polaris, the star near north celestial pole has a declination of almost + 90 degrees

Galaxy Clusters

Galaxy form small and big clusters and sometimes larger groups called superclusters (The Local Group is part of a supercluster of galaxies, called the Virgo Supercluster) Farther where other galaxies are too dim to see = quasars (These are brilliant centers of galaxies, glowing with the light of an extraordinarily energetic process) The enormous energy of the quasars is produced by gas that is heated to a temperature of millions of degrees as it falls toward a massive black hole and swirls around it. The brilliance of quasars makes them the most distant beacons we can see in the dark oceans of space. Quasars allow us to see Big Bang explosion that marks the beginning of time detected the feeble glow of the explosion itself, filling the universe and thus coming to us from all directions in space.

Galileo's Astronomical Observations

Galileo heard of the discovery an assembled the telescope with his own three power magnification which mean distant objects appear three times nearer and larger Galileo showed the telescope with a 9 times magnification to government officials in Venice and got great respect in compensation Galileo saw stars that were too faint to be seen with the unaided eye, some moons revolving around other planets--> proof that moon revolved around Earth Saw means crater is mountain ranges of valleys etc. After Galileo's work it became increasingly difficult to deny Copernican view--> Copernicus and Galileo began to revolutionize our conception of the cosmos---> Earth insignificance became prevalent

celestial sphere

Greeks regard it as a celestial sphere, an actual crystalline material with stars embedded like jewels A stick through North and South pole = axis --> rotates on it every 24 hours

Prime Meridian

Greenwich England = Prime Meridian ( the longitude = 0 degrees) Greenwich was selected because it was between continental Europe and the USA & b/c it was where more of the development of the measuring of longitude occurred

Hipparchus

H erected an observatory on the island of Rhodes around 150 BCE when Romans expanded into Mediterranean Pioneering star catalog with about 850 entries with their specific location in the sky (like longitude and latitude) Divided the stars into apparent magnitudes according to apparent brightness--> first magnitude (brightest), second magnitude.......etc. Made discovery: the position in the sky of the north celestial pole had altered over the previous century and a half--> direction around which the sky appears to rotate changes slowly but continuously

magnitude

He referred to the brightest stars in his catalog as first-magnitudes stars, whereas those so faint he could barely see them were sixth-magnitude stars.

helium vs. hydrogen

Helium has two protons and nucleus contains two neutrons

Gas Jets Flowing away from a Protostar

Here we see the neighborhood of a protostar, known to us as HH 34 because it is a Herbig-Haro object. The star is about 450 light-years away and only about 1 million years old. Light from the star itself is blocked by a disk, which is larger than 60 billion kilometers in diameter and is seen almost edge-on. Jets are seen emerging perpendicular to the disk. The material in these jets is flowing outward at speeds up to 580,000 kilometers per hour. The series of three images shows changes during a period of 5 years. Every few months, a compact clump of gas is ejected, and its motion outward can be followed. The changes in the brightness of the disk may be due to motions of clouds within the disk that alternately block some of the light and then let it through.

Measuring Parallaxes in Space

Hipparcos in 1989, which measured distances for thousands of stars out to about 300 light-years with an accuracy of 10 to 20% Gaia is expected to measure the position and distances to almost one billion stars with an accuracy of a few ten- millionths of an arcsecond. Gaia's distance limit will extend well beyond Hipparcos, studying stars out to 30,000 light-years (100 times farther than Hipparcos, covering nearly 1/3 of the galactic disk). Gaia will also be able to measure proper motions[2] for thousands of stars in the halo of the Milky Way—something that can only be done for the brightest stars right now. At the end of Gaia's mission, we will not only have a three-dimensional map of a large fraction of our own Milky Way Galaxy, but we will also have a strong link in the chain of cosmic distances that we are discussing in this chapter.

why happens when hot stars are nearby gas?

Hot stars are able to heat nearby gas to temperatures close to 10,000 K. The ultraviolet radiation from the stars also ionizes the hydrogen (remember that during ionization, the electron is stripped completely away from the proton). Such a detached proton won't remain alone forever when attractive electrons are around; it will capture a free electron, becoming a neutral hydrogen once more. However, such a neutral atom can then absorb ultraviolet radiation again and start the cycle over. At a typical moment, most of the atoms near a hot star are in the ionized state.

The Seasons & Sunshine

How does the Sun's favoring one hemisphere translate into making it warmer for us down on the surface of Earth? When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth's surface June is more direct and intense in NH and hence more effective at heating In September and March, Earth leans "sideways"—neither into the Sun nor away from it—so the two hemispheres are equally favored with sunshine.

human beings and stars

Human beings developed on a planet around a G-type star. This means that the Sun's stable main-sequence lifetime is so long that it afforded life on Earth plenty of time to evolve. When searching for intelligent life like our own on planets around other stars, it would be a pretty big waste of time to search around O- or B-type stars. These stars remain stable for such a short time that the development of creatures complicated enough to take astronomy courses is very unlikely.

The atom

Hydrogen simplest atom Mass of electron is 2000x smaller than the mass of a proton Electromagnetic force that holds proton and lector's together, just as gravity is the force that keep planets in orbit around the sun

The atom

Hydrogen simplest atom Mass of electron is 2000x smaller than the mass of a proton Electromagnetic force that holds proton and lector's together, just as gravity is the force that keep planets in orbit around the sun as protons increase the ratio of proton to neutrons goes up

why is stellar spectra not based on chemical composition of stars

Hydrogen, for example, is by far the most abundant element in most stars. However, lines of hydrogen are not seen in the spectra of the hottest and the coolest stars. In the atmospheres of the hottest stars, hydrogen atoms are completely ionized. Because the electron and the proton are separated, ionized hydrogen cannot produce absorption lines. In the atmospheres of the coolest stars, hydrogen atoms have their electrons attached and can switch energy levels to produce lines. However, practically all of the hydrogen atoms are in the lowest energy state (unexcited) in these stars and thus can absorb only those photons able to lift an electron from that first energy level to a higher level. Photons with enough energy to do this lie in the ultraviolet part of the electromagnetic spectrum, and there are very few ultraviolet photons in the radiation from a cool star. What this means is that if you observe the spectrum of a very hot or very cool star with a typical telescope on the surface of Earth, the most common element in that star, hydrogen, will show very weak spectral lines or none at all

how to look at new model today

Hypothesis proposed has be checked with what's already known --> heliocentric theory passes this test, planetary positions as well as geocentric Determine which predictions the new hypothesis makes that differ from these if competing ideas

luminosity classes.

Ia: Brightest supergiants Ib: Less luminous supergiants II: Bright giants III: Giants IV: Subgiants (intermediate between giants and main-sequence stars) V: Main-sequence stars

ionizaton

If enough energy is absorbed, the electron can be completely removed from the atom

Magnitude scale rules

If two stars differ by 0.75 magnitudes, they differ by a factor of about 2 in brightness. If they are 2.5 magnitudes apart, they differ in brightness by a factor of 10, and a 4-magnitude difference corresponds to a difference in brightness of a factor of 40

how do photographic plates work?

In a photographic plate, a light-sensitive chemical coating is applied to a piece of glass that, when developed, provides a lasting record of the image. At observatories around the world, vast collections of photographs preserve what the sky has looked like during the past 100 years.

National Radio Astronomy Observatory's Very Large Array (VLA)

It consists of 27 movable radio telescopes (on railroad tracks), each having an aperture of 25 meters, spread over a total span of about 36 kilometers. By electronically combining the signals from all of its individual telescopes, this array permits the radio astronomer to make pictures of the sky at radio wavelengths comparable to those obtained with a visible-light telescope, with a resolution of about 1 arcsecond.

Neutron Stars with Companions

It is possible that, under the right circumstances, a binary system can even survive the explosion of one of its members as a type II supernova. In that case, an ordinary star can eventually share a system with a neutron star. If material is then transferred from the "living" star to its "dead" (and highly compressed) companion, this material will be pulled in by the strong gravity of the neutron star. Such infalling gas will be compressed and heated to incredible temperatures. It will quickly become so hot that it will experience an explosive burst of fusion. The energies involved are so great that we would expect much of the radiation from the burst to emerge as X-rays. And indeed, high-energy observatories above Earth's atmosphere (see Astronomical Instruments) have recorded many objects that undergo just these types of X-ray bursts.

easiest way to measure diameter of Sun

Its angular diameter—that is, its apparent size on the sky—is about 1/2°. If we know the angle the Sun takes up in the sky and how far away it is, we can calculate its true (linear) diameter, which is 1.39 million kilometers, or about 109 times the diameter of Earth. **CANT DO IT For other stars they are too far away

Figure 23.6 Structure of an Old Massive Star

Just before its final gravitational collapse, the core of a massive star resembles an onion. The iron core is surrounded by layers of silicon and sulfur, oxygen, neon, carbon mixed with some oxygen, helium, and finally hydrogen. Outside the core, the composition is mainly hydrogen and helium. (Note that this diagram is not precisely to scale but is just meant to convey the general idea of what such a star would be like.) (credit: modification of work by ESO, Digitized Sky Survey)

Gravitational Contraction as a Source of Energy

Kelvin & Helmholtz-->Sun might produce energy by the conversion of gravitational energy into heat They suggested that the outer layers of the Sun might be "falling" inward because of the force of gravity. In other words, they proposed that the Sun could be shrinking in size, staying hot and bright as a result.

The selection effects (or biases) in the Kepler data are similar to those in Doppler observations.

Large planets are easier to find than small ones, and short-period planets are easier than long-period planets. If we require three transits to establish the presence of a planet, we are of course limited to discovering planets with orbital periods less than one-third of the observing interval. Thus, it was only in its fourth and final year of operation that Kepler was able to find planets with orbits like Earth's that require 1 year to go around their star.

Johannes Kepler

Learn the principles of the Copernican system and became converted to the heliocentric hypothesis Served as an assistant to Brahe who set him to work and trying to find a satisfactory theory of planetary motion didn't give Kepler any records until Brahe died in the fear that Kepler would get all the glory developed a series of principles, now known as Kepler's three laws, which described the behavior of planets based on their path through space--> the first 2 laws were published in 1609 in the new astronomy

Young Cluster NGC 2264

Located about 2600 light-years from us, this region of newly formed stars, known as the Christmas Tree Cluster, is a complex mixture of hydrogen gas (which is ionized by hot embedded stars and shown in red), dark obscuring dust lanes, and brilliant young stars.

circumpolar zone

Located or found in one of Earth's polar regions. Denoting a star that from a given observer's latitude does not go below the horizon during its diurnal motion. The closer an observer is to one of the poles, the greater the portion of the sky that contains circumpolar star Polaris: moves the least amount as the northern sky each day

Modern Visible and infrared telescope

Look at table 6.1 —> largest aperture 11.1 *9.9 South African large telescope

Kepler-62 System

Many have only two known planets, but a few have as many as five, and one has eight (the same number of planets as our own solar system). For the most part, these are very compact systems with most of their planets closer to their star than Mercury is to the Sun. The figure below shows one of the largest exoplanet systems: that of the star called Kepler-62

galileo & beginning of modern science

Many of the modern scientific concepts of observation experimentation in the testing of hypothesis through careful quantitative measurements were pioneered by Galileo Greatest contributions were in the field of mechanics the study of motion and the actions of forces on bodies--> rest is no more natural than motion reasoned that if all resisting effects could be removed the object would remain in steady motion indefinitely he argued that forces not only needed to start motion but also to stop it Accelerate: change in speed or direction--> objects accelerate uniformly in equal intervals of time they gain equal increments of speed Defended Copernicus heliocentric theory--> Church issued decree of absurdity to not defend it

Hot Jupiters

Many of these giant planets are orbiting close to their stars

Mass, Volume, Density

Mass which is a measure of the amount of material within an object Volume is measure of the physical space it occupies (cm ^3/mL) Density = MASS/Vol

what is the problem with the rutherford model

Maxwell's theory of electromagnetic radiation says that when electrons change either speed or the direction of motion, they must emit energy. Orbiting electrons constantly change their direction of motion, so they should emit a constant stream of energy. -Applying Maxwell's theory to Rutherford's model, all electrons should spiral into the nucleus of the atom as they lose energy, and this collapse should happen very quickly—in about 10-16 seconds.

angular momentum

Measure of the rotation of a body as it revolves around some fixed point defined as mass times velocity times distance from fixed point around which it revolves If the motion of a particular object takes place at a constant velocity at a fixed distance from the spin center angular momentum is also constant Kepler's second law--> conservation of angular momentum As the planet approaches the sun in its elliptical orbit the distance to the spin center degrees is the planet speeds up to conserve the angular moment

Orbits of Planets

Mercury has the shortest orbital period and the highest orbital speed All planets have orbits of rather low eccentricity --> most eccentric orbit, Mars has a eccentricity greater than that of many other planets--> Kepler ellipse deduction All major planets lie within 10 degrees of the common plane of the solar system

Mizar and complex star systems

Mizar has been known for centuries to have a faint companion called Alcor, which can be seen without a telescope. Mizar and Alcor form an optical double—a pair of stars that appear close together in the sky but do not orbit each other. Through a telescope, as Riccioli discovered in 1650, Mizar can be seen to have another, closer companion that does orbit it; Mizar is thus a visual binary. The two components that make up this visual binary, known as Mizar A and Mizar B, are both spectroscopic binaries. So, Mizar is really a quadruple system of stars.

Figure 23.2 Relating Masses and Radii of White Dwarfs.

Models of white-dwarf structure predict that as the mass of the star increases (toward the right), its radius gets smaller and smaller.

most of neutral hydrogen in the galaxy

Modern radio observations show that most of the neutral hydrogen in our Galaxy is confined to an extremely flat layer, less than 300 light-years thick, that extends throughout the disk of the Milky Way Galaxy. This gas has densities ranging from about 0.1 to about 100 atoms per cm3, and it exists at a wide range of temperatures, from as low as about 100 K (-173 °C) to as high as about 8000 K. These regions of warm and cold gas are interspersed with each other, and the density and temperature at any particular point in space are constantly changing.

interpretation of Newton's second law

Momentum depends on Speed (how fast a body moves or not) Direction of motion 3. mass ( amount of matter in a body) Momentum = Mass X velocity Friction can slow things down and act as an opposing force--> momentum of body can change all the under the action of an outside influence Newton's second law expresses force in terms of its ability to change momentum with time A force has both size and direction so that means a force is required to change either the speed or direction of a body Acceleration: rate of change in an object's velocity Proportional to the force being applied to it Greater the force greater the acceleration Those with less mass will accelerate more if you apply the same amount of force

RR Lyrae stars

More common than the cepheids, but less luminous, thousands of these pulsating variables are known in our Galaxy. The periods of RR Lyrae stars are always less than 1 day, and their changes in brightness are typically less than about a factor of two.

more on brown dwarfs

Most brown dwarfs start out with atmospheric temperatures and spectra like those of true stars with spectral classes of M6.5 and later, even though the brown dwarfs are not hot and dense enough in their interiors to fuse hydrogen. In fact, the spectra of brown dwarfs and true stars are so similar from spectral types late M through L that it is not possible to distinguish the two types of objects based on spectra alone.

open clusters are visible to the unaided eye

Most famous among them is the Pleiades (Figure 20.13), which appears as a tiny group of six stars (some people can see even more than six, and the Pleiades is sometimes called the Seven Sisters). This cluster is arranged like a small dipping spoon and is seen in the constellation of Taurus, the bull. A good pair of binoculars shows dozens of stars in the cluster, and a telescope reveals hundreds. (A car company, Subaru, takes its name from the Japanese term for this cluster; you can see the star group on the Subaru logo.) The Hyades is another famous open cluster in Taurus. To the naked eye, it appears as a V-shaped group of faint stars marking the face of the bull. Telescopes show that Hyades actually contains more than 200 stars.

Universal Compositin

Most of the universe is fanatically sparse and empty Molecules are the smallest particles into which any matter can be divided while still retaining its chemical properties Most abundant elements: hydrogen, helium & carbon--> bulk of matter in an atom in nucleus Everything in the universe can be explained by: gravity, electromagnetism and two forces that act on a nucleus level

variable stars

Most stars are constant in their luminosity, at least to within a percent or two. Like the Sun, they generate a steady flow of energy from their interiors However, some stars are seen to vary in brightness and, for this reason, are called variable stars. Many such stars vary on a regular cycle, like the flashing bulbs that decorate stores and homes during the winter holidays.

how does dust interact with color?

Much of the violet, blue, and green light from these stars has been scattered or absorbed by dust, so it does not reach Earth. Some of their orange and red light, with longer wavelengths, on the other hand, more easily penetrates the intervening dust and completes its long journey through space to enter Earth-based telescopes (Figure 20.15). Thus, the star looks redder from Earth than it would if you could see it from nearby. (Strictly speaking, reddening is not the most accurate term for this process, since no red color is added; instead, blues and related colors are subtracted, so it should more properly be called "deblueing.") In the most extreme cases, stars can be so reddened that they are entirely undetectable at visible wavelengths and can be seen only at infrared or longer wavelengths

nova

Novae fade away in a few months to a few years. Hundreds of novae have been observed, each occurring in a binary star system and each later showing a shell of expelled material. A number of stars have more than one nova episode, as more material from its neighboring star accumulates on the white dwarf and the whole process repeats. As long as the episodes do not increase the mass of the white dwarf beyond the Chandrasekhar limit (by transferring too much mass too quickly), the dense white dwarf itself remains pretty much unaffected by the explosions on its surface.

seven spectral classes

O, B, A, F, G, K, and M. Recently, astronomers have added three additional classes for even cooler objects—L, T, and Y. *** ORDER OF DECREASING TEMP

what about the atomic nuclei of a dying star?

Of course, the dying star also has atomic nuclei in it, not just electrons, but it turns out that the nuclei must be squeezed to much higher densities before their quantum nature becomes apparent. As a result, in white dwarfs, the nuclei do not exhibit degeneracy pressure. Hence, in the white dwarf stage of stellar evolution, it is the degeneracy pressure of the electrons, and not of the nuclei, that halts the collapse of the core.

why is it that even at high temps, it is difficult to force two protons to combine?

On average, a proton will rebound from other protons in the Sun's crowded core for about 14 billion years, at the rate of 100 million collisions per second, before it fuses with a second proton. This is, however, only the average waiting time. Some of the enormous numbers of protons in the Sun's inner region are "lucky" and take only a few collisions to achieve a fusion reaction: they are the protons responsible for producing the energy radiated by the Sun. Since the Sun is about 4.5 billion years old, most of its protons have not yet been involved in fusion reactions.

Herbig-Haro (HH) object

On occasion, the jets of high-speed particles streaming away from the protostar collide with a somewhat-denser lump of gas nearby, excite its atoms, and cause them to emit light. These glowing regions, each of which is known as a Herbig-Haro (HH) object after the two astronomers who first identified them, allow us to trace the progress of the jet to a distance of a light-year or more from the star that produced it.

Castor

One well-known binary star is Castor, located in the constellation of Gemini. By 1804, astronomer William Herschel, who also discovered the planet Uranus, had noted that the fainter component of Castor had slightly changed its position relative to the brighter component. (We use the term "component" to mean a member of a star system.) Here was evidence that one star was moving around another. It was actually the first evidence that gravitational influences exist outside the solar system.

Why is spectral analysis important

Only in this way can we "sample" the stars, which are too far away for us to visit. Encoded in the electromagnetic radiation from celestial objects is clear information about the chemical makeup of these objects. Only by understanding what the stars were made of could astronomers begin to form theories about what made them shine and how they evolved.

First Two Laws or Planetary Motion

Orbit: a path of an object through space Circular orbit didn't match up with Brahe's observation--> with Mars data, discovered that the orbit of the planet had a shape of a somewhat flatten circle, or ellipse (simplest kind of closed curve in conic sections) In an ellipse, the sum of the distance from two special points inside the ellipse to any point on the ellipse is always the same. These two points inside the ellipse are called its foci Widest diameter of the ellipse = major axis/ half this distance center to ellipse end = semimajor axis: used to specify the size of your lips The ratio of the distance between the foci to the length of the major axis = eccentricity--> rounds depends on this If both foci are in the same position= no distance between them = circle Max eccentricity = 1 = foci at ends of ellipse Kepler saw Mars elliptical orbit --> generalized that orbits of all planets are ellipses Second law deals with speed with which each planet moves along its ellipse= orbital speed Speed up as it gets closer to the sun and slower as it pulls away Covers the same amount of area in the same time interval Speed would be same in a circle but tends to vary because of elliptical orbit

Differences between Palomar telescope and the modern Gemini North telescope

Palomar—>is massive steel structure designed to hold 14.5 tons primary mirror with a 5 meter diameter. Glass tends to sag under its own weight; hence a huge steel structure is needed to hold the mirror Gemini north—> 8 meters in diameter, if it were built using same tech as Palomar telescope, would have to weigh at least 8 times S much and would require an enormous steel structure to support it

limitations to photography

Photographic films are inefficient: only about 1% of the light that actually falls on the film contributes to the chemical change that makes the image; the rest is wasted.

Earth's axis

Points where this line intersects that celestial sphere are called the north celestial pole and the south celestial pole Earth rotates about its axis --> sky turns in opposite direction around those celestial poles

Newton's Laws of Motion

Published The Principia published at Halley's expense--> in it proposed three laws of motion: Newton's first law: Every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force. Newton's second law: The change of motion of a body is proportional to and in the direction of the force acting on it. Newton's third law: For every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions).

how does a spectrometer work

REFER 6.16 Light from the source (actually, the image of a source produced by the telescope) enters the instrument through a small hole or narrow slit, and is collimated (made into a beam of parallel rays) by a lens. The light then passes through a prism, producing a spectrum: different wavelengths leave the prism in different directions because each wavelength is bent by a different amount when it enters and leaves the prism. A second lens placed behind the prism focuses the many different images of the slit or entrance hole onto a CCD or other detecting device. This collection of images (spread out by color) is the spectrum that astronomers can then analyze at a later point. As spectroscopy spreads the light out into more and more collecting bins, fewer photons go into each bin, so either a larger telescope is needed or the integration time must be greatly increased—usually both.

Period-Luminosity Relation for Cepheid Variables.

RR lyrae stars bottom left cepheids the rest

how are cosmic radio waves measured?

Radio waves can produce a current in conductors of electricity such as metals. An antenna is such a conductor: it intercepts radio waves, which create a feeble current in it. The current is then amplified in a radio receiver until it is strong enough to measure or record. astronomers use sophisticated data-processing techniques that allow thousands of separate frequency bands to be detected simultaneously.

how do we find the nearby stars we can't see by naked eye

Recent discoveries of nearby stars have relied heavily upon infrared telescopes that are able to find these many cool, low-mass stars.

Spectral Class for Stars

Refer to textbook 17. the table is slightly off

Just how different are these red giants and supergiants from a main-sequence star?

Relative to the Sun, this supergiant has a much larger radius, a much lower average density, a cooler surface, and a much hotter core. Table 22.2 Red giants can become so large that if we were to replace the Sun with one of them, its outer atmosphere would extend to the orbit of Mars or even beyond (Figure 22.4). This is the next stage in the life of a star as it moves (to continue our analogy to human lives) from its long period of "youth" and "adulthood" to "old age." (After all, many human beings today also see their outer layers expand a bit as they get older.) By considering the relative ages of the Sun and Betelgeuse, we can also see that the idea that "bigger stars die faster" is indeed true here. Betelgeuse is a mere 10 million years old, which is relatively young compared with our Sun's 4.5 billion years, but it is already nearing its death throes as a red supergiant.

Aristotle said earth round

Said earth is round as the Moon enters or emerges from Earth's shadow during an eclipse of the Moon, the shape of the shadow seen on the Moon is always round only sphere = round shadow Aristotle explained that travelers who go south a significant distance are able to observe stars that are not visible farther north. And the height of the North Star—the star nearest the north celestial pole—decreases as a traveler moves south. On a flat Earth, everyone would see the same stars overhead. The only possible explanation is that the traveler must have moved over a curved surface on Earth, showing stars from a different angle. sun farther away = moon hide sun from earth --> solar eclipse Aristarchus of Somas suggested earth move around the Sun ===> rejected by Aristotle and more ****b/c if this was true we would see stars from different places in Earth's orbit, as earth moved along nearby stars should shift their position in the sky relative distant ones

the sun is not cooling down

Scientists conclude that the temperature is highest at the center of a star, dropping to lower and lower values toward the stellar surface. (The high temperature of the Sun's chromosphere and corona may therefore appear to be a paradox. But remember from The Sun: A Garden-Variety Star that these high temperatures are maintained by magnetic effects, which occur in the Sun's atmosphere.) The outward flow of energy through a star robs it of its internal heat, and the star would cool down if that energy were not replaced. Similarly, a hot iron begins to cool as soon as it is unplugged from its source of electric energy. Therefore, a source of fresh energy must exist within each star. In the Sun's case, we have seen that this energy source is the ongoing fusion of hydrogen to form helium.

Model Stars

Scientists use the principles we have just described to calculate what the Sun's interior is like. These physical ideas are expressed as mathematical equations that are solved to determine the values of temperature, pressure, density, the efficiency with which photons are absorbed, and other physical quantities throughout the Sun. The solutions obtained, based on a specific set of physical assumptions, provide a theoretical model for the interior of the Sun.

Parallax Diagram

Seen from opposite sides of Earth's orbit, a nearby star shifts position when compared to a pattern of more distant stars. Astronomers actually define parallax to be one-half the angle that a star shifts when seen from opposite sides of Earth's orbit (the angle labeled P in Figure 19.6). The reason for this definition is just that they prefer to deal with a baseline of 1 AU instead of 2 AU. As Earth revolves around the Sun, the direction in which we see a nearby star varies with respect to distant stars. We define the parallax of the nearby star to be one half of the total change in direction, and we usually measure it in arcseconds.

fusion stars

Since only 0.7% of the hydrogen used in fusion reactions is converted into energy, fusion does not change the total mass of the star appreciably during this long period. It does, however, change the chemical composition in its central regions where nuclear reactions occur: hydrogen is gradually depleted, and helium accumulates. This change of composition changes the luminosity, temperature, size, and interior structure of the star. When a star's luminosity and temperature begin to change, the point that represents the star on the H-R diagram moves away from the zero-age main sequence.

Orbits of Asteroids & Comets

Small chunks of material left over from the formation process of the solar system Asteroids have orbits with smaller semimajor axes than do comets ( majority lie between 2.2 & 3.3 AU Asteroid belt is in the middle of Mars and Jupiter--> b/c they are so far apart Comets generally have orbits of larger size & greater eccentricity than those of the asteroids--> .8 eccentricity of higher They spend most of their time far away from Sun moving very slowly, as they approach perihelion--> comets stepped up and whip through inner parts of orbit faster

reflection nebula

Some dense clouds of dust are close to luminous stars and scatter enough starlight to become visible. Such a cloud of dust, illuminated by starlight, is called a reflection nebula, since the light we see is starlight reflected off the grains of dust.

colors vs spectral classes to estimate the temperature of a star

Spectra are harder to measure because the light has to be bright enough to be spread out into all colors of the rainbow, and detectors must be sensitive enough to respond to individual wavelengths. In order to measure colors, the detectors need only respond to the many wavelengths that pass simultaneously through the colored filters that have been chosen—that is, to all the blue light or all the yellow-green light.

Stars within 21 Light-Years of the Sun

Spectral Type. #of Stars A 2 F 1 G 7 K 17 M 94 White Dwarfs. 8 Brown Dwarfs. 33 Only three of the stars in our local neighborhood (one F type and two A types) are significantly more luminous and more massive than the Sun

Satellites

Sputnik the first artificial Earth satellite, was launched by what was then called the Soviet Union on 1957 Artificial and natural satellite act the same = if high enough to be free from atmospheric friction it will remain in orbit forever---> lot of energy to get off Earth Most satellites are launched into low Earth orbit, since this requires the minimum launch energy. At the orbital speed of 8 kilometers per second, they circle the planet in about 90 minute Some of the very low Earth orbits are not indefinitely stable because, as Earth's atmosphere swells from time to time, a frictional drag is generated by the atmosphere on these satellites, eventually leading to a loss of energy and "decay" of the orbit

how large can stars be?

Stars more massive than the Sun are rare. None of the stars within 30 light-years of the Sun has a mass greater than four times that of the Sun. Searches at large distances from the Sun have led to the discovery of a few stars with masses up to about 100 times that of the Sun, and a handful of stars (a few out of several billion) may have masses as large as 250 solar masses. However, most stars have less mass than the Sun.

westerlund 2

Stellar winds and pressure produced by the radiation from the hot stars within the cluster are blowing and sculpting the surrounding gas and dust. The nebula still contains many globules of dust. Stars are continuing to form within the denser globules and pillars of the nebula. This Hubble Space Telescope image includes near-infrared exposures of the star cluster and visible-light observations of the surrounding nebula. Colors in the nebula are dominated by the red glow of hydrogen gas, and blue-green emissions from glowing oxygen.

is the destruction of a white dwarf star binary system a supernova?

Such an explosion is also called a supernova, since, like the destruction of a high-mass star, it produces a huge amount of energy in a very short time. However, unlike the explosion of a high-mass star, which can leave behind a neutron star or black hole remnant, the white dwarf is completely destroyed in the process, leaving behind no remnant. We call these white dwarf explosions type Ia supernovae.

Suppose a life form has the misfortune to develop around a star that happens to lie near a massive star destined to become a supernova. What happens?

Such life forms may find themselves snuffed out when the harsh radiation and high-energy particles from the neighboring star's explosion reach their world. If, as some astronomers speculate, life can develop on many planets around long-lived (lower- mass) stars, then the suitability of that life's own star and planet may not be all that matters for its long-term evolution and survival. Life may well have formed around a number of pleasantly stable stars only to be wiped out because a massive nearby star suddenly went supernova. Just as children born in a war zone may find themselves the unjust victims of their violent neighborhood, life too close to a star that goes supernova may fall prey to having been born in the wrong place at the wrong time.

Plane that contains Earth's orbit around the Sun.

Sun takes an ecliptic path around Earth --> Sun rises about 4 minutes later each day with respect to the stars--> earth takes a little more than one rotation for Sun to come up again Whichever constellation is visible by the ecliptic at a certain period of time = sun crosses Gemini June 22-July 21 Ecliptic is inclined to an angle of 23.5 degrees

Characteristics of Main Sequence Stars

Table 18.3

Distance Range of Celestial Measurement Methods

Table 19.1 page 682

Interstellar Molecules

Table 20.1

Lifetimes of Main-Sequence Stars

Table 22.1

Figure 22.10 NGC 2264 H-R Diagram.

Table 22.10

Characteristics of Star Clusters

Table 22.3

Major Radio Observatories of the World

Table 6.2

Visible Light Interferometers

Table 6.3

Recent Observatories in Space

Table 6.4

using baseline surveyors and the moon

The Moon is the only object near enough that its distance can be found fairly accurately with measurements made without a telescope. Ptolemy determined the distance to the Moon correctly to within a few percent. He used the turning Earth itself as a baseline, measuring the position of the Moon relative to the stars at two different times of night.

Plasma

The Sun is so hot that all of the material in it is in the form of ionized gas, called a plasma. Plasma acts much like a hot gas, which is easier to describe mathematically than either liquids or solids. The particles that constitute a gas are in rapid motion, frequently colliding with one another. This constant bombardment is the pressure of the gas More particles within a given volume of gas produce more pressure because the combined impact of the moving particles increases with their number. The pressure is also greater when the molecules or atoms are moving faster. Since the molecules move faster when the temperature is hotter, higher temperatures produce higher pressure.

With a baseline of one AU, how far away would a star have to be to have a parallax of 1 arcsecond?

The answer turns out to be 206,265 AU, or 3.26 light-years. This is equal to 3.1 × 1013 kilometers (in other words, 31 trillion kilometers).

naming stars

The brightest stars have names that derive from the ancients. Bayer- he assigned a Greek letter to the brightest stars, roughly in order of brightness. In the constellation of Orion, for example, Betelgeuse is the brightest star, so it got the first letter in the Greek alphabet—alpha—and is known as Alpha Orionis BUT only 24 letters in greek alph

The Seasons

The difference between seasons gets more pronounced the farther north or south from the equator we travel, and the seasons in the Southern Hemisphere are the opposite of what we find on the northern half of Earth

first white dwarf

The first white dwarf star was detected in 1862. Called Sirius B, it forms a binary system with Sirius A, the brightest-appearing star in the sky. It eluded discovery and analysis for a long time because its faint light tends to be lost in the glare of nearby Sirius A

Radiation and Temperature

The hotter the solid or gas, the more rapid the motion of its molecules or atoms. The temperature of something is thus a measure of the average motion energy of the particles that make it up.

magnitude and brightness

The important fact to remember when using magnitude is that the system goes backward: the larger the magnitude, the fainter the object you are observing.

twinkling of stars

The light beams are bent enough that part of the time they reach your eye, and part of the time some of them miss, thereby making the star seem to vary in brightness. In space, however, the light of the stars is steady.

main- sequence turnoff.

The location in the H-R diagram where the stars have begun to leave the main sequence

Chandrasekhar limit

The maximum mass that a star can end its life with and still become a white dwarf—1.4 MSun—is called the Chandrasekhar limit. Stars with end-of-life masses that exceed this limit have a different kind of end in store

spectroscopic parallax

The method of determining a star's distance by comparing its apparent magnitude with its absolute magnitude, as estimated from its spectrum. The H-R diagram method allows astronomers to estimate distances to nearby stars, as well as some of the most distant stars in our Galaxy, but it is anchored by measurements of parallax. The distances measured using parallax are the gold standard for distances: they rely on no assumptions, only geometry. Once astronomers take a spectrum of a nearby star for which we also know the parallax, we know the luminosity that corresponds to that spectral type. Nearby stars thus serve as benchmarks for more distant stars because we can assume that two stars with identical spectra have the same intrinsic luminosity

doppler Method of Detecting Planets

The motion of a star around a common center of mass with an orbiting planet can be detected by measuring the changing speed of the star. When the star is moving away from us, the lines in its spectrum show a tiny redshift; when it is moving toward us, they show a tiny blueshift. The change in color (wavelength) has been exaggerated here for illustrative purposes. In reality, the Doppler shifts we measure are extremely small and require sophisticated equipment to be detected.

Orbits in the Solar System

The path of any object under the influence of gravity through space = orbit The place where the planet is closest to the Sun and moves the fastest in called the perihelion of its orbit The place where is farthest away and moved the most slowly is that aphelion For the moon the terms are perigee & apogee Moon = a natural object that goes around a planet and the word satellite to mean a human made object that revolves around a plane)

zodiac

The sun, moon and earth on the same plane with an 18 degree belt, centered on the ecliptic called

Isotopes

The various types of hydrogen nuclei with different numbers of neutrons are called isotopes of hydrogen closely related but with different characteristics and behaviors.

elements & lines on spectra

These gases turned out not to be transparent at all colors: they were quite opaque at a few sharply defined wavelengths. Something in each gas had to be absorbing just a few colors of light and no others. clear that certain lines in the spectrum "go with" certain elements

22.14 Cluster 47 Tucanae. T

This H-R diagram is for the globular cluster 47. Note that the scale of luminosity differs from that of the other H-R diagrams in this chapter. We are only focusing on the lower portion of the main sequence, the only part where stars still remain in this old cluster.

first successful detections of stellar parallax

Thomas Henderson, a Scottish astronomer working at the Cape of Good Hope, and Friedrich Struve in Russia independently measured the parallaxes of the stars 61 Cygni, Alpha Centauri, and Vega, respectively. Even the closest star, Alpha Centauri, showed a total displacement of only about 1.5 arcseconds during the course of a year.

Interplanetary Spacecraft

To escape earth these crafts must achieve escape speed, the speed needed to move away from earth forever, which is about 11 kilometers per second In interplanetary flight, these spacecraft follow orbits around the sun that are modified only when they pass near one of the planets As it comes closer to its target a spacecraft is deflected by the planet's gravitational force into a modified orbit either gaining or losing energy in the process--> the gravitational force can be used to redirect a spacecraft was having target in order to orbit a planet you must slow the spacecraft allowing it to be captured into an elliptical orbit and additional rocket thrust to bring the vehicle down from orbit and onto a surface

But stars do not have the same luminosity so...

To measure the luminosities of stars, we must first compensate for the dimming effects of distance on light, and to do that, we must know how far away they are. Distance is among the most difficult of all astronomical measurements.

what electromagnetic waves can only be observed from space?

Ultraviolet, X-ray, and direct gamma-ray (high-energy electromagnetic wave) observations can be made only from space.

airborne & space infrared telescopes

Water vapor, the main source of atmospheric interference for making infrared observations, is concentrated in the lower part of Earth's atmosphere. --> need to go to space or on a plane to avoid this

how to determine mass of the stars in a spectroscopic binary

We can analyze a radial velocity curve We measure the speeds of the stars from the Doppler effect. We then determine the period—how long the stars take to go through an orbital cycle—from the velocity curve. Knowing how fast the stars are moving and how long they take to go around tells us the circumference of the orbit and, hence, the separation of the stars in kilometers or astronomical units. From Kepler's law, the period and the separation allow us to calculate the sum of the stars' masses.

What is the difference type 1 and type 2 supernovae?

We distinguish type I supernovae from those of supernovae of type II originating from the death of massive stars discussed earlier by the absence of hydrogen in their observed spectra. Hydrogen is the most common element in the universe and is a major component of massive, evolved stars. However, as we learned earlier, hydrogen is absent from the white dwarf remnant, which is primarily composed of carbon and oxygen for masses comparable to the Chandrasekhar mass limit.

parsec

We give this unit a special name, the parsec (pc)—derived from "the distance at which we have a parallax of one second." The distance (D) of a star in parsecs is just the reciprocal of its parallax (p) in arcseconds; D = 1/p Thus, a star with a parallax of 0.1 arcsecond would be found at a distance of 10 parsecs, and one with a parallax of 0.05 arcsecond would be 20 parsecs away. 1 parsec = 3.26 light-year, and 1 light-year = 0.31 parsec.

sun is a representative star

We saw that what stars such as the Sun "do for a living" is to convert protons into helium deep in their interiors via the process of nuclear fusion, thus producing energy. The fusion of protons to helium is an excellent, long-lasting source of energy for a star because the bulk of every star consists of hydrogen atoms, whose nuclei are protons.

Young Cluster H-R Diagram

We see an H-R diagram for a hypothetical young cluster with an age of 3 million years. Note that the high-mass (high-luminosity) stars have already arrived at the main-sequence stage of their lives, while the lower-mass (lower-luminosity) stars are still contracting toward the zero-age main sequence (the red line) and are not yet hot enough to derive all of their energy from the fusion of hydrogen. There are real star clusters that fit this description. The first to be studied (in about 1950) was NGC 2264, which is still associated with the region of gas and dust from which it was born

What would happen if there were no continuous spectrum for our gases to remove light from?

What if, instead, we heated the same thin gases until they were hot enough to glow with their own light? When the gases were heated, a spectrometer revealed no continuous spectrum, but several separate bright lines. That is, these hot gases emitted light only at certain specific wavelengths or colors.

what happens to radius of white dwarf in its final stages?

When Chandrasekhar made his calculation about white dwarfs, he found something very surprising: the radius of a white dwarf shrinks as the mass in the star increases (the larger the mass, the more tightly packed the electrons can become, resulting in a smaller radius). According to the best theoretical models, a white dwarf with a mass of about 1.4 MSun or larger would have a radius of zero. What the calculations are telling us is that even the force of degenerate electrons cannot stop the collapse of a star with more mass than this.

how two stars move around center of mass

When one star is approaching us relative to the center of mass, the other star is receding from us. In the top left illustration, star A is moving toward us, so the line in its spectrum is Doppler-shifted toward the blue end of the spectrum. Star B is moving away from us, so its line shows a redshift. When we observe the composite spectrum of the two stars, the line appears double. When the two stars are both moving across our line of sight (neither away from nor toward us), they both have the same radial velocity (that of the pair's center of mass); hence, the spectral lines of the two stars come together. *****REFER TO FIGURE 18.6

How does the Sun favoring a hemisphere make it warmer for us down on the surface?

When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth's surface June is more direct and intense in NH and hence more effective at heating Second effect has to do with the length of the time the Sun spends above the horizon--> hours of daylight increase in summer and decrease in winter In June the Sun is north of the celestial equator and spends more time with those who live in the Northern Hemisphere--> it rises high and is above the horizon for as long as 15 hours Thus not only direct rays but also a longer period of time opposite in the Southern Hemisphere

florescent light vs H II region

When you turn on the current, electrons collide with atoms of mercury vapor in the tube. The mercury is excited to a high-energy state because of these collisions. When the electrons in the mercury atoms return to lower-energy levels, some of the energy they emit is in the form of ultraviolet photons. These, in turn, strike a phosphor-coated screen on the inner wall of the light tube. The atoms in the screen absorb the ultraviolet photons and emit visible light as they cascade downward among the energy levels. (The difference is that these atoms give off a wider range of light colors, which mix to give the characteristic white glow of fluorescent lights, whereas the hydrogen atoms in an H II region give off a more limited set of colors.)

why do dust clouds glow brightly in the infrared?

While dust clouds are too cold to radiate a measurable amount of energy in the visible part of the spectrum, they glow brightly in the infrared (Figure 20.11). The reason is that small dust grains absorb visible light and ultraviolet radiation very efficiently. The grains are heated by the absorbed radiation, typically to temperatures from 10 to about 500 K, and re-radiate this heat at infrared wavelengths.

so are white dwarfs degenerate stars?

White dwarfs, then, are stable, compact objects with electron-degenerate cores that cannot contract any further. Calculations showing that white dwarfs are the likely end state of low-mass stars Subrahmanyan Chandrasekhar. He was able to show how much a star will shrink before the degenerate electrons halt its further contraction and hence what its final diameter will be

infrared orion nebula

With near-infrared radiation, we can see more detail within the dusty nebula since infrared can penetrate dust more easily than can visible light.

Kepler results

You can see the wide range of sizes, including planets substantially larger than Jupiter and smaller than Earth. The absence of Kepler- discovered exoplanets with orbital periods longer than a few hundred days is a consequence of the 4-year lifetime of the mission. (Remember that three evenly spaced transits must be observed to register a discovery.) At the smaller sizes, the absence of planets much smaller than one earth radius is due to the difficulty of detecting transits by very small planets. In effect, the "discovery space" for Kepler was limited to planets with orbital periods less than 400 days and sizes larger than Mars.

visual binary

a binary star system in which both of the stars can be seen with a telescope

Planet

a body of significant size that orbits a star and does not produce its own light ( if it consistently produces light it's a star) We are able to see nearby planets because they reflect the light of our local star, too far away the small amount of light emitted would not make them visible to us

spectral class

a classification system for stars based the patterns of lines observed in stellar spectra Because a star's temperature determines which absorption lines are present in its spectrum, these spectral classes are a measure of its surface temperature. There are seven standard spectral classes.

spectrometer

a device that spreads light into its different colors Upon leaving the opposite face of the prism, the light is bent again and further dispersed. If the light leaving the prism is focused on a screen, the different wavelengths or colors that make up white light are lined up side by side just like a rainbow

H-R diagram

a graph that shows the relationship between a star's surface temperature and absolute magnitude

zenith

a point directly above your head

properties of common particles

16.1 <2 * 10^-36 (uncertain)

Photometry

The process of measuring the apparent brightness of stars is called photometry

what stars end up as white dwarfs?

stars with starting masses up to at least 8 MSun (and perhaps even more)

doppler shift

17.10 When the spectral lines of a moving star shift toward the red end of the spectrum, we know that the star is moving away from us. If they shift toward the blue end, the star is moving toward us.

apparent magnitudes of well-known objects

17.2 table

Omega Centauri

(a) Located at about 16,000 light-years away, Omega Centauri is the most massive globular cluster in our Galaxy. It contains several million stars. (b) This image, taken with the Hubble Space Telescope, zooms in near the center of Omega Centauri. The image is about 6.3 light-years wide. The most numerous stars in the image, which are yellow-white in color, are main-sequence stars similar to our Sun. The brightest stars are red giants that have begun to exhaust their hydrogen fuel and have expanded to about 100 times the diameter of our Sun. The blue stars have started helium fusion.

brown dwarf

"Failed" star; Star not massive enough to sustain nuclear fusion. -->masses intermediate between stars and planets very difficult to observe because they are extremely faint and cool, and they put out most of their light in the infrared part of the spectrum. It was only after the construction of very large telescopes, like the Keck telescopes in Hawaii, and the development of very sensitive infrared detectors, that the search for brown dwarfs succeeded.

spring tides

( Spring tides are approximately the same, whether the Sun and Moon are on the same or opposite sides of Earth, because tidal bulges occur on both sides. When the Moon is at first quarter or last quarter (at right angles to the Sun's direction), the tides produced by the Sun partially cancel the tides of the Moon, making them lower than usual. These are called neap tides. However, the presence of land masses stopping the flow of water, the friction in the oceans and between oceans and the ocean floors, the rotation of Earth, the wind, the variable depth of the ocean, and other factors all complicate the picture. --> some places have small tides and some have high tides one set of tide predictions doesn't work for the whole planet.

what does telescope ability to show fine detail depend on

--> aperture -->depends upon the wavelength of the radiation that the telescope is gathering. ------>The longer the waves, the harder it is to resolve fine detail in the images or maps we make. ------->because radio waves have long wavelengths poses a challenge for good resolution

how do we know that sun is actually this hot ?

--> calculates the temperature and pressure at every point inside the Sun and determines what nuclear reactions, if any, are taking place. For some calculations, we can use observations to determine whether the computer program is producing results that match what we see. -->can also calculate how the Sun will change with time. After all, the Sun must change. In its center, the Sun is slowly depleting its supply of hydrogen and creating helium instead. Will the Sun get hotter? Cooler? Larger? Smaller? Brighter? Fainter? -->changes in center: since eventually all the hydrogen fuel hot enough for fusion will be exhausted. Either a new source of energy must be found, or the Sun will cease to shine.

Maxwell's Theory of Electromagnetism

-deals with the electrical charge of subatomic particles and their effect; especially when they are moving -When charges are not in motion, we observe only this electric attraction or repulsion. If charges are in motion, however (as they are inside every atom and in a wire carrying a current), then we measure another force called magnetism

The Challenge of the Calendar

2 purposes: keep track of time over the course of long spans to honor holidays and anniversaries & useful to a large number of people The natural units of our calendar are the day, based on the period of rotation of Earth; the month, based on the cycle of the Moon's phases (see later in this chapter) about Earth; and the year, based on the period of revolution of Earth about the Sun. Difficulties have resulted from the fact that these three periods are not commensurable; that's a fancy way of saying that one does not divide evenly into any of the others he basic period of revolution of Earth, called the tropical year, is 365.2422 days.

Giants

A giant star has a large, extended photosphere. Because it is so large, a giant star's atoms are spread over a great volume, which means that the density of particles in the star's photosphere is low. As a result, the pressure in a giant star's photosphere is also low.

What do we mean, exactly, by "discovery" of transiting exoplanets?

A single transit shows up as a very slight drop in the brightness of the star, lasting several hours. However, astronomers must be on guard against other factors that might produce a false transit, especially when working at the limit of precision of the telescope. We must wait for a second transit of similar depth. But when another transit is observed, we don't initially know whether it might be due to another planet in a different orbit. The "discovery" occurs only when a third transit is found with similar depth and the same spacing in time as the first pair.

what causes stars to form

Although we do not know what initially caused stars to begin forming in Orion, there is good evidence that the first generation of stars triggered the formation of additional stars, which in turn led to the formation of still more stars

Tycho Brahe's observatory

Early observations and study of exploding star that flared up to great brilliance in the a night Sky Brahe was able to establish a fine astronomical Observatory on the North Sea island of Hven Made a continuous record of the positions of the sun moon and planets for almost 20 years enabled him to note that the positions of the planets varied from those given and published tables which were based on the work of Ptolemy made enemies of government officials and then took up residence in Prague and found a mathematician Johannes Kepler to assist him in analyzing his extensive planetary data He was the last and greatest of the pre telescopic observers in Europe

how does the sun maintain its stability

If the internal pressure in such a star were not great enough to balance the weight of its outer parts, the star would collapse somewhat, contracting and building up the pressure inside. On the other hand, if the pressure were greater than the weight of the overlying layers, the star would expand, thus decreasing the internal pressure. Expansion would stop, and equilibrium would again be reached when the pressure at every internal point equaled the weight of the stellar layers above that point. ***hydrostatic equilibrium

If we can observe the spectrum of a star, we can estimate

If we can observe the spectrum of a star, we can estimate its distance from our understanding of the H-R diagram. As discussed in Analyzing Starlight, a detailed examination of a stellar spectrum allows astronomers to classify the star into one of the spectral types indicating surface temperature. (The types are O, B, A, F, G, K, M, L, T, and Y; each of these can be divided into numbered subgroups.) In general, however, the spectral type alone is not enough to allow us to estimate luminosity. BUTTT . A G2 star could be a main- sequence star with a luminosity of 1 LSun, or it could be a giant with a luminosity of 100 LSun, or even a supergiant with a still higher luminosity.

where do molecules form?

It is in these dark regions of space, protected from starlight, that molecules can form. Chemical reactions occurring both in the gas and on the surface of dust grains lead to much more complex compounds, hundreds of which have been identified in interstellar space. Among the simplest of these are water (H2O), carbon monoxide (CO), which is produced by fires on Earth, and ammonia (NH3), whose smell you recognize in strong home cleaning products. Carbon monoxide is particularly abundant in interstellar space and is the primary tool that astronomers use to study giant molecular clouds. Unfortunately, the most abundant molecule, H2, is particularly difficult to observe directly because in most giant molecular clouds, it is too cold to emit even at radio wavelengths. CO, which tends to be present wherever H2 is found, is a much better emitter and is often used by astronomers to trace molecular hydrogen

2 step in fusion in sun : what happens to the positron formed in step one?

Since it is antimatter, this positron will instantly collide with a nearby electron, and both will be annihilated, producing electromagnetic energy in the form of gamma-ray photons. This gamma ray, which has been created in the center of the Sun, finds itself in a world crammed full of fast-moving nuclei and electrons. The gamma ray collides with particles of matter and transfers its energy to one of them. The particle later emits another gamma-ray photon, but often the emitted photon has a bit less energy than the one that was absorbed Such interactions happen to gamma rays again and again and again as they make their way slowly toward the outer layers of the Sun, until their energy becomes so reduced that they are no longer gamma rays but X- rays Later, as the photons lose still more energy through collisions in the crowded center of the Sun, they become ultraviolet photons.

Propagating Star Formation

Star formation can move progressively through a molecular cloud. The oldest group of stars lies to the left of the diagram and has expanded because of the motions of individual stars. Eventually, the stars in the group will disperse and no longer be recognizable as a cluster. The youngest group of stars lies to the right, next to the molecular cloud. This group of stars is only 1 to 2 million years old. The pressure of the hot, ionized gas surrounding these stars compresses the material in the nearby edge of the molecular cloud and initiates the gravitational collapse that will lead to the formation of more stars.

Beginnings of Astrology

The Babylonians, believing the planets and their motions influenced the fortunes of kings and nations, used their knowledge of astronomy to guide their rulers. When the Babylonian culture was absorbed by the Greeks, astrology gradually came to influence the entire Western world and eventually spread to Asia as well. Natal astrology: believed sun, moon and planets at the moment of birth affected a person's personality and fortune--> peaked with Ptolemy in 400 years Ptolemy compiled the Tetrabiblos, a treatise on astrology that remains the "bible" of the subject

Right Ascension

The arbitrarily chosen point where we start counting is the vernal equinox (a point in the sky where the ecliptic (the sun's path) crosses the celestial equator)--> like longitude Can be expression as units of angle (degrees) or units of time--> b/c celestial sphere appears to turn around Earth once a day as our planet turns on its axis Thus the 360° of RA that it takes to go once around the celestial sphere can just as well be set equal to 24 hours. Then each 15° of arc is equal to 1 hour of time. For example, the approximate celestial coordinates of the bright star Capella are RA 5h = 75° and declination +50°

concave primary mirror

The mirror is curved like the inner surface of a sphere, and it reflects light in order to form an image coated with a shiny metal, usually silver, aluminum, or, occasionally, gold, to make them highly reflective. If the mirror has the correct shape, all parallel rays are reflected back to the same point, the focus of the mirror. Thus, images are produced by a mirror exactly as they are by a lens.

what do we find when we do this?

The model stars with the largest masses are the hottest and most luminous, and they are located at the upper left of the diagram. The least-massive model stars are the coolest and least luminous, and they are placed at the lower right of the plot. The other model stars all lie along a line running diagonally across the diagram. In other words, the main sequence turns out to be a sequence of stellar masses.

How to use triangulation

The parallax is also the angle that lines AC and BC make—in mathematical terms, the angle subtended by the baseline. A knowledge of the angles at A and B and the length of the baseline, AB, allows the triangle ABC to be solved for any of its dimensions—say, the distance AC or BC. The solution could be reached by constructing a scale drawing or by using trigonometry to make a numerical calculation. If the tree were farther away, the whole triangle would be longer and skinnier, and the parallax angle would be smaller. Thus, we have the general rule that the smaller the parallax, the more distant the object we are measuring must be.

ionization from collisions

The rate at which such collisional ionizations occur depends on the speeds of the atoms and hence on the temperature of the gas—the hotter the gas, the more of its atoms will be ionized.

Diameters of eclipsing binary stars

The technique involves making a light curve of an eclipsing binary, a graph that plots how the brightness changes with time. Even though we cannot see the two stars separately in such a system, the light curve can tell us what is happening. When the smaller star just starts to pass behind the larger star (a point we call first contact), the brightness begins to drop. The eclipse becomes total (the smaller star is completely hidden) at the point called second contact. At the end of the total eclipse (third contact), the smaller star begins to emerge. When the smaller star has reached last contact, the eclipse is completely over

Disks around Protostars

These Hubble Space Telescope infrared images show disks around young stars in the constellation of Taurus, in a region about 450 light-years away. In some cases, we can see the central star (or stars—some are binaries). In other cases, the dark, horizontal bands indicate regions where the dust disk is so thick that even infrared radiation from the star embedded within it cannot make its way through. The brightly glowing regions are starlight reflected from the upper and lower surfaces of the disk, which are less dense than the central, dark regions

giant molecular clouds

These clouds have cold interiors with characteristic temperatures of only 10-20 K; most of their gas atoms are bound into molecules. These clouds turn out to be the birthplaces of most stars in our Galaxy.

Jewel Box

This open cluster of young, bright stars is about 6400 light-years away from the Sun. Note the contrast in color between the bright yellow supergiant and the hot blue main-sequence stars. The name comes from John Herschel's nineteenth-century description of it as "a casket of variously colored precious stones

Darwin & The Slowing of the Earth

What Darwin calculated for the Earth-Moon system was that the Moon will slowly spiral outward, away from Earth. As it moves farther away, it will orbit less quickly (just as planets farther from the Sun move more slowly in their orbits). Thus, the month will get longer. Also, because the Moon will be more distant, total eclipses of the Sun will no longer be visible from Earth Moving away at 3.8 cm each year = billions of year day and month will be the same length

why is the most common type absent before Kepler survey

What a remarkable discovery it is that the most common types of planets in the Galaxy are completely absent from our solar system and were unknown until Kepler's survey. However, recall that really small planets were difficult for the Kepler instruments to find. So, to estimate the frequency of Earth-size exoplanets, we need to correct for this sampling bias. The result is the corrected size distribution shown in Figure 21.24. Notice that in this graph, we have also taken the step of showing not the number of Kepler detections but the average number of planets per star for solar-type stars (spectral types F, G, and K).

astronomical unit

When Earth and the Sun are closest, they are about 147.1 million kilometers apart; when Earth and the Sun are farthest, they are about 152.1 million kilometers apart. The average of these two distances is called the astronomical unit (AU). The length of 1 AU can be expressed in light travel time as 499.004854 light-seconds, or about 8.3 light-minutes. If we use the definition of the meter given previously, this is equivalent to 1 AU = 149,597,870,700 meters. These distances are, of course, given here to a much higher level of precision than is normally astronomical unit: AU = 1.50 × 1011 m = 1.50 × 108 km = 500 light-seconds

refractor

a telescope that uses a lens to collect and focus light.

stellar wind

consists mainly of protons (hydrogen nuclei) and electrons streaming away from the star at speeds of a few hundred kilometers per second (several hundred thousand miles per hour). When the wind first starts up, the disk of material around the star's equator blocks the wind in its direction. Where the wind particles can escape most effectively is in the direction of the star's poles.

the Orion molecular cloud:what happens in regions of star formation by considering a nearby site where stars are forming

constellation of Orion, The Hunter, about 1500 light- years away The Orion molecular cloud is much larger than the star pattern and is truly an impressive structure. In its long dimension, it stretches over a distance of about 100 light-years. The total quantity of molecular gas is about 200,000 times the mass of the Sun. Most of the cloud does not glow with visible light but betrays its presence by the radiation that the dusty gas gives off at infrared and radio wavelengths.

cold interstellar clouds

contain cyanoacetylene (HC3N) and acetaldehyde (CH3CHO), generally regarded as starting points for amino acid formation. These are building blocks of proteins, which are among the fundamental chemicals from which living organisms on Earth are constructed

would the contractions of the sun be able to maintain the rate of energy production for a long time?

contraction of the Sun at a rate of only about 40 meters per year would be enough to produce the amount of energy that it is now radiating. -->the decrease in the Sun's size from such a slow contraction would be undetectable. --> we can calculate how much energy has been radiated by the Sun during its entire lifetime as it has contracted from a very large diameter to its present size.--> sun 4 * 10^26 watts = 100 million years

why determines the light gathering ability of a telescope?

determined by the area of the device acting as the light-gathering "bucket."

Dispersion

different wavelengths (or colors of light) are bent by different amounts and therefore follow slightly different paths through the prism. The violet light is bent more than the red.

The Birth of Modern Astronomy

expansion of Islam after 7th century led to the flowering of Arabic and Jewish cultures that preserve translated and added many astronomical ideas of the Greeks As European culture began to reemerge and trade with Arab countries --> rediscovery of ancient text such as the Almagest and reawakening of interest in astronomy

particles shooting out in opposite directions

from the popular regions of newly formed stars. In many cases, these beams point back to the location of a protostar that is still so completely shrouded in dust that we cannot yet see it

Alpha particles are

helium atoms without the electrons and are positively charged

what does a star turning into a white dwarf depend on?

how much mass is lost in the red-giant and earlier phases of evolution. All stars that have masses below the Chandrasekhar limit when they run out of fuel will become white dwarfs, no matter what mass they were born with.

The Gemini North Telescope

mirror is only about 8 inches thick and weighs 24.5 tons, less than twice as much as the Palomar mirror. The Gemini North telescope was completed about 50 years after the Palomar telescope use active control to correct sag

To be truly representative, the stellar population, an H-R diagram should be plotted...

or all stars within a certain distance. Unfortunately, our knowledge is reasonably complete only for stars within 10 to 20 light-years of the Sun, among which there are no giants or supergiants. Still, from many surveys (and more can now be done with new, more powerful telescopes), we estimate that about 90% of the true stars overall (excluding brown dwarfs) in our part of space are main-sequence stars, about 10% are white dwarfs, and fewer than 1% are giants or supergiants.

the redefinition of the Meter

the meter was redefined to equal 1,650,763.73 wavelengths of a particular atomic transition in the element krypton-86. The advantage of this redefinition is that anyone with a suitably equipped laboratory can reproduce a standard meter, without reference to any particular metal bar.

key reasons that measuring distances to the stars is such a struggle

variety of intrinsic luminosities apparent brightness --> we can calculate how far it is

what happens when the source of waves moves away from you?

wavelength gets longer, we call the change in colors a redshift

Planets with Known Densities

we have been able to measure both the size of the planet from transit data and its mass from Doppler data, yielding an estimate of its density

planetarium

we project a simulation of the stars & planets into a white dome As the celestial sphere rotates, the objects on it maintain their positions with respect to one another ( only comets because they are not stars they are cosmic dust)--> help set up systems for keeping track of what things are visible in the sky and where they happen to be at a given time

Speed and Temperature

when the temperature is higher, so are the speed and energy of the collisions. The hotter the gas, therefore, the more likely that electrons will occupy the outermost orbits, which correspond to the highest energy levels. This means that the level where electrons start their upward jumps in a gas can serve as an indicator of how hot that gas is absorption lines in a spectrum give astronomer info about the temperature of the regions where the lines originate

formula for doppler shift of light

λ is the wavelength emitted by the source, Δλ is the difference between λ and the wavelength measured by the observer, c is the speed of light, and v is the relative speed of the observer and the source in the line of sight. The variable v is counted as positive if the velocity is one of recession, and negative if it is one of approach. Solving this equation for the velocity, we find v = c × Δλ/λ.

Example 1: Sirius Binary system

(20)3 = (M1 + M2)(50)^2 8000 = (M1 + M2)(2500) M1+M2 = 8000 =3.2 Therefore, the sum of masses of the two stars in the Sirius binary system is 3.2 times the Sun's mass. In order to determine the individual mass of each star, we would need the velocities of the two stars and the orientation of the orbit relative to our line of sight.

exoplanet

(a planet outside our solar system) orbiting a main-sequence star, and today we know that most stars form with planets.

formation of a star

(a) Dense cores form within a molecular cloud. (b) A protostar with a surrounding disk of material forms at the center of a dense core, accumulating additional material from the molecular cloud through gravitational attraction. (c) A stellar wind breaks out but is confined by the disk to flow out along the two poles of the star. (d) Eventually, this wind sweeps away the cloud material and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes observable. These sketches are not drawn to the same scale. The diameter of a typical envelope that is supplying gas to the newly forming star is about 5000 AU. The typical diameter of the disk is about 100 AU or slightly larger than the diameter of the orbit of Pluto.

Figure 22.2 Star Layers during and after the Main Sequence

(a) During the main sequence, a star has a core where fusion takes place and a much larger envelope that is too cold for fusion. (b) When the hydrogen in the core is exhausted (made of helium, not hydrogen), the core is compressed by gravity and heats up. The additional heat starts hydrogen fusion in a layer just outside the core. Note that these parts of the Sun are not drawn to scale.

How to Use a Cepheid to Measure Distance

(a) Find a cepheid variable star and measure its period. (b) Use the period- luminosity relation to calculate the star's luminosity. (c) Measure the star's apparent brightness. (d) Compare the luminosity with the apparent brightness to calculate the distance.

continuous spectra

(formed when a solid or very dense gas gives off radiation) is an array of all wavelengths or colors of the rainbow. A continuous spectrum can serve as a backdrop from which the atoms of much less dense gas can absorb light. When we have a hot, thin gas, each particular chemical element or compound produces its own characteristic pattern of spectral lines—its spectral signature. No two types of atoms or molecules give the same patterns. In other words, each particular gas can absorb or emit only certain wavelengths of the light peculiar to that gas.

radial velocity

(motion toward or away from us) changes by about 13 meters per second with a period of 12 years because of the gravitational pull of Jupiter. This corresponds to about 30 miles per hour, roughly the speed at which many of us drive around town. Detecting motion at this level in a star's spectrum presents an enormous technical challenge, but several groups of astronomers around the world, using specialized spectrographs designed for this purpose, have succeeded. Note that the change in speed does not depend on the distance of the star from the observer. Using the Doppler effect to detect planets will work at any distance, as long as the star is bright enough to provide a good spectrum and a large telescope is available to make the observations

the value of stellar spectra

, William Wollaston built an improved spectrometer that included a lens to focus the Sun's spectrum on a screen. With this device, Wollaston saw that the colors were not spread out uniformly, but instead, some ranges of color were missing, appearing as dark bands in the solar spectrum. He mistakenly attributed these lines to natural boundaries between the colors. . In 1815, German physicist Joseph Fraunhofer, upon a more careful examination of the solar spectrum, found about 600 such dark lines (missing colors), which led scientists to rule out the boundary hypothesis

what happens when a stable white dwarf can no longer contract or produce energy through fusion?

, its only energy source is the heat represented by the motions of the atomic nuclei in its interior. The light it emits comes from this internal stored heat, which is substantial. Gradually, however, the white dwarf radiates away all its heat into space. After many billions of years, the nuclei will be moving much more slowly, and the white dwarf will no longer shine (Figure 23.5). It will then be a black dwarf—a cold stellar corpse with the mass of a star and the size of a planet. It will be composed mostly of carbon, oxygen, and neon, the products of the most advanced fusion reactions of which the star was capable.

at lower planet masses

, notice that as the mass of these hypothetical planets increases, the radius also increases. That makes sense—if you were building a model of a planet out of clay, your toy planet would increase in size as you added more clay. However, for the highest mass planets (M > 1000 MEarth) in Figure 21.25, notice that the radius stops increasing and the planets with greater mass are actually smaller. This occurs because increasing the mass also increases the gravity of the planet, so that compressible materials (even rock is compressible) will become more tightly packed, shrinking the size of the more massive planet. In reality, planets are not pure compositions like the hypothetical water or iron planet. Earth is composed of a solid iron core, an outer liquid-iron core, a rocky mantle and crust, and a relatively thin atmospheric layer. Exoplanets are similarly likely to be differentiated into compositional layers. The theoretical lines in Figure 21.25 are simply guides that suggest a range of possible compositions.

eclipsing binary stars

- Two close stars that appear to be a single star varying in brightness Some binary stars are lined up in such a way that, when viewed from Earth, each star passes in front of the other during every revolution (Figure 18.10). When one star blocks the light of the other, preventing it from reaching Earth, the luminosity of the system decreases, and astronomers say that an eclipse has occurred.

how does Davis's result explained later

- three types of neutrinos -Solar fusion produces only one type of neutrino, the so-called electron neutrino, and the initial experiments to detect solar neutrinos were designed to detect this one type. Subsequent experiments showed that these neutrinos change to a different type during their journey from the center of the Sun through space to Earth in a process called neutrino oscillation. An experiment, conducted at the Sudbury Neutrino Observatory in Canada, was the first one designed to capture all three types of neutrinos (Figure 16.20). The experiment was located in a mine 2 kilometers underground. The neutrino detector consisted of a 12-meter-diameter transparent acrylic plastic sphere, which contained 1000 metric tons of heavy water. Remember that an ordinary water nucleus contains two hydrogen atoms and one oxygen atom. Heavy water instead contains two deuterium atoms and one oxygen atom, and incoming neutrinos can occasionally break up the loosely bound proton and neutron that make up the deuterium nucleus. The sphere of heavy water was surrounded by a shield of 1700 metric tons of very pure water, which in turn was surrounded by 9600 photomultipliers, devices that detect flashes of light produced after neutrons interact with the heavy water.

When does fission occur?

--> atomic bombs --> nuclear reactors -->occurs spontaneously in some unstable nuclei through the process of natural radioactivity. -->fission requires big, complex nuclei, whereas we know that the stars are made up predominantly of small, simple nuclei--> fusion to explain the energy of the Sun & the stars

radio waves

--> cannot be heard --> if translated to sound = static -->information that can tell us about the chemistry and physical conditions of the sources of the waves. -->world's largest radio reflectors that can be pointed to any direction in the sky have apertures of 100 meters.

Hipparchus

--> photometry began with him he erected an observatory on the island of Rhodes in the Mediterranean. There he prepared a catalog of nearly 1000 stars that included not only their positions but also estimates of their apparent brightnesses.

Hubble Space Telescope

-->2.4 meter aperture= largest telescope into space so far -->named after Edwin Hubble who discovered the expansion of the universe in the 1920s --> first orbiting observatory designed to be serviced by Shuttle astronauts --> Hubble Ultra Deep Field: an image of a small region of the sky observed for almost 100 hours. It contains views of about 10,000 galaxies, some of which formed when the universe was just a few percent of its current age. --> error in the primary mirror added corrective optics in front of their eyes

Neutrino

-->Energy seemed to disappear when certain types of nuclear reactions took place, violating the law of conservation of energy. -->a so-far-undetected particle, which was given the name neutrino carried away the "missing" energy. -->neutrinos were particles with zero mass, and that like photons, they moved with the speed of light.

proper motion

-->cannot be detected with stellar spectra -->, we do not notice any change in the positions of the bright stars during the course of a human lifetime -->which is along our line of sight (i.e., toward or away from Earth), this motion, called proper motion, is transverse: that is, across our line of sight. We see it as a change in the relative positions of the stars on the celestial sphere (Figure 17.11). These changes are very slow. Even the star with the largest proper motion takes 200 years to change its position in the sky by an amount equal to the width of the full Moon, and the motions of other stars are smaller yet.

why is contraction not the primary source of solar energy?

-->sun is much older than 100 million years -->contraction is an important source of energy while stars are being born

X-rays

.01 to 20 nanometers can penetrate a short length of human flesh, they are stopped by the large numbers of atoms in Earth's atmosphere with which they interact. also can only study in space

with adaptive optics, ground-based telescopes can achieve resolutions of.....

0.1 arcsecond or a little better in the infrared region of the spectrum. This impressive figure is the equivalent of the resolution that the Hubble Space Telescope achieves in the visible-light region of the spectrum.

what are the three basic components of a modern system for measuring radiation from astronomical sources.

1. telescope: serves as a "bucket" for collecting visible light (or radiation at other wavelengths, as shown 2. wavelength sorting device: attached to the telescope that sorts the incoming radiation by wavelength (we might simply want to separate blue light from red light so that we can determine the temperature of a star. But at other times, we want to see individual spectral lines to determine what an object is made of, or to measure its speed) 3. Detector: a device that senses the radiation in the wavelength regions we have chosen and permanently records the observations.

first step of fusion in sun

1. the high temperatures inside the Sun's core, two protons combine to make a deuterium nucleus, which is an isotope (or version) of hydrogen that contains one proton and one neutron.one of the original protons has been converted into a neutron in the fusion reaction. Electric charge has to be conserved in nuclear reactions, and it is conserved in this one. A positron (antimatter electron) emerges from the reaction and carries away the positive charge originally associated with one of the protons. FIGURE 16.6

the smallest mass that a true star can have is....

1/12 that of the Sun. By a "true" star, astronomers mean one that becomes hot enough to fuse protons to form helium Objects with masses between roughly 1/100 and 1/12 that of the Sun may produce energy for a brief time by means of nuclear reactions involving deuterium, but they do not become hot enough to fuse protons. Such objects are intermediate in mass between stars and planets and have been given the name brown dwarfs

length of light-second

3 × 108 m = 3 × 105 km

speed of light

3 × 108 m/s = 3 × 105 km/s

Large Sypnotic Survey Telescope (LSST)

8.4-meter telescope with a significantly larger field of view than any existing telescopes. It will rapidly scan the sky to find transients, phenomena that change quickly, such as exploding stars and chunks of rock that orbit near Earth. The LSST is expected to see first light in 2021.

Fusion on Earth

= cannot be controlled =Fusion energy would have many advantages: it would use hydrogen (or deuterium, which is heavy hydrogen) as fuel, and there is abundant hydrogen in Earth's lakes and oceans. Water is much more evenly distributed around the world than oil or uranium, meaning that a few countries would no longer hold an energy advantage over the others. And unlike fission, which leaves dangerous byproducts, the nuclei that result from fusion are perfectly safe. =HOWEVER, it takes extremely high temperatures for nuclei to overcome their electrical repulsion and undergo fusion. -Interactions at such temperatures are difficult to sustain and control.

H II Regions

A cloud of ionized hydrogen Scientists who work with spectra use the Roman numeral I to indicate that an atom is neutral; successively higher Roman numerals are used for each higher stage of ionization. H II thus refers to hydrogen that has lost its one electron; Fe III is iron with two electrons missing

supernova

A gigantic explosion in which a massive star collapses and throws its outer layers into space When these explosions happen close by, they can be among the most spectacular celestial events, as we will discuss in the next section. (Actually, there are at least two different types of supernova explosions: the kind we have been describing, which is the collapse of a massive star, is called, for historical reasons, a type II supernova. We will describe how the types differ later in this chapter).

cepheid light curve

A graph that shows how the brightness of a variable star changes with time is called a light curve (Figure 19.9). The maximum is the point of the light curve where the star has its greatest brightness; the minimum is the point where it is faintest. If the light variations repeat themselves periodically, the interval between the two maxima is called the period of the star.

The horoscope

A key to Natal astrology is the horoscope, a chart showing the positions of the planets in the Sky at the moment of an individual's birth ( marker of the hour) At the time astrology was set up a Zodiac was divided into 12 sectors called signs each 30 degrees long, each sign was named after a constellation and the Sky through which the sun moon and planets seemed to pass Sun sign = zodiac --> because of procession the constellation Zodiac slid westward along the elliptic by about ½ of the Zodiac about the width of one sign A complete horoscope shows the sun moon at each planet by indicating its position in the appropriate sign of the Zodiac but the position in the Sky or the house must also be calculated because this celestial sphere turned and the entire Zodiac moves across the Sky to the West

What is a safe distance to be from a supernova explosion?

A lot depends on the violence of the particular explosion, what type of supernova it is (see The Evolution of Binary Star Systems), and what level of destruction we are willing to accept. Calculations suggest that a supernova less than 50 light-years away from us would certainly end all life on Earth, and that even one 100 light-years away would have drastic consequences for the radiation levels here. One minor extinction of sea creatures about 2 million years ago on Earth may actually have been caused by a supernova at a distance of about 120 light-years.

Appearance of the Total Eclipse

A solar eclipse starts when the Moon just begins to silhouette itself against the edge of the Sun's disk. A partial phase follows, during which more and more of the Sun is covered by the Moon. About an hour after the eclipse begins, the Sun becomes completely hidden behind the Moon. During totality, the sky is dark enough that planets become visible in the sky, and usually the brighter stars do as well. The corona is the Sun's outer atmosphere, consisting of sparse gases that extend for millions of miles in all directions from the apparent surface of the Sun. Only when the brilliant glare from the Sun's visible disk is blotted out by the Moon during a total eclipse is the pearly white corona visible

Newton's Law of Universal Gravitation

A straight line that defined the most natural state of motion --> but planets move in ellipses so some force must be bending their paths --> Newton proposed gravity Gravity was only a concept related to Earth--> extended it to moon --> all material bodies, so attractive force between the Sun and each of the planets could keep them in orbit Mathematical equation = follower Kepler planet model & predict correct behavior of falling bodies on Earth Fgravity = G(M1 M2)/ R2 --> universal law of gravitation Objects accelerate downward at 9.8 meters per second on earth \ What we weigh --> depends on the local force of gravity Greater the mass, greater the attraction --> attraction gets weaker but never truly 0

Milky Way Galaxy

All the stars visible in our night sky - Milky Way Galaxy We can't through to Milky Way's rim because the space between stars are occupied by a mixture of hydrogen gas and solid particles we call interstellar dust This gas and dust collect into enormous clouds in many places in the Galaxy, becoming the raw material for future generations of stars

absorption

Although gas does not absorb much light, we know from everyday experience that tiny solid or liquid particles can be very efficient absorbers. Water vapor in the air is quite invisible. When some of that vapor condenses into tiny water droplets, however, the resulting cloud is opaque. Dust storms, smoke, and smog offer familiar examples of the efficiency with which solid particles absorb light. On the basis of arguments like these, astronomers have concluded that widely scattered solid particles in interstellar space must be responsible for the observed dimming of starlight.

Eclipses of the Moon

Although the Sun is about 400 times larger in diameter than the Moon, it is also about 400 times farther away, so both the Sun and the Moon have the same angular size—about 1/2° When the Moon's shadow strikes Earth, people within that shadow see the Sun at least partially covered by the Moon; that is, they witness a solar eclipse. When the Moon passes into the shadow of Earth, people on the night side of Earth see the Moon darken in what is called a lunar eclipse. The shadows of Earth and the Moon consist of two parts: a cone where the shadow is darkest, called the umbra, and a lighter, more diffuse region of darkness called the penumbra. As you can imagine, the most spectacular eclipses occur when an object enters the umbra. If the path of the Moon in the sky were identical to the path of the Sun (the ecliptic), we might expect to see an eclipse of the Sun and the Moon each month—whenever the Moon got in front of the Sun or into the shadow of Earth. However, as we mentioned, the Moon's orbit is tilted relative to the plane of Earth's orbit about the Sun by about 5° (imagine two hula hoops with a common center, but tilted a bit). As a result, during most months, the Moon is sufficiently above or below the ecliptic plane to avoid an eclipse. But when the two paths cross (twice a year), it is then "eclipse season" and eclipses are possible.

hydrostatic equilibrium

An analogy is an inflated balloon, which will expand or contract until an equilibrium is reached between the pressure of the air inside and outside. The technical term for this condition is hydrostatic equilibrium. Stable stars are all in hydrostatic equilibrium; so are the oceans of Earth as well as Earth's atmosphere. The air's own pressure keeps it from falling to the ground.

Excitation

An atom can absorb energy, which raises it to a higher energy level (corresponding, in the simple Bohr picture, to an electron's movement to a larger orbit) Generally, an atom remains excited for only a very brief time. After a short interval, typically a hundred-millionth of a second or so, it drops back spontaneously to its ground state, with the simultaneous emission of light With each jump, it emits a photon of the wavelength that corresponds to the energy difference between the levels at the beginning and end of that jump

Joseph Fraunhofer

Analyzed solar spectrum of sun with spectroscope Saw more that 800 dark lines on top of the continuous spectrum Caused by absorbtion of some of the wavelengths by solar atmosphere Called Fraunhofer lines

seven days a week

Ancient Greeks study seven celestial wanderers to which they dedicated a unit of time

astrology

Ancient cultures believed that sun, moon and five visible planets connect directly to life and death and require an deep understanding of gods actions Astrology: positions of these bodies among the stars of the zodiac are thought to hold the key to understanding what we can expect from life

what can be used as radar telescope?

Any radio dish can be used as a radar telescope if it is equipped with a powerful transmitter as well as a receiver. largest radar telescope 1000-foot (305-meter) telescope at Arecibo in Puerto Rico

Distance to Stars

As Earth travels from one side of its orbit to the other, it graciously provides us with a baseline of 2 AU, or about 300 million kilometers. Although this is a much bigger baseline than the diameter of Earth, the stars are so far away that the resulting parallax shift is still not visible to the naked eye—not even for the closest stars

binary system: one white dwarf & other

As fresh hydrogen from the outer layers of its companion accumulates on the surface of the hot white dwarf, it begins to build up a layer of hydrogen. As more and more hydrogen accumulates and heats up on the surface of the degenerate star, the new layer eventually reaches a temperature that causes fusion to begin in a sudden, explosive way, blasting much of the new material away.

the end of all fusion reactions to be the time of a star's death

As the core is stabilized by degeneracy pressure, a last shudder of fusion passes through the outside of the star, consuming the little hydrogen still remaining. Now the star is a true white dwarf: nuclear fusion in its interior has ceased

why is all directions on a spinning sphere are not created equal Why?

As the protostar rotates, it is much easier for material to fall right onto the poles (which spin most slowly) than onto the equator (where material moves around most rapidly). Therefore, gas and dust falling in toward the protostar's equator are "held back" by the rotation and form a whirling extended disk around the equator (part b in Figure 21.8). You may have observed this same "equator effect" on the amusement park ride in which you stand with your back to a cylinder that is spun faster and faster. As you spin really fast, you are pushed against the wall so strongly that you cannot possibly fall toward the center of the cylinder. Gas can, however, fall onto the protostar easily from directions away from the star's equator.

how do the electrons act as the star dies

As the star's core contracts, electrons are squeezed closer and closer together. Eventually, a star like the Sun becomes so dense that further contraction would in fact require two or more electrons to violate the rule against occupying the same place and moving in the same way. Such a dense gas is said to be degenerate

Astrology Today

Astrology is cast horoscopes and suggest interpretations, sun sign astrology is a simple verified variant of Natal astrology (also emphasis on planet configuration but does being born 15 min later make you a diff person) no known forces or rules that could attribute or provide any proof that sun and the movement of the planets have anything to do with their personality or future No evidence that Natal astrology has any predictive power, even in the statistical sens

Why do astronauts aboard Space shuttle appear to have no gravitational forces on them?

Astronauts feel weightless because they are in free fall --> they are in free fall and accelerate at the same rate as everything around them--> they are falling around Earth as a result they will continue to fall and said to be in orbit around Earth

locating places in the sky

Astronomers use coordinates called declination & right ascension to measure positions in the sky sky appears to rotate about points above the North and South Poles of Earth—points in the sky called the north celestial pole and the south celestial pole. Halfway between the celestial poles, and thus 90° from each pole, is the celestial equator, a great circle on the celestial sphere that is in the same plane as Earth's equator. We can use these markers in the sky to set up a system of celestial coordinates

But if all the stars on the main sequence are doing the same thing (fusing hydrogen), why are they distributed along a sequence of points? That is, why do they differ in luminosity and surface temperature (which is what we are plotting on the H-R diagram)?

Astrophysicists have been able to show that the structure of stars that are in equilibrium and derive all their energy from nuclear fusion is completely and uniquely determined by just two quantities: the total mass and the composition of the star. This fact provides an interpretation of many features of the H-R diagram. Imagine a cluster of stars forming from a cloud of interstellar "raw material" whose chemical composition is similar to the Sun's. In such a cloud, all the clumps of gas and dust that become stars begin with the same chemical composition and differ from one another only in mass. Now suppose that we compute a model of each of these stars for the time at which it becomes stable and derives its energy from nuclear reactions, but before it has time to alter its composition appreciably as a result of these reactions. The models calculated for these stars allow us to determine their luminosities, temperatures, and sizes. If we plot the results from the models—one point for each model star—on the H-R diagram, we get something that looks just like the main sequence we saw for real stars.

The Season At Different Latitudes

At equator all seasons are much the same = every day of the year the sun is half up and half down--> wet & dry seasons rather than based on sunlight Extreme north or extreme south like the artic & Antarctic seasons become more pronounces At the North Pole, all celestial objects that are north of the celestial equator are always above the horizon and, as Earth turns, circle around parallel to it. The Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches the vernal equinox and sets when it reaches the autumnal equinox. Each year there are 6 months of sunshine at each pole, followed by 6 months of darkness. LOOK AT EXAMPLE 4.2

what happens at the every end to these massive stars?

At this stage of its evolution, a massive star resembles an onion with an iron core. As we get farther from the center, we find shells of decreasing temperature in which nuclear reactions involve nuclei of progressively lower mass—silicon and sulfur, oxygen, neon, carbon, helium, and finally, hydrogen

how did we discover Ultra-Hot Interstellar Gas?

Before the launch of astronomical observatories into space, which could see radiation in the ultraviolet and X-ray parts of the spectrum, astronomers assumed that most of the region between stars was filled with hydrogen at temperatures no warmer than those found in H II regions. But telescopes launched above Earth's atmosphere obtained ultraviolet spectra that contained interstellar lines produced by oxygen atoms that have been ionized five times.

how to get mass of each star from mass of both stars

But the relative orbital speeds of the two stars can tell us how much of the total mass each star has. As we saw in our seesaw analogy, the more massive star is closer to the center of mass and therefore has a smaller orbit. Therefore, it moves more slowly to get around in the same time compared to the more distant, lower-mass star. If we sort out the speeds relative to each other, we can sort out the masses relative to each other

temperature and density in the inner region

Calculations show that the temperature and density in the inner region slowly increase as helium accumulates in the center of a star. As the temperature gets hotter, each proton acquires more energy of motion on average; this means it is more likely to interact with other protons, and as a result, the rate of fusion also increases. For the proton-proton cycle described in, the rate of fusion goes up roughly as the temperature to the fourth power. If the rate of fusion goes up, the rate at which energy is being generated also increases, and the luminosity of the star gradually rises. Initially, however, these changes are small, and stars remain within the main-sequence band on the H-R diagram for most of their lifetimes.

coRoT

CoRoT discovered 32 transiting exoplanets, including the first transiting planet with a size and density similar to Earth. In 2012, the spacecraft suffered an onboard computer failure, ending the mission. Meanwhile, NASA built a much more powerful transit observatory called Kepler.

distance within solar system

Copernicus and Kepler established the relative distances of the planets -->couldn't establish absolute distances , to establish absolute distances, astronomers -->had to measure one distance in the solar system directly.--> measured estimate of Venus

Why couldn't it be the interstellar gas that reddens distant stars and not the dust?

Despite its very high density compared with that of interstellar gas, it is so transparent as to be practically invisible. (Gas does have a few specific spectral lines, but they absorb only a tiny fraction of the light as it passes through.)The quantity of gas required to produce the observed absorption of light in interstellar space would have to be enormous. The gravitational attraction of so great a mass of gas would affect the motions of stars in ways that could easily be detected. Such motions are not observed, and thus, the interstellar absorption cannot be the result of gases.

how are the spectral classes divided

Each of these spectral classes, except possibly for the Y class which is still being defined, is further subdivided into 10 subclasses designated by the numbers 0 through 9. A B0 star is the hottest type of B star; a B9 star is the coolest type of B star and is only slightly hotter than an A0 star.

why are observations outside of earth better?

Earth's atmosphere blocks most radiation at wavelengths shorter than visible light, so we can only make direct ultraviolet, X-ray, and gamma ray observations from space (though indirect gamma ray observations can be made from Earth). Getting above the distorting effects of the atmosphere is also an advantage at visible and infrared wavelengths amount of detail you can observe is limited only by the size of the instrument & the cost

Locating places on earth

Earth's axis of rotation defined the locations of its North & South Poles & its equator halfway between East is the directions toward which the Earth rotates, and west is its opposite NESW is well defined except at the North & South Poles, East and West are ambiguous

Earth satellite

Earth's nearest satellite is the moon (takes one month to revolve around the earth) 1.3 seconds for light from moon to earth and 3 seconds for radio wave to cover that distance

astronomical unit

Earth-Sun distance = astronomical unit (light takes roughly 8 min to take one AU) One year ( 3 * 10 ^7 seconds) for earth to go around the sun Because gravity holds us firmly to Earth and there is no resistance to Earth's motion in the vacuum of space, we participate in this extremely fast-moving trip without being aware of it day to day ( 8 planets)

Astronomy Around the World

Egyptians: adopted a calendar 365 days and kept track of Sirius because this cycle corresponds with the Nile River Chinese determined the same year length + comets, meteors and dark spots on the Sun Recorded guest stars that are usually invisible but sometimes brighten up for a little time --> still use these records Mayan Culture in Mexico and Central America --> calendar based on Venus and made observations from site dedicated to this cause Polynesians learned to navigate by the stars over hundreds of kilometers of open ocean Britain people used stone circles to track of motions of the sun and moon > Stonehedge Early Greek & Roman Cosmology

light curve of an edge-on eclipsing binary

FIGURE 18.11 During the time interval between the first and second contacts, the smaller star has moved a distance equal to its own diameter. During the time interval from the first to third contacts, the smaller star has moved a distance equal to the diameter of the larger star. If the spectral lines of both stars are visible in the spectrum of the binary, then the speed of the smaller star with respect to the larger one can be measured from the Doppler shift. But knowing the speed with which the smaller star is moving and how long it took to cover some distance can tell the span of that distance—in this case, the diameters of the stars. The speed multiplied by the time interval from the first to second contact gives the diameter of the smaller star. We multiply the speed by the time between the first and third contacts to get the diameter of the larger star. **: orbits are generally not seen exactly edge-on, and the light from each star may be only partially blocked by the other. Furthermore, binary star orbits, just like the orbits of the planets, are ellipses, not circles. However, all these effects can be sorted out from very careful measurements of the light curve.

radial velocity curve

FIGURE 18.7 These curves plot the radial velocities of two stars in a spectroscopic binary system, showing how the stars alternately approach and recede from Earth. Note that positive velocity means the star is moving away from us relative to the center of mass of the system, which in this case is 40 kilometers per second. Negative velocity means the star is moving toward us relative to the center of mass.

Luminosity Classes Stars

FIGURE 19.15 Stars of the same temperature (or spectral class) can fall into different luminosity classes on the Hertzsprung-Russell diagram. By studying details of the spectrum for each star, astronomers can determine which luminosity class they fall in (whether they are main-sequence stars, giant stars, or supergiant stars).

Scattering of Light by Dust.

Figure 20.15 Interstellar dust scatters blue light more efficiently than red light, thereby making distant stars appear redder and giving clouds of dust near stars a bluish hue. Here, a red ray of light from a star comes straight through to the observer, whereas a blue ray is shown scattering. A similar scattering process makes Earth's sky look blue.

Formation of the 21-Centimeter Line.

Figure 20.5 When the electron in a hydrogen atom is in the orbit closest to the nucleus, the proton and the electron may be spinning either (a) in the same direction or (b) in opposite directions. When the electron flips over, the atom gains or loses a tiny bit of energy by either absorbing or emitting electromagnetic energy with a wavelength of 21 centimeters.

Interpretation of Newton's first law

First law is a restatement of one of Galileo's discoveries, called the conservation of momentum In the absence of any outside influence, there is a measure of a body's motion, called its momentum, that remains unchanged First law also called law of inertia, where inertia is the tendency of objects to keep doing what they are already doing Velocity: describe the speed and direction of motion

The Gregorian Calendar

First, 10 days had to be dropped out of the calendar to bring the vernal equinox back to March 21; by proclamation, the day following October 4, 1582, became October 15 The second feature of the new Gregorian calendar was a change in the rule for leap year, making the average length of the year more closely approximate the tropical year. Gregory decreed that three of every four century years—all leap years under the Julian calendar— (1600,2000) The average length of this Gregorian year, 365.2425 mean solar days, is correct to about 1 day in 3300 years.

how does low pressure affect spectrum of giant

First, a star with a lower-pressure photosphere shows narrower spectral lines than a star of the same temperature with a higher-pressure photosphere (Figure 17.9). The difference is large enough that careful study of spectra can tell which of two stars at the same temperature has a higher pressure (and is thus more compressed) and which has a lower pressure (and thus must be extended). This effect is due to collisions between particles in the star's photosphere—more collisions lead to broader spectral lines. Collisions will, of course, be more frequent in a higher-density environment. Think about it like traffic—collisions are much more likely during rush hour, when the density of cars is high. Second, more atoms are ionized in a giant star than in a star like the Sun with the same temperature. The ionization of atoms in a star's outer layers is caused mainly by photons, and the amount of energy carried by photons is determined by temperature. But how long atoms stay ionized depends in part on pressure. Compared with what happens in the Sun (with its relatively dense photosphere), ionized atoms in a giant star's photosphere are less likely to pass close enough to electrons to interact and combine with one or more of them, thereby becoming neutral again. Ionized atoms, as we discussed earlier, have different spectra from atoms that are neutral.

The discovery of neptune

Herschel --> Uranus 1781--> some planets could be too dim to visible to naked eye Even after allowance for perturbations was made for Uranus, there was a .03 degree difference in predicted vs actual--> Couch Adams proposed another planet and told George Airy where to find the new planet in the sky Verrier published it--> airy told Challis to search for new object -->Challis was going to repeatedly observe faint stars over multiple day and hoped the planet would distinguish itself from stars based on motion--> failed to do so Verrier -> Galle + possessing new charts of the Aquarius region, found and identified the planet that very night. It was less than a degree from the position Le verrier predicted Discovery of Neptune (Adams & Le Verrier) was a triumph for gravitational theory for it dramatically confirmed the generality of Newton's laws

doppler effect on stars

If a star approaches or recedes from us, the wavelengths of light in its continuous spectrum appear shortened or lengthened, respectively, as do those of the dark lines. However, unless its speed is tens of thousands of kilometers per second, the star does not appear noticeably bluer or redder than normal. The Doppler shift is thus not easily detected in a continuous spectrum and cannot be measured accurately in such a spectrum. The wavelengths of the absorption lines can be measured accurately, however, and their Doppler shift is relatively simple to detect.

Doppler effect to measure how fast a star rotates.

If an object is rotating, then one of its sides is approaching us while the other is receding (unless its axis of rotation happens to be pointed exactly toward us). This is clearly the case for the Sun or a planet; we can observe the light from either the approaching or receding edge of these nearby objects and directly measure the Doppler shifts that arise from the rotation.

The Doppler method allows us to estimate the mass of a planet

If the same object can be studied by both the Doppler and transit techniques, we can measure both the mass and the size of the exoplanet. This is a powerful combination that can be used to derive the average density (mass/volume) of the planet. In 1999, using measurements from ground-based telescopes, the first transiting planet was detected orbiting the star HD 209458. The planet transits its parent star for about 3 hours every 3.5 days as we view it from Earth. Doppler measurements showed that the planet around HD 209458 has about 70% the mass of Jupiter, but its radius is about 35% larger than Jupiter's. This was the first case where we could determine what an exoplanet was made of—with that mass and radius, HD 209458 must be a gas and liquid world like Jupiter or Saturn.

The interaction of many bodies

If you have a cluster and you know the position of each start at any given instant you can calculate the combined gravitational force of the entire group on any one member of the cluster you can find how will accelerate--> thus tracking its motion we must simultaneously calculate the acceleration of each star produced by the combination of the gravitational attractions of all of all others in order to track the motions of all of them and hence of any one The dominant gravitational attraction of the Sun --> on orbit of planets--> treat the effects of other bodies as small perturbations (disturbances)--> lead to discovery of new planet in 1846

why doesn't this reemitted light quickly "fill in" the darker absorption lines?

Imagine a beam of white light coming toward you through some cooler gas. Some of the reemitted light is actually returned to the beam of white light you see, but this fills in the absorption lines only to a slight extent. The reason is that the atoms in the gas reemit light in all directions, and only a small fraction of the reemitted light is in the direction of the original beam (toward you). In a star, much of the reemitted light actually goes in directions leading back into the star, which does observers outside the star no good whatsoeve

interiors of rocky planets make the simplifying assumption that the planet consists of two or three layers.

In Figure 21.25, the two green triangles with roughly 1 MEarth and 1 REarth represent Venus and Earth. Notice that these planets fall between the models for a pure iron and a pure rock planet, consistent with what we would expect for the known mixed-chemical composition of Venus and Earth. In the case of gaseous planets, the situation is more complex. Hydrogen is the lightest element in the periodic table, yet many of the detected exoplanets in Figure 21.25 with masses greater than 100 MEarth have radii that suggest they are lower in density than a pure hydrogen planet. Hydrogen is the lightest element, so what is happening here? Why do some gas giant planets have inflated radii that are larger than the fictitious pure hydrogen planet? Many of these planets reside in short-period orbits close to the host star where they intercept a significant amount of radiated energy. If this energy is trapped deep in the planet atmosphere, it can cause the planet to expand. Planets that orbit close to their host stars in slightly eccentric orbits have another source of energy: the star will raise tides in these planets that tend to circularize the orbits. This process also results in tidal dissipation of energy that can inflate the atmosphere. It would be interesting to measure the size of gas giant planets in wider orbits where the planets should be cooler—the expectation is that unless they are very young, these cooler gas giant exoplanets (sometimes called "cold Jupiters") should not be inflated. But we don't yet have data on these more distant exoplanets.

why does an object at a higher temp emit more power at wavelength than does a cooler one?

In a hot gas, for example, the atoms have more collisions and give off more energy. In the real world of stars, this means that hotter stars give off more energy at every wavelength than do cooler stars

prime focus

In a reflecting telescope, the concave mirror is placed at the bottom of a tube or open framework. The mirror reflects the light back up the tube to form an image near the front end at a location called the prime focus. Since an astronomer at the primer focus can block much of the light coming to the main mirror, the use of a small secondary mirror allows more light to get through the system

The wave-like characteristics of light

In both cases, the disturbance travels rapidly outward from the point of origin and can use its energy to disturb other things farther away. For example, in water, the expanding ripples moving away from our frog could disturb the peace of a dragonfly resting on a leaf in the same pool. electromagnetic waves: the radiation generated by a transmitting antenna full of charged particles and moving electrons at your local radio station can, sometime later, disturb a group of electrons in your car radio antenna and bring you the news and weather while you are driving to class or work in the morning.

individual stars in an open cluster can survive for billions of years, they typically remain together as a cluster for only a few million years, or at most, a few hundred million years. WHY?

In small open clusters, the average speed of the member stars within the cluster may be higher than the cluster's escape velocity,[1] and the stars will gradually "evaporate" from the cluster. Close encounters of member stars may also increase the velocity of one of the members beyond the escape velocity. Every few hundred million years or so, the cluster may have a close encounter with a giant molecular cloud, and the gravitational force exerted by the cloud may tear the cluster apart.

why even the densest interstellar clouds are sill less dense than on Earth?

In some interstellar clouds, the density of gas and dust may exceed the average by as much as a thousand times or more, but even this density is more nearly a vacuum than any we can make on Earth. To show what we mean, let's imagine a vertical tube of air reaching from the ground to the top of Earth's atmosphere with a cross-section of 1 square meter. Now let us extend the same-size tube from the top of the atmosphere all the way to the edge of the observable universe—over 10 billion light-years away. Long though it is, the second tube would still contain fewer atoms than the one in our planet's atmosphere.

outflows from protostars

In the HH47 image, a protostar 1500 light-years away (invisible inside a dust disk at the left edge of the image) produces a very complicated jet. The star may actually be wobbling, perhaps because it has a companion. Light from the star illuminates the white region at the left because light can emerge perpendicular to the disk (just as the jet does). At right, the jet is plowing into existing clumps of interstellar gas, producing a shock wave that resembles an arrowhead.

What would it be like to live inside a globular cluster?

In the dense central regions, the stars would be roughly a million times closer together than in our own neighborhood. If Earth orbited one of the inner stars in a globular cluster, the nearest stars would be light-months, not light-years, away. They would still appear as points of light, but would be brighter than any of the stars we see in our own sky. The Milky Way would probably be difficult to see through the bright haze of starlight produced by the cluster. About 150 globular clusters are known in our Galaxy. Most of them are in a spherical halo (or cloud) surrounding the flat disk formed by the majority of our Galaxy's stars. All the globular clusters are very far from the Sun, and some are found at distances of 60,000 light-years or more from the main disk of the Milky Way. The diameters of globular star clusters range from 50 light-years to more than 450 light-years.

reddening

In the early part of the twentieth century, astronomers discovered that some stars look red even though their spectral lines indicate that they must be extremely hot (and thus should look blue). The solution to this seeming contradiction turned out to be that the light from these hot stars is not only dimmed but also reddened by interstellar dust, a phenomenon known as interstellar reddening.

what happens to the white dwarf in this binary system?

In this way, the white dwarf quickly (but only briefly) becomes quite bright, hundreds or thousands of times its previous luminosity.

Keck Telescope

Instead of a single primary mirror 10 meters in diameter, each Keck telescope achieves its larger aperture by combining the light from 36 separate hexagonal mirrors, each 1.8 meters wide (Figure 6.9). Computer-controlled actuators (motors) constantly adjust these 36 mirrors so that the overall reflecting surface acts like a single mirror with just the right shape to collect and focus the light into a sharp image. --> steel structure designed so entire telescope can be pointed toward anywhere on the sky --> motorized drive system that moves it very smoothly from east to west at exactly the same rate that Earth is rotating from west to east, so it can continue to point at the object being observed. ->housed in a dome to protect elements and moved so light from objected isn't blocked

Interstellar clouds lifetime

Interstellar clouds do not last for the lifetime of the universe. Instead, they are like clouds on Earth, constantly shifting, merging with each other, growing, or dispersing. Some become dense and massive enough to collapse under their own gravity, forming new stars. When stars die, they, in turn, eject some of their material into interstellar space. This material can then form new clouds and begin the cycle over again.

Estimating the masses of binary star systems

Kepler found that the time a planet takes to go around the Sun is related by a specific mathematical formula to its distance from the Sun. In our binary star situation, if two objects are in mutual revolution, then the period (P) with which they go around each other is related to the semimajor axis (D) of the orbit of one with respect to the other, according to this equation

Orbital Mass & Motion

Kepler third law --> relationship between orbital period of a revolution & its distance from the Sun a 3 = ⎝M 1 + M 2 ⎞ × P 2 Because the mass of planets are so insignificant next to the sun, Kepler did not realize that both masses had to be included in calculation

Kepler's Three Laws of Planetary Motion

Kepler's first law: Each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse. Kepler's second law: The straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time Kepler's third law: The square of a planet's orbital period is directly proportional to the cube of the semimajor axis of its orbit Provide a precise geometric description of planetary motion within the framework of the Copernican system--> except possible to calculate planetary positions with great improved precision still do not help us understand what forces of nature constrain the planets to these rules

To determine the relative sizes of the two stars, we take the ratio of the corresponding luminosities::

L(sirius)/L(companion) = (Asirius * Asirius)/(Acompanion *Fcompanions = Asirius/Acomp = 4piR^2Sirius/4piR^2comp = R^2sirius/R^2comp L(sirius)/L(companion) = 8200 = = R^2sirius/R^2comp Therefore, the relative sizes of the two stars can be found by taking the square root of the relative luminosity. Since 8200 = 91 , the radius of Sirius is 91 times larger than the radium of its faint companion. The method for determining the radius shown here requires both stars be visible, which is not always the case.

class T brown dwarfs

Lines of steam (hot water vapor) are present, along with lines of carbon monoxide and neutral sodium, potassium, cesium, and rubidium. Methane (CH4) lines are strong in class-T brown dwarfs, as methane exists in the atmosphere of the giant planets in our own solar system.

magnitude scale

Measurements showed that we receive about 100 times more light from a first-magnitude star than from a sixth-magnitude star. Based on this measurement, astronomers then defined an accurate magnitude system in which a difference of five magnitudes corresponds exactly to a brightness ratio of 100:1. it has a magnitude of 2.0 (or 2.1, 2.3, and so forth). So what number is it that, when multiplied together five times, gives you this factor of 100? Play on your calculator and see if you can get it. The answer turns out to be about 2.5, which is the fifth root of 100. This means that a magnitude 1.0 star and a magnitude 2.0 star differ in brightness by a factor of about 2.5. Likewise, we receive about 2.5 times as much light from a magnitude 2.0 star as from a magnitude 3.0 star. What about the difference between a magnitude 1.0 star and a magnitude 3.0 star? Since the difference is 2.5 times for each "step" of magnitude, the total difference in brightness is 2.5 × 2.5 = 6.25 times.

Eclipses of the Sun & Moon

Much of the time, the Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned. In this type of "annular eclipse," there is a ring of light around the dark sphere of the Moon. If moon is somewhat nearer and moon completely covers Sun = total solar eclipse total eclipse of the Sun occurs at those times when the umbra of the Moon's shadow reaches the surface of Earth If the Sun and Moon are properly aligned, then the Moon's darkest shadow intersects the ground at a small point on Earth's surface. Anyone on Earth within the small area covered by the tip of the Moon's shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse. observers on a larger area of Earth's surface who are in the penumbra will see only a part of the Sun eclipsed by the Moon: we call this a partial solar eclipse. Between Earth's rotation and the motion of the Moon in its orbit, the tip of the Moon's shadow sweeps eastward at about 1500 kilometers per hour along a thin band across the surface of Earth. The thin zone across Earth within which a total solar eclipse is visible (weather permitting) is called the eclipse path. Within a region about 3000 kilometers on either side of the eclipse path, a partial solar eclipse is visible. It does not take long for the Moon's shadow to sweep past a given point on Earth. The duration of totality may be only a brief instant; it can never exceed about 7 minutes.

Pleiades Cluster

One of the best-known examples is the nebulosity around each of the brightest stars in the Pleiades cluster (see Figure 20.1). The dust grains are small, and such small particles turn out to scatter light with blue wavelengths more efficiently than light at red wavelengths. A reflection nebula, therefore, usually appears bluer than its illuminating star reflection nebula shines only because the dust within it scatters light from a nearby bright source. The Pleiades cluster is currently passing through an interstellar cloud that contains dust grains, which scatter the light from the hot blue stars in the cluster. The Pleiades cluster is about 400 light-years from the Sun

two neutron stars

One such system has the stars in very close orbits to one another, so much that they continually alter each other's orbit. Another binary neutron star system includes two pulsars that are orbiting each other every 2 hours and 25 minutes. As we discussed earlier, pulsars radiate away their energy, and these two pulsars are slowly moving toward one another, such that in about 85 million years, they will actually merge (see Gravitational Wave Astronomy for our first observations of such a merger).

two ways to detect orbital motion

One way would be to look for changes in the Sun's position on the sky. The second would be to use the Doppler effect to look for changes in its velocity. Let's discuss each of these in turn. The diameter of Jupiter's apparent orbit viewed from Alpha Centauri is 10 seconds of arc, and that of the Sun's orbit is 0.010 seconds of arc. (Remember, 1 second of arc is 1/3600 degree.) If they could measure the apparent position of the Sun (which is bright and easy to detect) to sufficient precision, they would describe an orbit of diameter 0.010 seconds of arc with a period equal to that of Jupiter, which is 12 years. , if they watched the Sun for 12 years, they would see it wiggle back and forth in the sky by this minuscule fraction of a degree. From the observed motion and the period of the "wiggle," they could deduce the mass of Jupiter and its distance using Kepler's laws.

why the higher the temp the shorter the wavelength at which maximum power is emitted

Remember that a shorter wavelength means a higher frequency and energy. It makes sense, then, that hot objects give off a larger fraction of their energy at shorter wavelengths (higher energies) than do cool objects. You may have observed examples of this rule in everyday life. When a burner on an electric stove is turned on low, it emits only heat, which is infrared radiation, but does not glow with visible light. If the burner is set to a higher temperature, it starts to glow a dull red.

What does theory predict for the H-R diagram of a cluster whose stars have recently condensed from an interstellar cloud?

Remember that at every stage of evolution, massive stars evolve more quickly than their lower-mass counterparts. After a few million years ("recently" for astronomers), the most massive stars should have completed their contraction phase and be on the main sequence, while the less massive ones should be off to the right, still on their way to the main sequence. illustrated by HR diagram

Copernicus

Renaissance: displacement of earth from the center of the universe by Copernicus--> new sun centered, heliocentric model of the solar system De Revolutionibus Orbium Coelestium (On the Revolution of Celestial Orbs), published in 1543, the year of his death. --> develop an improved theory from which to calculate planetary positions, but in doing so, he was himself not free of all traditional prejudices --> proved the circular orbit and defense of heliocentrism with evidence Biggest reservation was that if earth was moving we would feel it --> however we have seen trees move while we are the ones that are moving He also reasoned that the apparent rotation of the celestial sphere could be explained by assuming that Earth rotates while the celestial sphere is stationary If rotation would break earth to pieces this idea would be worse for the sun

nuclear attraction versus electrical repulsion

Since like charges repel via the electrical force (postive charges from the two nuclei we are trying tojoin) , the closer we get two nuclei to each other, the more they repel. get them within "striking distance" (which is very tiny about the size of a nucleus) of the nuclear force, they will then come together with a much stronger attraction.

Basics of Stars

Stable (main-sequence) stars such as our Sun maintain equilibrium by producing energy through nuclear fusion in their cores. The ability to generate energy by fusion defines a star. Each second in the Sun, approximately 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning into energy in the process. This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel. Stars come with many different masses, ranging from 1/12 solar masses (MSun) to roughly 100-200 MSun. There are far more low-mass than high-mass stars. The most massive main-sequence stars (spectral type O) are also the most luminous and have the highest surface temperature. The lowest-mass stars on the main sequence (spectral type M or L) are the least luminous and the coolest. A galaxy of stars such as the Milky Way contains enormous amounts of gas and dust—enough to make billions of stars like the Sun.

Sun changes in position

Stars continue to circle during the day, but the Sun makes them difficult to see Sun changes position gradually on the celestial sphere ~ 1 degree to the east relative to the stars 1 year to make a full circle --> as Earth goes around the sun

Early Calendar

Stonehedge: tones are aligned with the directions of the Sun and Moon during their risings and settings at critical times of the year (such as the summer and winter solstices), and it is generally believed that at least one function of the monument was connected with the keeping of a calendar. Mayan Calendar: did not attempt to correlate their calendar accurately with the length of the year or lunar month. Rather, their calendar was a system for keeping track of the passage of days and for counting time far into the past or future. Among other purposes, it was useful for predicting astronomical events, such as the position of Venus in the sky Ancient Chinese: In addition to the motions of Earth and the Moon, they were able to fit in the approximately 12-year cycle of Jupiter, which was central to their system of astrology. The Chinese still preserve some aspects of this system in their cycle of 12 "years"—the Year of the Dragon, the Year of the Pig, and so on—that are defined by the position of Jupiter in the zodiac. Western Calendars: --> Sumerians --> Egyptians and Greeks --> Julian Calendar-->which approximated the year at 365.25 days, fairly close to the actual value of 365.2422. The Romans achieved this approximation by declaring years to have 365 days each, with the exception of every fourth year. The leap year was to have one extra day, bringing its length to 366 days, and thus making the average length of the year in the Julian calendar 365.25 days --> dropped trying to base calendars off of moon and sun

Lunar Phases (figure 4.14)

Sun moves 1/12 its path around the sky each month, but we can assume the Sun' light is constant through a moon's four week cycle Move moves completely around earth in that time how much of moon face we see illuminated depends on the angle the sun makes with the Moon The moon = new when it is in the same general direction in the sky as the Sun ( A)--> moon invisible to us because new mon is the same part of the sky as the sun, it rises at sunrise & sets at sunset Moves 12 degrees in the sky each day (24 times its own diameter) A day or two later = thin crescent first appears, as we begin to see a small part of the Moon's illuminated hemisphere--> reflects a little sunlight towards us along one sides--> increases in size on successive days as Moon moves farther & farther around the sky away from Sun (B)--> moon is moving eastward away from the Sun, it rises later & later each day the Moon is one-quarter of the way around its orbit (position C) and so we say it is at the first quarter phase. Half of the Moon's illuminated side is visible to Earth observers. Because of its eastward motion, the Moon now lags about one-quarter of the day behind the Sun, rising around noon and setting around midnight During the week after the first quarter phase, we see more and more of the Moon's illuminated hemisphere (position D), a phase that is called waxing (or growing) gibbous. Eventually, the Moon arrives at position E in our figure, where it and the Sun are opposite each other in the sky--> full moon Moon rises at sunset and setting at sunrise Highest and most noticeable in midnight More likely to notice or remember b/c of bright celestial light During the two weeks following the full moon, the Moon goes through the same phases again in reverse order returning to new phase after about 29.5 days. About a week after the full moon, for example, the Moon is at third quarter, meaning that it is three-quarters of the way around (not that it is three-quarters illuminated—in fact, half of the visible side of the Moon is again dark). At this phase, the Moon is now rising around midnight and setting around noon. And, since the Moon's orbit is tilted relative to the path of the Sun in the sky, Earth's shadow misses the Moon most months. That's why we regularly get treated to a full moon. The times when Earth's shadow does fall on the Moon are called lunar eclipses

source of many of the high-energy cosmic ray particles?

Supernovae Trapped by the magnetic field of the Galaxy, the particles from exploded stars continue to circulate around the vast spiral of the Milky Way. Scientists speculate that high-speed cosmic rays hitting the genetic material of Earth organisms over billions of years may have contributed to the steady mutations—subtle changes in the genetic code—that drive the evolution of life on our planet.

How the bohr model explains why atoms absorb or emit only specific energies or wavelengths of light.

Suppose a beam of white light (which consists of photons of all visible wavelengths) shines through a gas of atomic hydrogen. A photon of wavelength 656 nanometers has just the right energy to raise an electron in a hydrogen atom from the second to the third orbit. Thus, as all the photons of different energies (or wavelengths or colors) stream by the hydrogen atoms, photons with this particular wavelength can be absorbed by those atoms whose electrons are orbiting on the second level. When they are absorbed, the electrons on the second level will move to the third level, and a number of the photons of this wavelength and energy will be missing from the general stream of white light. only photons with these exact energies can be absorbed. All of the other photons will stream past the atoms untouched. Thus, hydrogen atoms absorb light at only certain wavelengths and produce dark lines at those wavelengths in the spectrum we see.

Direct Detection

Suppose, for example, you were a great distance away and wished to detect reflected light from Earth. Earth intercepts and reflects less than one billionth of the Sun's radiation, so its apparent brightness in visible light is less than one billionth that of the Sun. Compounding the challenge of detecting such a faint speck of light, the planet is swamped by the blaze of radiation from its parent star. Even today, the best telescope mirrors' optics have slight imperfections that prevent the star's light from coming into focus in a completely sharp point.

Measuring the Characteristics of Stars

Surface temperature 1. Determine the color (very rough).2. Measure the spectrum and get the spectral type. Chemical composition Determine which lines are present in the spectrum. Luminosity Measure the apparent brightness and compensate for distance. Radial Velocity Measure the Doppler shift in the spectrum. Mass Measure the period and radial velocity curves of spectroscopic binary stars. Rotation Measure the width of spectral lines. Diameter 1. Measure the way a star's light is blocked by the Moon.2. Measure the light curves and Doppler shifts for eclipsing binary stars.

interstellar grains

Thanks to their small sizes and low temperatures, interstellar grains radiate most of their energy at infrared to microwave frequencies, with wavelengths of tens to hundreds of microns. Earth's atmosphere is opaque to radiation at these wavelengths, so emission by interstellar dust is best measured from space. Observations from above Earth's atmosphere show that dust clouds are present throughout the plane of the Milky Way

why are white dwarf explosions called Ia supernovae?

The "a" subdesignation of type Ia supernovae further refers to the presence of strong silicon absorption lines, which are absent from supernovae originating from the collapse of massive stars. Silicon is one of the products that results from the fusion of carbon and oxygen, which bears out the scenario we described above—that there is a sudden onset of the fusion of the carbon (and oxygen) of which the white dwarf was made.

reddening of the Sun

The Sun appears much redder at sunset than it does at noon. The lower the Sun is in the sky, the longer the path its light must travel through the atmosphere. Over this greater distance, there is a greater chance that sunlight will be scattered. Since red light is less likely to be scattered than blue light, the Sun appears more and more red as it approaches the horizon.

ionization energy

The atom is then said to be ionized. The minimum amount of energy required to remove one electron from an atom in its ground state

why is direct imaging in characterizing exoplanets

The brightness of the planet can be measured at different wavelengths. These observations provide an estimate for the temperature of the planet's atmosphere; in the case of HR 8799 planet 1, the color suggests the presence of thick clouds. Spectra can also be obtained from the faint light to analyze the atmospheric constituents. A spectrum of HR 8799 planet 1 indicates a hydrogen-rich atmosphere, while the closer planet 4 shows evidence for methane in the atmosphere.

Eventually, all the hydrogen in a star's core, where it is hot enough for fusion reactions, is used up. What happens ?

The core then contains only helium, "contaminated" by whatever small percentage of heavier elements the star had to begin with. The helium in the core can be thought of as the accumulated "ash" from the nuclear "burning" of hydrogen during the main-sequence stage. Energy can no longer be generated by hydrogen fusion in the stellar core because the hydrogen is all gone and, as we will see, the fusion of helium requires much higher temperatures. Since the central temperature is not yet high enough to fuse helium, there is no nuclear energy source to supply heat to the central region of the star. The long period of stability now ends, gravity again takes over, and the core begins to contract. Once more, the star's energy is partially supplied by gravitational energy, in the way described by Kelvin and Helmholtz. As the star's core shrinks, the energy of the inward- falling material is converted to heat.

how do the degenerate electrons act?

The degenerate electrons do not require an input of heat to maintain the pressure they exert, and so a star with this kind of structure, if nothing disturbs it, can last essentially forever. (Note that the repulsive force between degenerate electrons is different from, and much stronger than, the normal electrical repulsion between charges that have the same sign.) The electrons in a degenerate gas do move about, as do particles in any gas, but not with a lot of freedom. A particular electron cannot change position or momentum until another electron in an adjacent stage gets out of the way. The situation is much like that in the parking lot after a big football game. Vehicles are closely packed, and a given car cannot move until the one in front of it moves, leaving an empty space to be filled.

why does the core only shrink a little bit at the beginning?

The electrons at first resist being crowded closer together, and so the core shrinks only a small amount. Ultimately, however, the iron core reaches a mass so large that even degenerate electrons can no longer support it. When the density reaches 4 × 1011 g/cm3 (400 billion times the density of water), some electrons are actually squeezed into the atomic nuclei, where they combine with protons to form neutrons and neutrinos. This transformation is not something that is familiar from everyday life, but becomes very important as such a massive star core collapses. Some of the electrons are now gone, so the core can no longer resist the crushing mass of the star's overlying layers. The core begins to shrink rapidly. More and more electrons are now pushed into the atomic nuclei, which ultimately become so saturated with neutrons that they cannot hold onto them.

degenerate gas

The electrons in a degenerate gas resist further crowding with tremendous pressure. (It's as if the electrons said, "You can press inward all you want, but there is simply no room for any other electrons to squeeze in here without violating the rules of our existence.")

what is the problem with eyes as detectors?

The eye also suffers from having a very short integration time; it takes only a fraction of a second to add light energy together before sending the image to the brain. One important advantage of modern detectors is that the light from astronomical objects can be collected by the detector over longer periods of time; this technique is called "taking a long exposure." Exposures of several hours are required to detect very faint objects in the cosmos.

baseline surveyors

The farther away an astronomical object lies, the longer the baseline has to be to give us a reasonable chance of making a measurement. Unfortunately, nearly all astronomical objects are very far away. To measure their distances requires a very large baseline and highly precise angular measurements.

what we see vs actual distance

The farther out in space we look, the longer the light has taken to get here, and the longer ago it left its place of origin. By looking billions of light-years out into space, astronomers are actually seeing billions of years into the past. Strive for telescopes that collect more faint light--> so we can observe fainter objects

how is the structure of molecular clouds maintained

The force of gravity, pulling inward, tries to make a star collapse. Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. When a star is first forming, low temperature (and hence, low pressure) and high density (hence, greater gravitational attraction) both work to give gravity the advantage. In order to form a star—that is, a dense, hot ball of matter capable of starting nuclear reactions deep within—we need a typical core of interstellar atoms and molecules to shrink in radius and increase in density by a factor of nearly 1020. It is the force of gravity that produces this drastic collapse.

metallicity

The fraction of a star's mass that is composed of these elements is referred to as the star's metallicity. The metallicity of the Sun, for example, is 0.02, since 2% of the Sun's mass is made of elements heavier than helium.

what is the limit to which building up elements by fusion can go on.?

The fusion of silicon into iron turns out to be the last step in the sequence of nonexplosive element production. Up to this point, each fusion reaction has produced energy because the nucleus of each fusion product has been a bit more stable than the nuclei that formed it. light nuclei give up some of their binding energy in the process of fusing into more tightly bound, heavier nuclei. It is this released energy that maintains the outward pressure in the core so that the star does not collapse. But of all the nuclei known, iron is the most tightly bound and thus the most stable.

where does the energy of the inward falling material is converted to heat flow?

The heat generated in this way, like all heat, flows outward to where it is a bit cooler. In the process, the heat raises the temperature of a layer of hydrogen that spent the whole long main-sequence time just outside the core. Like an understudy waiting in the wings of a hit Broadway show for a chance at fame and glory, this hydrogen was almost (but not quite) hot enough to undergo fusion and take part in the main action that sustains the star. Now, the additional heat produced by the shrinking core puts this hydrogen "over the limit," and a shell of hydrogen nuclei just outside the core becomes hot enough for hydrogen fusion to begin. New energy produced by fusion of this hydrogen now pours outward from this shell and begins to heat up layers of the star farther out, causing them to expand. Meanwhile, the helium core continues to contract, producing more heat right around it. This leads to more fusion in the shell of fresh hydrogen outside the core (Figure 22.2). The additional fusion produces still more energy, which also flows out into the upper layer of the star.

fullerenes

The largest compounds yet discovered in interstellar space are fullerenes, molecules in which 60 or 70 carbon atoms are arranged in a cage-like configuration

zero-age main sequence

The left-hand edge of the main-sequence band in the H-R diagram We use the term zero-age to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core. The zero-age main sequence is a continuous line in the H-R diagram that shows where stars of different masses but similar chemical composition can be found when they begin to fuse hydrogen.

pulsating variables & doppler effect

The lines in the spectrum shift toward the blue as the surface of the star moves toward us and then shift to the red as the surface shrinks back. As the star pulsates, it also changes its overall color, indicating that its temperature is also varying. And, most important for our purposes, the luminosity of the pulsating variable also changes in a regular way as it expands and contracts.

Figure 23.18 Evolution of a Binary System.

The more massive star evolves first to become a red giant and then a white dwarf. The white dwarf then begins to attract material from its companion, which in turn evolves to become a red giant. Eventually, the white dwarf acquires so much mass that it is pushed over the Chandrasekhar limit and becomes a type Ia supernova.

how much more luminous than the sun the most luminous of bright stars?

The most luminous of the bright stars listed in Appendix J emit more than 50,000 times more energy than does the Sun. These highly luminous stars are missing from the solar neighborhood because they are very rare. None of them happens to be in the tiny volume of space immediately surrounding the Sun, and only this small volume was surveyed to get the data shown in Table 18.1.

why does this make sense

The most massive stars have the most gravity and can thus compress their centers to the greatest degree. This means they are the hottest inside and the best at generating energy from nuclear reactions deep within. As a result, they shine with the greatest luminosity and have the hottest surface temperatures. The stars with lowest mass, in turn, are the coolest inside and least effective in generating energy. Thus, they are the least luminous and wind up being the coolest on the surface. Our Sun lies somewhere in the middle of these extremes (as you can see in Figure 18.14). The characteristics of representative main-sequence stars (excluding brown dwarfs, which are not true stars) are listed in Table 18.3.

air limitations on usefulness of telescopes

The most obvious limitation is weather conditions such as clouds, wind, and rain. At the best sites, the weather is clear as much as 75% of the time. Even on a clear night, the atmosphere filters out a certain amount of starlight, especially in the infrared, where the absorption is due primarily to water vapor. Astronomers therefore prefer dry sites, generally found at high altitudes. The sky above the telescope should be dark. Near cities, the air scatters the glare from lights, producing an illumination that hides the faintest stars and limits the distances that can be probed by telescopes. (Astronomers call this effect light pollution.) Observatories are best located at least 100 miles from the nearest large city. Finally, the air is often unsteady; light passing through this turbulent air is disturbed, resulting in blurred star images. Astronomers call these effects "bad seeing." When seeing is bad, images of celestial objects are distorted by the constant twisting and bending of light rays by turbulent air.

Orion Nebula

The red glow that pervades the great Orion Nebula is produced by the first line in the Balmer series of hydrogen. Hydrogen emission indicates that there are hot young stars nearby that ionize these clouds of gas. When electrons then recombine with protons and move back down into lower energy orbits, emission lines are produced. The blue color seen at the edges of some of the clouds is produced by small particles of dust that scatter the light from the hot stars. Dust can also be seen silhouetted against the glowing gas.

period-luminosity relation

The relation that describes how the luminosity of a Cepheid variable star is related to the period between peaks in its brightness; the longer the period, the more luminous the star.--> discovered by Henrietta Leavitt Leavitt discovered hundreds of variable stars in the Large Magellanic Cloud and Small Magellanic Cloud, two great star systems that are actually neighboring galaxies (although they were not known to be galaxies then). A small fraction of these variables were cepheids

second step of fusion in sun

The second step in forming helium from hydrogen is to add another proton to the deuterium nucleus to create a helium nucleus that contains two protons and one neutron (Figure 16.7). In the process, some mass is again lost and more gamma radiation is emitted. Such a nucleus is helium because an element is defined by its number of protons; any nucleus with two protons is called helium. But this form of helium, which we call helium-3 (and write in shorthand as 3He) is not the isotope we see in the Sun's atmosphere or on Earth. That helium has two neutrons and two protons and hence is called helium-4 (4He). To get to helium-4 in the Sun, helium-3 must combine with another helium-3 in the third step of fusion (illustrated in Figure 16.8). Note that two energetic protons are left over from this step; each of them comes out of the reaction ready to collide with other protons and to start step 1 in the chain of reactions all over again.

detection of a few of those elusive neutrinos created during nuclear fusion

The second technique for obtaining information about the Sun's interior involves the detection of a few of those elusive neutrinos created during nuclear fusion. Recall from our earlier discussion that neutrinos created in the center of the Sun make their way directly out of the Sun and travel to Earth at nearly the speed of light. As far as neutrinos are concerned, the Sun is transparent. About 3% of the total energy generated by nuclear fusion in the Sun is carried away by neutrinos. So many protons react and form neutrinos inside the Sun's core that, scientists calculate, 35 million billion (3.5 × 1016) solar neutrinos pass through each square meter of Earth's surface every second. If we can devise a way to detect even a few of these solar neutrinos, then we can obtain information directly about what is going on in the center of the Sun. Unfortunately for those trying to "catch" some neutrinos, Earth and everything on it are also nearly transparent to passing neutrinos, just like the Sun. On very, very rare occasions, however, one of the billions and billions of solar neutrinos will interact with another atom. The first successful detection of solar neutrinos made use of cleaning fluid (C2Cl4), which is the least expensive way to get a lot of chlorine atoms together. The nucleus of a chlorine (Cl) atom in the cleaning fluid can be turned into a radioactive argon nucleus by an interaction with a neutrino. Because the argon is radioactive, its presence can be detected. However, since the interaction of a neutrino with chlorine happens so rarely, a huge amount of chlorine is needed.

Figure 22.5 Evolutionary Tracks of Stars of Different Masses.

The solid black lines show the predicted evolution from the main sequence through the red giant or supergiant stage on the H-R diagram. Each track is labeled with the mass of the star it is describing. The numbers show how many years each star takes to become a giant. The red line is the zero-age main sequence. While theorists debate the exact number of years shown here, our main point should be clear. The more massive the star, the shorter time it takes for each stage in its life.

Orion's Belt stars

The stars in Orion's belt are typically about 5 million years old, whereas the stars near the middle of the "sword" hanging from Orion's belt are only 300,000 to 1 million years old. The region about halfway down the sword where star formation is still taking place is called the Orion Nebula. About 2200 young stars are found in this region, which is only slightly larger than a dozen light-years in diameter. The Orion Nebula also contains a tight cluster of stars called the Trapezium (Figure 21.5). The brightest Trapezium stars can be seen easily with a small telescope Compare this with our own solar neighborhood, where the typical spacing between stars is about 3 light-years. Only a small number of stars in the Orion cluster can be seen with visible light, but infrared images—which penetrate the dust better—detect the more than 2000 stars that are part of the group

cosmic distance ladder

The succession of methods by which astronomers determine the distances to celestial objects. Parallaxes are the foundation of all stellar distance estimates, spectroscopic methods use nearby stars to calibrate their H-R diagrams, and RR Lyrae and cepheid distance estimates are grounded in H-R diagram distance estimates (and even in a parallax measurement to a nearby cepheid, Delta Cephei). This chain of methods allows astronomers to push the limits when looking for even more distant stars. Recent work, for example, has used RR Lyrae stars to identify dim companion galaxies to our own Milky Way out at distances of 300,000 light-years. The H-R diagram method was recently used to identify the two most distant stars in the Galaxy: red giant stars way out in the halo of the Milky Way with distances of almost 1 million light- years

electrons in a star for most of its life

The temperature in the interior of a star is always so high that the atoms are stripped of virtually all their electrons. For most of a star's life, the density of matter is also relatively low, and the electrons in the star are moving rapidly. This means that no two of them will be in the same place moving in exactly the same way at the same time. But this all changes when a star exhausts its store of nuclear energy and begins its final collapse.

interstellar extinction

The tiny interstellar dust grains absorb some of the starlight they intercept. But at least half of the starlight that interacts with a grain is merely scattered, that is, it is redirected rather than absorbed. Since neither the absorbed nor the scattered starlight reaches us directly, both absorption and scattering make stars look dimmer. The effects of both processes are called interstellar extinction

But there are stars whose core masses are greater than 3 MSun when they exhaust their fuel supplies. What becomes of them

The truly bizarre result of the death of such massive stellar cores (called a black hole)

Exoplanet discoveries through 2015 (figure 21.22)

The vertical axis shows the radius of each planet compared to Earth. Horizontal lines show the size of Earth, Neptune, and Jupiter. The horizontal axis shows the time each planet takes to make one orbit (and is given in Earth days). Recall that Mercury takes 88 days and Earth takes a little more than 365 days to orbit the Sun. The yellow and red dots show planets discovered by transits, and the blue dots are the discoveries by the radial velocity (Doppler) technique.

Most of the volume of the interstellar medium is filled with neutral (nonionized) hydrogen. How do we go about looking for it?

The very hot stars required to produce H II regions are rare, and only a small fraction of interstellar matter is close enough to such hot stars to be ionized by them. neutral hydrogen atoms at temperatures typical of the gas in interstellar space neither emit nor absorb light in the visible part of the spectrum. Nor, for the most part, do the other trace elements that are mixed with the interstellar hydrogen. However, some of these other elements can absorb visible light even at typical interstellar temperatures. This means that when we observe a bright source such as a hot star or a galaxy, we can sometimes see additional lines in its spectrum produced when interstellar gas absorbs light at particular frequencies . Some of the strongest interstellar absorption lines are produced by calcium and sodium, but many other elements can be detected as well in sufficiently sensitive observations

Wien's Law

The wavelength at which maximum power is emitted where the wavelength is in nanometers (one billionth of a meter) and the temperature is in K (the constant 3 x 10^6 has units of nm × K).

Magellanic Clouds

These two "large" and "small" irregular galaxies are named for a 16thC navigator. Visible in the southern hemisphere, they form part of the Milky Way subgroup. are two irregular dwarf galaxies visible in the Southern Celestial Hemisphere; they are members of the Local Group and are orbiting the Milky Way galaxy. All three of these small galaxies are satellites of the Milky Way Galaxy, interacting with it through the force of gravity *****Nearest galaxy is a spiral like our own called Andromeda galaxy (50 galaxies make up Local Group)

Pillars of Dust & Dense Globules in M16

This Hubble Space Telescope image of the central regions of M16 (also known as the Eagle Nebula) shows huge columns of cool gas, (including molecular hydrogen, H2) and dust. These columns are of higher density than the surrounding regions and have resisted evaporation by the ultraviolet radiation from a cluster of hot stars just beyond the upper-right corner of this image. The tallest pillar is about 1 light-year long, and the M16 region is about 7000 light-years away from us.

kepler discoveries graph

This bar graph shows the number of planets of each size range found among the first 2213 Kepler planet discoveries. Sizes range from half the size of Earth to 20 times that of Earth. On the vertical axis, you can see the fraction that each size range makes up of the total. Note that planets that are between 1.4 and 4 times the size of Earth make up the largest fractions, yet this size range is not represented among the planets in our solar system.

Figure 23.4 Evolutionary Track for a Star Like the Sun

This diagram shows the changes in luminosity and surface temperature for a star with a mass like the Sun's as it nears the end of its life. After the star becomes a giant again (point A on the diagram), it will lose more and more mass as its core begins to collapse. The mass loss will expose the hot inner core, which will appear at the center of a planetary nebula. In this stage, the star moves across the diagram to the left as it becomes hotter and hotter during its collapse (point B). At first, the luminosity remains nearly constant, but as the star begins to cool off, it becomes less and less bright (point C). It is now a white dwarf and will continue to cool slowly for billions of years until all of its remaining store of energy is radiated away. (This assumes the Sun will lose between 46-50% of its mass during the giant stages, based upon various theoretical models).

Measurement of Earth by Eratosthenes

This indicated that the Sun was directly over the well—meaning that Syene was on a direct line from the center of Earth to the Sun. At the corresponding time and date in Alexandria, Eratosthenes observed the shadow a column made and saw that the Sun was not directly overhead, but was slightly south of the zenith, so that its rays made an angle with the vertical equal to about 1/50 of a circle (7°). Because the Sun's rays striking the two cities are parallel to one another, why would the two rays not make the same angle with Earth's surface? Shadow of a column--> 7 degrees because of curvature--> Alexandria must be 1/50 Earth's circumference north of Syene --> Alexandria was 5000 stadia north of Syene--> 50 * 5000= 250000 stadia-> accuracy undeterminable because stadia measurement unknown

neutron star

This means the collapsing core can reach a stable state as a crushed ball made mainly of neutrons, which astronomers call a neutron star. We don't have an exact number (a "Chandrasekhar limit") for the maximum mass of a neutron star, but calculations tell us that the upper mass limit of a body made of neutrons might only be about 3 MSun. So if the mass of the core were greater than this, then even neutron degeneracy would not be able to stop the core from collapsing further. The dying star must end up as something even more extremely compressed, which until recently was believed to be only one possible type of object—the state of ultimate compaction known as a black hole (which is the subject of our next chapter). This is because no force was believed to exist that could stop a collapse beyond the neutron star stage.

Figure 22.15 H-R Diagrams for Clusters of Different Ages.

This sketch shows how the turn-off point from the main sequence gets lower as we make H-R diagrams for clusters that are older and older.

Kepler Space Telescope

This spacecraft stared continuously at more than 150,000 stars in a small patch of sky near the constellation of Cygnus—just above the plane of our Milky Way Galaxy (Figure 21.20). Kepler's cameras and ability to measure small changes in brightness very precisely enabled the discovery of thousands of exoplanets, including many multi-planet systems. The spacecraft required three reaction wheels—a type of wheel used to help control slight rotation of the spacecraft—to stabilize the pointing of the telescope and monitor the brightness of the same group of stars over and over again. Kepler was launched with four reaction wheels (one a spare), but by May 2013, two wheels had failed and the telescope could no longer be accurately pointed toward the target area. Kepler had been designed to operate for 4 years, and ironically, the pointing failure occurred exactly 4 years and 1 day after it began observing. However, this failure did not end the mission. The Kepler telescope continued to observe for two more years, looking for short-period transits in different parts of the sky. A new NASA mission called TESS (Transiting Exoplanet Survey Satellite) will carry out a survey all over the sky of the nearer (and therefore brighter) stars, starting in 2018.

Type Ia supernovae

This type of supernova is brighter than supernovae produced by the collapse of a massive star. Thus, type Ia supernovae can be seen at very large distances, and they are found in all types of galaxies. The energy output from most type Ia supernovae is consistent, with little variation in their maximum luminosities, or in how their light output initially increases and then slowly decreases over time. These properties make type Ia supernovae extremely valuable "standard bulbs" for astronomers looking out at great distances—well beyond the limits of our own Galaxy. In contrast, type II supernovae are about 5 times less luminous than type Ia supernovae and are only seen in galaxies that have recent, massive star formation. Type II supernovae are also less consistent in their energy output during the explosion and can have a range a peak luminosity values.

Orion in Visible & Infrared

This wide-angle, infrared view of the same area was taken with the Infrared Astronomical Satellite. Heated dust clouds dominate in this false-color image, and many of the stars that stood out on part (a) are now invisible. An exception is the cool, red-giant star Betelgeuse, which can be seen as a yellowish point at the left vertex of the blue triangle (at Orion's left armpit). The large, yellow ring to the right of Betelgeuse is the remnant of an exploded star. The infrared image lets us see how large and full of cooler material the Orion molecular cloud really is This wide-angle, infrared view of the same area was taken with the Infrared Astronomical Satellite. Heated dust clouds dominate in this false-color image, and many of the stars that stood out on part (a) are now invisible. An exception is the cool, red-giant star Betelgeuse, which can be seen as a yellowish point at the left vertex of the blue triangle (at Orion's left armpit). The large, yellow ring to the right of Betelgeuse is the remnant of an exploded star. The infrared image lets us see how large and full of cooler material the Orion molecular cloud really is

Just how old are the different clusters we have been discussing?

To get their actual ages (in years), we must compare the appearances of our calculated H-R diagrams of different ages to observed H-R diagrams of real clusters. In practice, astronomers use the position at the top of the main sequence (that is, the luminosity at which stars begin to move off the main sequence to become red giants) as a measure of the age of a cluster (the main-sequence turnoff we discussed previously). For example, we can compare the luminosities of the brightest stars that are still on the main sequence in Figure 22.10 and Figure 22.13. Using this method, some associations and open clusters turn out to be as young as 1 million years old, while others are several hundred million years old. Once all of the interstellar matter surrounding a cluster has been used to form stars or has dispersed and moved away from the cluster, star formation ceases, and stars of progressively lower mass move off the main sequence, as shown in Figure 22.10, Figure 22.12, and Figure 22.13. To our surprise, even the youngest of the globular clusters in our Galaxy are found to be older than the oldest open cluster. All of the globular clusters have main sequences that turn off at a luminosity less than that of the Sun. Star formation in these crowded systems ceased billions of years ago, and no new stars are coming on to the main sequence to replace the ones that have turned off (see Figure 22.15).

Blackbody

To understand, in more quantitative detail, the relationship between temperature and electromagnetic radiation,......... A hypothetical object that absorbs all of the radiation that strikes it. The energy that is absorbed causes the atoms and molecules in it to vibrate or move around at increasing speeds. As it gets hotter, this object will radiate electromagnetic waves until absorption and radiation are in balance. We want to discuss such an idealized object because, as you will see, stars behave in very nearly the same way.

stars with different masses vs Sun

Tracks are shown for stars with different masses (from 0.5 to 15 times the mass of our Sun) and with chemical compositions similar to that of the Sun. The red line is the initial or zero-age main sequence. The numbers along the tracks indicate the time, in years, required for each star to reach those points in their evolution after leaving the main sequence. Once again, you can see that the more massive a star is, the more quickly it goes through each stage in its life. most massive star in this diagram has a mass similar to that of Betelgeuse, and so its evolutionary track shows approximately the history of Betelgeuse. The track for a 1-solar-mass star shows that the Sun is still in the main-sequence phase of evolution, since it is only about 4.5 billion years old. It will be billions of years before the Sun begins its own "climb" away from the main sequence—the expansion of its outer layers that will make it a red giant.

Examples of Ia supernovae?

Tycho's Supernova, and Kepler's Supernova For instance, in contrast to the case of SN 1054, which yielded the spinning pulsar in the Crab Nebula, none of these historical supernovae shows any evidence of stellar remnants that have survived their explosions.

how do binary stars affect each other's evolution?

Under the right circumstances, stars can exchange material, especially during the stages when one of them swells up into a giant or supergiant, or has a strong wind. When this happens and the companion stars are sufficiently close, material can flow from one star to another, decreasing the mass of the donor and increasing the mass of the recipient. Such mass transfer can be especially dramatic when the recipient is a stellar remnant such as a white dwarf or a neutron star.

Conclusion from Rutherford a gold foil experiment

Was that nearly all of the mass as well as all of the positive charge in each individual good atom is concentrated in the nucleus - Rutherfords model placed electrons in orbit around this nucleus - his model requires that the electrons be in motion because if stationary they would fall into each other Because electrons and nucleus are extremely small, most of the atom is empty

wavelength and frequency

Waves with longer wavelengths have lower frequencies-->vice versa

Doppler Shifts and Motions of Stars (FIGURE 18.6).

We see changes in velocity because when one star is moving toward Earth, the other is moving away; half a cycle later, the situation is reversed. Doppler shifts cause the spectral lines to move back and forth. In diagrams 1 and 3, lines from both stars can be seen well separated from each other. When the two stars are moving perpendicular to our line of sight (that is, they are not moving either toward or away from us), the two lines are exactly superimposed, and so in diagrams 2 and 4, we see only a single spectral line. Note that in the diagrams, the orbit of the star pair is tipped slightly with respect to the viewer (or if the viewer were looking at it in the sky, the orbit would be tilted with respect to the viewer's line of sight). If the orbit were exactly in the plane of the page or screen (or the sky), then it would look nearly circular, but we would see no change in radial velocity (no part of the motion would be toward us or away from us.) If the orbit were perpendicular to the plane of the page or screen, then the stars would appear to move back and forth in a straight line, and we would see the largest-possible radial velocity variations.

Figure 22.13 H-R Diagram for an Older Cluster

We see the H-R diagram for a hypothetical older cluster at an age of 4.24 billion years. Note that most of the stars on the upper part of the main sequence have turned off toward the red-giant region. And the most massive stars in the cluster have already died and are no longer on the diagram. The oldest clusters of all are the globular clusters. Figure 22.14 shows the H-R diagram of globular cluster 47 Tucanae. Notice that the luminosity and temperature scales are different from those of the other H-R diagrams in this chapter. In Figure 22.13, for example, the luminosity scale on the left side of the diagram goes from 0.1 to 100,000 times the Sun's luminosity. But in Figure 22.14, the luminosity scale has been significantly reduced in extent. So many stars in this old cluster have had time to turn off the main sequence that only the very bottom of the main sequence remains.

Dopler Shift in Stars

We should see all the spectral lines of moving stars shifted toward the red end of the spectrum if the star is moving away from us, or toward the blue (violet) end if it is moving toward us (Figure 17.10). The greater the shift, the faster the star is moving. along the line of sight between the star and the observer, is called radial velocity and is usually measured in kilometers per second.

igure 21.24 Size Distribution of Planets for Stars Similar to the Sun.

We show the average number of planets per star in each planet size range. (The average is less than one because some stars will have zero planets of that size range.) This distribution, corrected for biases in the Kepler data, shows that Earth-size planets may actually be the most common type of exoplanets. (credit: modification of work by NASA/Kepler mission)

The second method for indirect detection of exoplanets: brightness

When the orbital plane of the planet is tilted or inclined so that it is viewed edge-on, we will see the planet cross in front of the star once per orbit, causing the star to dim slightly; this event is known as transit. Figure 21.19 shows a sketch of the transit at three time steps: (1) out of transit, (2) the start of transit, and (3) full transit, along with a sketch of the light curve, which shows the drop in the brightness of the host star. The amount of light blocked—the depth of the transit—depends on the area of the planet (its size) compared to the star. If we can determine the size of the star, the transit method tells us the size of the planet.

It is even possible to learn something about the planet's atmosphere.

When the planet passes in front of HD 209458, the atoms in the planet's atmosphere absorb starlight. Observations of this absorption were first made at the wavelengths of yellow sodium lines and showed that the atmosphere of the planet contains sodium; now, other elements can be measured as well.

Instead of observing the evolution of a single star, we can look at

a group or cluster of stars. We look for a group of stars that is very close together in space, held together by gravity, often moving around a common center. Then it is reasonable to assume that the individual stars in the group all formed at nearly the same time, from the same cloud, and with the same composition. We expect that these stars will differ only in mass. And their masses determine how quickly they go through each stage of their lives. Since stars with higher masses evolve more quickly, we can find clusters in which massive stars have already completed their main-sequence phase of evolution and become red giants, while stars of lower mass in the same cluster are still on the main sequence, or even—if the cluster is very young—undergoing pre-main- sequence gravitational contraction. We can see many stages of stellar evolution among the members of a single cluster, and we can see whether our models can explain why the H-R diagrams of clusters of different ages look the way they do.

why does it mean when saying "energy is emitted when the electron does a flip?"

a hydrogen atom consists of a proton and an electron bound together. Both the proton and the electron act is if they were spinning like tops, and spin axes of the two tops can either be pointed in the same direction (aligned) or in opposite directions (anti-aligned). If the proton and electron were spinning in opposite directions, the atom as a whole would have a very slightly lower energy than if the two spins were aligned If an atom in the lower-energy state (spins opposed) acquired a small amount of energy, then the spins of the proton and electron could be aligned, leaving the atom in a slightly excited state. If the atom then lost that same amount of energy again, it would return to its ground state. The amount of energy involved corresponds to a wave with a wavelength of 21 centimeters; hence, it is known as the 21-centimeter line.

five electrons from their orbits around an oxygen nucleus requires

a lot of energy. Subsequent observations with orbiting X-ray telescopes revealed that the Galaxy is filled with numerous bubbles of X-ray-emitting gas. To emit X-rays, and to contain oxygen atoms that have been ionized five times, gas must be heated to temperatures of a million degrees or more.

eclipse of the moon

a lunar eclipse is visible to everyone who can see the Moon. Because a lunar eclipse can be seen (weather permitting) from the entire night side of Earth, lunar eclipses are observed far more frequently from a given place on Earth than are solar eclipses An eclipse of the Moon is total only if the Moon's path carries it though Earth's umbra. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon. because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses The Moon is opposite the Sun, which means the Moon will be in full phase before the eclipse, making the darkening even more dramatic. As the Moon begins to dip into the shadow, the curved shape of Earth's shadow upon it soon becomes apparent. Moon is red == sunlight that has been bent into Earth's shadow For an eclipse where the Moon goes through the center of Earth's shadow, each partial phase consumes at least 1 hour, and totality can last as long as 1 hour and 40 minutes. Not dangerous to look at On arrival, make sure the scene is saved enter, and then perform a rapid scan of the patient, noting whether any blood or bodily fluids are present period select the proper PPE according to the task you are likely to perform. Typically, globally used for all patient Contacts.

If the neutron star and its companion are positioned the right way,

a significant amount of material can be transferred to the neutron star and can set it spinning faster (as spin energy is also transferred). The radius of the neutron star would also decrease as more mass was added. Astronomers have found pulsars in binary systems that are spinning at a rate of more than 500 times per second! (These are sometimes called millisecond pulsars since the pulses are separated by a few thousandths of a second.) Such a rapid spin could not have come from the birth of the neutron star; it must have been externally caused. (Recall that the Crab Nebula pulsar, one of the youngest pulsars known, was spinning "only" 30 times per second.) Indeed, some of the fast pulsars are observed to be part of binary systems, while others may be alone only because they have "fully consumed" their former partner stars through the mass transfer process. (These have sometimes been called " black widow pulsars.")

Cluster m41

a) Cluster M41 is older than NGC 2264 (see Figure 22.10) and contains several red giants. Some of its more massive stars are no longer close to the zero-age main sequence (red line). (b) This ground-based photograph shows the open cluster M41. Note that it contains several orange-color stars. These are stars that have exhausted hydrogen in their centers, and have swelled up to become red giants a) Cluster M41 is older than NGC 2264 (see Figure 22.10) and contains several red giants. Some of its more massive stars are no longer close to the zero-age main sequence (red line). (b) This ground-based photograph shows the open cluster M41. Note that it contains several orange-color stars. These are stars that have exhausted hydrogen in their centers, and have swelled up to become red giants.

The collapse that takes place when electrons are...

absorbed into the nuclei is very rapid. In less than a second, a core with a mass of about 1 MSun, which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers. The speed with which material falls inward reaches one-fourth the speed of light. The collapse halts only when the density of the core exceeds the density of an atomic nucleus (which is the densest form of matter we know). A typical neutron star is so compressed that to duplicate its density, we would have to squeeze all the people in the world into a single sugar cube! This would give us one sugar cube's worth (one cubic centimeter's worth) of a neutron star. The neutron degenerate core strongly resists further compression, abruptly halting the collapse. The shock of the sudden jolt initiates a shock wave that starts to propagate outward. However, this shock alone is not enough to create a star explosion. The energy produced by the outflowing matter is quickly absorbed by atomic nuclei in the dense, overlying layers of gas, where it breaks up the nuclei into individual neutrons and protons.

Raymond Davis Jr.

aced a tank containing nearly 400,000 liters of cleaning fluid 1.5 kilometers beneath Earth's surface in a gold mine at Lead, South Dakota. A mine was chosen so that the surrounding material of Earth would keep cosmic rays (high- energy particles from space) from reaching the cleaning fluid and creating false signals. (Cosmic-ray particles are stopped by thick layers of Earth, but neutrinos find them of no significance.) Calculations show that solar neutrinos should produce about one atom of radioactive argon in the tank each day. hey counted argon atoms about once per month—and remember, they were looking for a tiny handful of argon atoms in a massive tank of chlorine atoms. When all was said and done, Davis' experiment, begun in 1970, detected only about one-third as many neutrinos as predicted by solar models! This was a shocking result because astronomers thought they had a pretty good understanding of both neutrinos and the Sun's interior. For many years, astronomers and physicists wrestled with Davis' results, trying to find a way out of the dilemma of the "missing" neutrinos.

what does the downward-flowing cool material acts as a

acts as a kind of plug that block the upward flow of hot material, which is then diverted sideways and eventually reaches the solar surface in the region around the sunspot. This outward flow of hot material accounts for the paradox that we described in The Sun: A Garden-Variety Star—namely, that the Sun emits slightly more energy when more of its surface is covered by cool sunspots.

doppler effect: if the light source is moving

all light waves travel at the speed of light this means that motion cannot affect the speed, but only the wavelength and the frequency. As the wavelength decreases, the frequency must increase. If the waves are shorter, more will be able to move by during each second

Once a star has reached the main-sequence stage of its life, it derives its energy...

almost entirely from the conversion of hydrogen to helium via the process of nuclear fusion in its core Since hydrogen is the most abundant element in stars, this process can maintain the star's equilibrium for a long time. Thus, all stars remain on the main sequence for most of their lives. Some astronomers like to call the main-sequence phase the star's "prolonged adolescence" or "adulthood"

geo-centrism

almost everyone believed that the Earth was the center of the universe until the European Renaissance Philosophical and religious systems that taught the unique of human beings as the central focus of the cosmos

Tides are caused by

an actual flow of water over Earth's surface toward the two regions below and opposite the Moon, causing the water to pile up to greater depths at those places The rotation of Earth would carry an observer at any given place alternately into regions of deeper and shallower water. An observer being carried toward the regions under or opposite the Moon, where the water was deepest, would say, "The tide is coming in"; when carried away from those regions, the observer would say, "The tide is going out." During a day, the observer would be carried through two tidal bulges (one on each side of Earth) and so would experience two high tides and two low tides. The actual tides we experiences are a combo of the large effect of the Moon & the smaller effect of the Sun When sun & moon are lines up = new or full moon = tides produces reinforce each other and are greater than normal

The protostar and disk at this stage are embedded in

an envelope of dust and gas from which material is still falling onto the protostar. This dusty envelope blocks visible light, but infrared radiation can get through. As a result, in this phase of its evolution, the protostar itself is emitting infrared radiation and so is observable only in the infrared region of the spectrum. Once almost all of the available material has been accreted and the central protostar has reached nearly its final mass, it is given a special name: it is called a T Tauri star, named after one of the best studied and brightest members of this class of stars, which was discovered in the constellation of Taurus. (Astronomers have a tendency to name types of stars after the first example they discover or come to understand. It's not an elegant system, but it works.) Only stars with masses less than or similar to the mass of the Sun become T Tauri stars. Massive stars do not go through this stage,

Proxima Centauri

an example of the most common type of star, and our most common type of stellar neighbor (as we saw in Stars: A Celestial Census.) Low-mass red M dwarfs make up about 70% of all stars and dominate the census of stars within 10 parsecs (33 light-years) of the Sun. if you wanted to see an M dwarf with your naked eye, you would be out of luck. These stars only produce a fraction of the Sun's light, and nearly all of them require a telescope to be detected.

before the light reaches the detector, astronomers use some type of

an instrument to sort the light according to wavelength. The instrument may be as simple as colored filters, which transmit light within a specified range of wavelength OR the instrument between telescope and detector may be one of several devices that spread the light out into its full rainbow of colors so that astronomers can measure individual lines in the spectrum SPECTROMETER. WHETHER FILTER OR SPECTROMETER still have to use detectors to record and measure the properties of light. EX: A red transparent plastic is an everyday example of a filter that transmits only the red light and blocks the other colors. After the light passes through a filter, it forms an image that astronomers can then use to measure the apparent brightness and color of objects.

Borexino experiment

an international experiment conducted in Italy, detected neutrinos coming from the Sun that were identified as coming from different reactions. Whereas the p-p chain is the reaction producing most of the Sun's energy, it is not the only nuclear reaction occurring in the Sun's core. There are side reactions involving nuclei of such elements as beryllium and boron. By probing the number of neutrinos that come from each reaction, the Borexino experiment has helped us confirm in detail our understanding of nuclear fusion in the Sun. In 2014, the Borexino experiment also identified neutrinos that were produced by the first step in the p-p chain, confirming the models of solar astronomers.

Parallax

apparent shift in the directions of an object as a result of the motion of the observer-> shift in direction of a stars due to Earth's orbital motion = stellar parallax Brighter stars did not seems to shift as Greeks presumed--> either earth was not moving or stars were so far away that shift was miniscule--> retreated to Geo-centrism

retrograde motion

apparent westward motion of a planet as Earth swings between it and the Sun (more difficult to explains back when Earth was considered unmoving and Greek only believed in circular celestial motion--> Ptolemy had to work with this!) Planets move eastward as they orbit the Sun (B--> D) as earth passes planets in our example, appears to drift backwards, moving west in the sky Even though its moving East, faster moving Earth has overtaken it and seems to be leaving it behind As Earth rounds to E, the planet again takes up its apparent eastward motion in the sky

emission spectrum

appears as a pattern or series of bright lines; it consists of light in which only certain discrete wavelengths are present.

Infared

are absorbed by water and carbon dioxide molecules, which are more concentrated low in Earth's atmosphere. infrared astronomy is best done from high mountaintops, high-flying airplanes, and spacecraft. he nerve endings in our skin are sensitive to this band of the electromagnetic spectrum.

energy levels of an ionized atom

are entirely different from those of the same atom when it is neutral. Each time an electron is removed from the atom, the energy levels of the ion, and thus the wavelengths of the spectral lines it can produce, change. Ionized hydrogen, having no electron, can produce no absorption lines.

Open clusters

are found in the disk of the Galaxy. They have a range of ages, some as old as, or even older than, our Sun. The youngest open clusters are still associated with the interstellar matter from which they formed. Open clusters are smaller than globular clusters, usually having diameters of less than 30 light-years, and they typically contain only several dozen to several hundreds of stars (Figure 22.7). The stars in open clusters usually appear well separated from one another, even in the central regions, which explains why they are called "open." Our Galaxy contains thousands of open clusters, but we can see only a small fraction of them. Interstellar dust, which is also concentrated in the disk, dims the light of more distant clusters so much that they are undetectable

area of aperture

area of circle A = pi * r^2 or .25*pi*d^2

stellar convection

as currents of hot gas flow up and down through the star (Figure 16.12). Such currents travel at moderate speeds and do not upset the overall stability of the star. They don't even result in a net transfer of mass either inward or outward because, as hot material rises, cool material falls and replaces it. This results in a convective circulation of rising and falling cells as seen in Figure 16.12. In much the same way, heat from a fireplace can stir up air currents in a room, some rising and some falling, without driving any air into or out the room. Convection currents carry heat very efficiently outward through a star. In the Sun, convection turns out to be important in the central regions and near the surface.

electrons moving energy level: why is it easier to see electromagnetic radiation as photons

as electrons move from one level to another, they give off or absorb little packets of energy. When an electron moves to a higher level, it absorbs a photon of just the right energy (provided one is available). When it moves to a lower level, it emits a photon with the exact amount of energy it no longer needs in its "lower-cost living situation." E=hf h = 6.626 *10^-34 Js Higher-energy photons correspond to higher-frequency waves (which have a shorter wavelength); lower-energy photons are waves of lower frequency.

how did ptolemy solve retrograde motion

assuming a stationary Earth. Ptolemy solved this problem by having each planet revolve in a small orbit called an epicycle which revolved around each in deferent Because earth was considered unmoving Ptolemy had to introduce inform circular movement around another axis called the equant point ( model accepted in Muslim world and later in Christian Europe)

in order to address the mystery of the absent companion stars...

astronomers have recently begun to investigate alternative mechanisms of generating type Ia supernovae. All proposed mechanisms rely upon white dwarfs composed of carbon and oxygen, which are needed to meet the observed absence of hydrogen in the type Ia spectrum. And because any isolated white dwarf below the Chandrasekhar mass is stable, all proposed mechanisms invoke a binary companion to explode the white dwarf. The leading alternative mechanism scientists believe creates a type Ia supernova is the merger of two white dwarf stars in a binary system. The two white dwarfs may have unstable orbits, such that over time, they would slowly move closer together until they merge. If their combined mass is greater than the Chandrasekhar limit, the result could also be a type Ia supernova explosion

how to detect using infrared spectrum?

astronomers must protect the infrared detector from nearby radiation, just as you would shield photographic film from bright daylight. Since anything warm radiates infrared energy, the detector must be isolated in very cold surroundings; often, it is held near absolute zero (1 to 3 K) by immersing it in liquid helium. The second step is to reduce the radiation emitted by the telescope structure and optics, and to block this heat from reaching the infrared detector.

The Atacama Large Millimeter/submillimeter array (ALMA)

at an altitude of 16,400 feet, consists of 12 7-meter and 54 12-meter telescopes, and can achieve baselines up to 16 kilometers. Since it became operational in 2013, it has made observations at resolutions down to 6 milliarcseconds (0.006 arcseconds), a remarkable achievement for radio astronomy.

Williamina Fleming devised a system to classify stars

based on the strength of hydrogen absorption lines. Spectra with the strongest lines were classified as "A" stars, the next strongest "B," and so on down the alphabet to "O" stars, in which the hydrogen lines were very weak. But we saw above that hydrogen lines alone are not a good indicator for classifying stars, since their lines disappear from the visible light spectrum when the stars get too hot or too cold.

Gustav Kirchhoff

became the first person to use spectroscopy to identify an element in the Sun when he found the spectral signature of sodium gas. In the years that followed, astronomers found many other chemical elements in the Sun and stars. In fact, the element helium was found first in the Sun from its spectrum and only later identified on Earth.

why do blackbodies emit radiation (photons) of all wavelength(all colors) because...

because in any solid or denser gas, some molecules or atoms vibrate or move between collisions slower than average and some move faster than average. So when we look at the electromagnetic waves emitted, we find a broad range, or spectrum, of energies and wavelengths. More energy is emitted at the average vibration or motion rate (the highest part of each curve), but if we have a large number of atoms or molecules, some energy will be detected at each wavelength.

why is difficult to plot an H-R diagram

because most stars are so faint that we cannot see those outside our immediate neighborhood. The stars plotted in Figure 18.14 were selected because their distances are known. This sample omits many intrinsically faint stars that are nearby but have not had their distances measured, so it shows fewer faint main- sequence stars than a "fair" diagram would.

Refraction

bending of light passing through air or water that allow us to see little over the horizon the Sun appears to rise earlier and to set later than it would if no atmosphere were present the atmosphere scatters light and provides some twilight illumination even when the Sun is below the horizon. Astronomers define morning twilight as beginning when the Sun is 18° below the horizon, and evening twilight extends until the Sun sinks more than 18° below the horizon Leadesr to small correction in many of our statements about the seasons. At equinoxes sun above the horizon for a little longer than 12 hours and & below for a little less than 12 Most dramatic at the poles--> more than a week before it reaches the equator Warmer and colder are a little after the times when we get the most/least sunlight because it takes time for Earth to warm up

Leavitt: brighter-appearing cepheids

brighter-appearing cepheids always have the longer periods of light variation. Thus, she reasoned, the period must be related to the luminosity of the stars. When Leavitt did this work, the distance to the Magellanic Clouds was not known, so she was only able to show that luminosity was related to period. She could not determine exactly what the relationship is. To define the period-luminosity relation with actual numbers (to calibrate it), astronomers first had to measure the actual distances to a few nearby cepheids in another way. (This was accomplished by finding cepheids associated in clusters with other stars whose distances could be estimated from their spectra, as discussed in the next section of this chapter.) But once the relation was thus defined, it could give us the distance to any cepheid, wherever it might be located

how can we calculates the energy these reactions generate?

by calculating the difference in the initial and final masses. he masses of hydrogen and helium atoms in the units normally used by scientists are 1.007825 u and 4.00268 u, respectively. (The unit of mass, u, is defined to be 1/12 the mass of an atom of carbon, or approximately the mass of a proton.) Here, we include the mass of the entire atom, not just the nucleus, because electrons are involved as well. When hydrogen is converted into helium, two positrons are created (remember, the first step happens twice), and these are annihilated with two free electrons, adding to the energy produced. 4 × 1.007825 = 4.03130 u (mass of initial hydrogen atoms) − 4.00268 u (mass of final helium atoms) = 0.02862 u (mass lost in the transformation) The mass lost, 0.02862 u, is 0.71% of the mass of the initial hydrogen. Thus, if 1 kilogram of hydrogen is converted into helium, then the mass of the helium is only 0.9929 kilograms, and 0.0071 kilograms of material is converted into energy. The speed of light (c) is 3 × 108 meters per second, so the energy released by the conversion of just 1 kilogram of hydrogen into helium is: E=mc2 E = .0071 kg * ( 3* 10^8 m/s)^2 = 6.4 * 10^14 J

When nuclear reactions stop, the core of a massive star is supported.....

by degenerate electrons, just as a white dwarf is. For stars that begin their evolution with masses of at least 10 MSun, this core is likely made mainly of iron. (For stars with initial masses in the range 8 to 10 MSun, the core is likely made of oxygen, neon, and magnesium, because the star never gets hot enough to form elements as heavy as iron. The exact composition of the cores of stars in this mass range is very difficult to determine because of the complex physical characteristics in the cores, particularly at the very high densities and temperatures involved.)

how can Gamma-ray detections be made from earths' surface?

by using the atmosphere as the primary detector. When a gamma ray hits our atmosphere, it accelerates charged particles (mostly electrons) in the atmosphere. Those energetic particles hit other particles in the atmosphere and give off their own radiation. The effect is a cascade of light and energy that can be detected on the ground. The VERITAS array in Arizona and the H.E.S.S. array in Namibia are two such ground-based gamma-ray observatories.

The first evidence for absorption by interstellar clouds

came from the analysis of a spectroscopic binary star While most of the lines in the spectrum of this binary shifted alternately from longer to shorter wavelengths and back again, as we would expect from the Doppler effect for stars in orbit around each other, a few lines in the spectrum remained fixed in wavelength. . The lines were also peculiar in that they were much, much narrower than the rest of the lines, indicating that the gas producing them was at a very low pressure. Subsequent work demonstrated that these lines were not formed in the star's atmosphere at all, but rather in a cold cloud of gas located between Earth and the binary star.

why do we need bigger and bigger telescopes?

celestial objects—such as planets, stars, and galaxies—send much more light to Earth than any human eye (with its tiny opening) can catch, and bigger telescopes can detect fainter objects.

pulsating variables

cepheid and RR Lyrae variables, both of which are pulsating variable stars. Such a star actually changes its diameter with time—periodically expanding and contracting, as your chest does when you breathe.

the collapse of a star

collapse is the final event in the life of the core. Because the star's mass is relatively low, it cannot push its core temperature high enough to begin another round of fusion (in the same way larger-mass stars can). The core continues to shrink until it reaches a density equal to nearly a million times the density of water! At this extreme density, a new and different way for matter to behave kicks in and helps the star achieve a final state of equilibrium. In the process, what remains of the star becomes one of the strange white dwarfs

Neutral hydrogen atoms can acquire small amounts of energy through...

collisions with other hydrogen atoms or with free electrons. Such collisions are extremely rare in the sparse gases of interstellar space. An individual atom may wait centuries before such an encounter aligns the spins of its proton and electron. Nevertheless, over many millions of years, a significant fraction of the hydrogen atoms are excited by a collision. (Out there in cold space, that's about as much excitement as an atom typically experiences.)

interferometer array

combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers

molecular cloud structure

complex filamentary structure, similar to cirrus clouds in Earth's atmosphere, but much less dense. The molecular cloud filaments can be up to 1000 light-years long. Within the clouds are cold, dense regions with typical masses of 50 to 500 times the mass of the Sun; we give these regions the highly technical name clumps. Within these clumps, there are even denser, smaller regions called cores. The cores are the embryos of stars. The conditions in these cores—low temperature and high density—are just what is required to make stars.

To understand how a planet can move its host star

consider a single Jupiter-like planet. Both the planet and the star actually revolve about their common center of mass. Remember that gravity is a mutual attraction. The star and the planet each exert a force on the other, and we can find a stable point, the center of mass, between them about which both objects move. The smaller the mass of a body in such a system, the larger its orbit. A massive star barely swings around the center of mass, while a low-mass planet makes a much larger "tour."

absoption spectrum

consists of a series or pattern of dark lines—missing colors—superimposed upon the continuous spectrum of a source In contrast, absorption spectra occur when passing white light through a cool, thin gas. The temperature and other conditions determine whether the lines are bright or dark (whether light is absorbed or emitted), but the wavelengths of the lines for any element are the same in either case. It is the precise pattern of wavelengths that makes the signature of each element unique

what happens after iron ?

content being iron; it requires payment (must absorb energy) to change its stable nuclear structure. This is the exact opposite of what has happened in each nuclear reaction so far: instead of providing energy to balance the inward pull of gravity, any nuclear reactions involving iron would remove some energy from the core of the star. Unable to generate energy, the star now faces catastrophe.

selection effect

contrast between stars that are close to us and those that can be seen with the unaided eye When a population of objects (stars in this example) includes a great variety of different types, we must be careful what conclusions we draw from an examination of any particular subgroup (this one is heavily weighted to most luminous stars) t requires much more effort to assemble a complete data set for the nearest stars, since most are so faint that they can be observed only with a telescope. However, it is only by doing so that astronomers are able to know about the properties of the vast majority of the stars, which are actually much smaller and fainter than our own SUn

Convection

currents of warm material rise, carrying their energy with them to cooler layers. A good example is hot air rising from a fireplace.

asterism

denote an especially noticeable star pattern within a constellation i.e.. Big dipper is an asterism in ursa major

interstellar gas temperature

depending on where it is located, can be as cold as a few degrees above absolute zero or as hot as a million degrees or more.

the rate at which ions and electrons recombine....

depends on their relative speeds/temp also depends on density, he higher the density, the greater the chance for recapture, because the different kinds of particles are crowded more closely together.

what does the resolution of the interferometer depend on?

depends upon the separation of the telescopes, not upon their individual apertures. Two telescopes separated by 1 kilometer provide the same resolution as would a single dish 1 kilometer across (although they are not, of course, able to collect as much radiation as a radio-wave bucket that is 1 kilometer across).

Fermi-Gamma Ray Space Telescope

designed to measure cosmic gamma rays at energies greater than any previous telescope, and thus able to collect radiation from some of the most energetic events in the universe.

Low Mass Brown Dwarfs vs High Mass Planets

deuterium fusion. Although brown dwarfs do not sustain regular (proton-proton) hydrogen fusion, they are capable of fusing deuterium (a rare form of hydrogen with one proton and one neutron in its nucleus). The fusion of deuterium can happen at a lower temperature than the fusion of hydrogen. If an object has enough mass to fuse deuterium (about 13 MJ or 0.012 MSun), it is a brown dwarf. Objects with less than 13 MJ do not fuse deuterium and are usually considered planets.

apertures

diameter of the opening through which light travels or reflects bigger apertures = greater light capturing power

Color Index

difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B-V) magnitudes

Phases

different appearances with the Moon starting dark and getting more and more illuminated by sunlight over the course of two weeks --> moon disks fade becoming dark again two weeks later

for visible light, we perceive different wavelengths as....

different colors: red, for example, is the longest visible wavelength, and violet is the shortest.

spectral signatures

different substances showed distinctive spectral signatures by which their presence could be detected unique pattern of colors for each type of atom (its spectrum) can help us identify which element or elements are in a gas.

Edward C. Pickering: MIZAR

discovered a second class of binary stars in 1889—a class in which only one of the stars is actually seen directly. He was examining the spectrum of Mizar and found that the dark absorption lines in the brighter star's spectrum were usually double. Not only were there two lines where astronomers normally saw only one, but the spacing of the lines was constantly changing. At times, the lines even became single. Pickering correctly deduced that the brighter component of Mizar, called Mizar A, is itself really two stars that revolve about each other in a period of 104 days.

First Binary stars

discovered in 1650, less than half a century after Galileo began to observe the sky with a telescope. John Baptiste Riccioli (1598-1671), an Italian astronomer, noted that the star Mizar, in the middle of the Big Dipper's handle, appeared through his telescope as two stars.

double stars

discovered in 1650, less than half a century after Galileo began to observe the sky with a telescope. John Baptiste Riccioli (1598-1671), an Italian astronomer, noted that the star Mizar, in the middle of the Big Dipper's handle, appeared through his telescope as two stars.

JJ Thomson

discovered the electron—> flow of this causes electricity

Light year

distance light travels during one year (because light always travels at the same speed, and because its speed turns out to be the fastest possible in the universe)-------> 9.46* 10^12 km = 1 light year Info about the universe comes to us exclusively through various forms or light and all light travels at the speed of light- 1 light-year every year Information about a star "now" is when light reaches us despite the star having moved away a 100 years ago

Henry Norris Russell

embarked on a lifelong quest to ascertain the physical conditions inside stars from the clues in their spectra

X-ray, and gamma-ray astronomy measure

energy per area per second rather than magnitudes to express the results of their measurements.

Sudhury experiment agrees with astronomer agrees

experiment detected about 1 neutrino per hour and has shown that the total number of neutrinos reaching the heavy water is just what solar models predict. Only one-third of these, however, are electron neutrinos. It appears that two-thirds of the electron neutrinos produced by the Sun transform themselves into one of the other types of neutrinos as they make their way from the core of the Sun to Earth. This is why the earlier experiments saw only one-third the number of neutrinos expected. Although it is not intuitively obvious, such neutrino oscillations can happen only if the mass of the electron neutrino is not zero. Other experiments indicate that its mass is tiny (even compared to the electron). The 2015 Nobel Prize in physics was awarded to researchers Takaaki Kajita and Arthur B. McDonald for their work establishing the changeable nature of neutrinos. (Raymond Davis shared the 2002 Nobel Prize with Japan's Masatoshi Koshiba for the experiments that led to our understanding of the neutrino problem in the first place.) But the fact that the neutrino has mass at all has deep implications for both physics and astronomy. For example, we will look at the role that neutrinos play in the inventory of the mass of the universe

spectra vs temp

figure 17.5 in the hottest O stars (those with temperatures over 28,000 K), only lines of ionized helium and highly ionized atoms of other elements are conspicuous. Hydrogen lines are strongest in A stars with atmospheric temperatures of about 10,000 K. Ionized metals provide the most conspicuous lines in stars with temperatures from 6000 to 7500 K (spectral type F). In the coolest M stars (below 3500 K), absorption bands of titanium oxide and other molecules are very strong. By the way, the spectral class assigned to the Sun is G2

spectra of stars with different spectral classes

figure 17.6 The strongest four lines seen at spectral type A1 (one in the red, one in the blue-green, and two in the blue) are Balmer lines of hydrogen. Note how these lines weaken at both higher and lower temperatures, as Figure 17.5 also indicates. The strong pair of closely spaced lines in the yellow in the cool stars is due to neutral sodium (one of the neutral metals in Figure 17.5).

Barnards 68

figure 20.9 his object, first catalogued by E. E. Barnard, is a dark interstellar cloud. Its striking appearance is due to the fact that, since it is relatively close to Earth, there are no bright stars between us and it, and its dust obscures the light from the stars behind it. barnard 68 is an example of a relatively dense cloud or dark nebula containing tiny, solid dust grains. Such opaque clouds are conspicuous on any photograph of the Milky Way, the galaxy in which the Sun is located

One of the primary objectives of the Kepler mission was to

find out how many stars hosted planets and especially to estimate the frequency of earthlike planets. Although Kepler looked at only a very tiny fraction of the stars in the Galaxy, the sample size was large enough to draw some interesting conclusions. While the observations apply only to the stars observed by Kepler, those stars are reasonably representative, and so astronomers can extrapolate to the entire Galaxy.

first step in the process of creating stars is

formation of dense cores within a clump of gas and dust It is generally thought that all the material for the star comes from the core, the larger structure surrounding the forming star. Eventually, the gravitational force of the infalling gas becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores. The material then undergoes a rapid collapse, and the density of the core increases greatly as a result. During the time a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostatr

amount of energy a photon has depends on its....

frequency--> high freq high energy

elementary Particles

fundamentals of atom = proton, neutron, electron or each kind of particle, there is a corresponding but opposite antiparticle. If the particle carries a charge, its antiparticle has the opposite charge. anti electron = positron

While no energy is being generated within the white dwarf core of the star....

fusion still occurs in the shells that surround the core. As the shells finish their fusion reactions and stop producing energy, the ashes of the last reaction fall onto the white dwarf core, increasing its mass. As Figure 23.2 shows, a higher mass means a smaller core. The core can contract because even a degenerate gas is still mostly empty space. Electrons and atomic nuclei are, after all, extremely small. The electrons and nuclei in a stellar core may be crowded compared to the air in your room, but there is still lots of space between them.

In this section, you were introduced to some very dense objects. How would those objects' gravity affect you?

g = (G × M)/ R^2 G= 6.67 * 10 ^-11 m^2/kgs^2 g on EARTH = 9.8 m/s^2

About 99 percent of the material betwen the stars is a form of

gas—that is, it consists of individual atoms or molecules. The most abundant elements in this gas are hydrogen and helium (which we saw are also the most abundant elements in the stars), but the gas also includes other elements. Some of the gas is in the form of molecules—combinations of atoms.

Ptolemy's greatest contribution

geometric representation of the solar system that predicted the positions of the planes for any desired date and time ( Hipparchus observational material + Ptolemy cosmological model) The complicating factor in explaining the motions of the planets is that their apparent wandering in the sky results from the combination of their own motions with Earth's orbital revolution.

dark lines in the solar system

give evidence of certain chemical elements between us and the Sun absorbing those wavelengths of sunlight. Because the space between us and the Sun is pretty empty, astronomers realized that the atoms doing the absorbing must be in a thin atmosphere of cooler gas around the Sun. This outer atmosphere is not all that different from the rest of the Sun, just thinner and cooler. Thus, we can use what we learn about its composition as an indicator of what the whole Sun is made of. use the presence of absorption and emission lines to analyze the composition of other stars and clouds of gas in space.

age of the universe globular cluster

globular clusters are the oldest structures in our Galaxy (and in other galaxies as well). The youngest have ages of about 11 billion years and some appear to be even older. Since these are the oldest objects we know of, this estimate is one of the best limits we have on the age of the universe itself—it must be at least 11 billion years old.

how do close spaced systems like this planets interact...

gravitationally with each other. The result is that the observed transits occur a few minutes earlier or later than would be predicted from simple orbits. These gravitational interactions have allowed the Kepler scientists to calculate masses for the planets, providing another way to learn about exoplanets.

ion

greater amounts of energy must be absorbed by the now-ionized atom (called an ion) to remove an additional electron deeper in the structure of the atom. Successively greater energies are needed to remove the third, fourth, fifth—and so on—electrons from the atom. If enough energy is available, an atom can become completely ionized, losing all of its electrons.

Helioseismology

has shown that convection extends inward from the surface 30% of the way toward the center; we have used this information in drawing Figure 16.15. Pulsation measurements also show that the differential rotation that we see at the Sun's surface, with the fastest rotation occurring at the equator, persists down through the convection zone. Below the convection zone, however, the Sun, even though it is gaseous throughout, rotates as if it were a solid body like a bowling ball. Another finding from helioseismology is that the abundance of helium inside the Sun, except in the center where nuclear reactions have converted hydrogen into helium, is about the same as at its surface. That result is important to astronomers because it means we are correct when we use the abundance of the elements measured in the solar atmosphere to construct models of the solar interior. Helioseismology also allows scientists to look beneath a sunspot and see how it works. In The Sun: A Garden- Variety Star, we said that sunspots are cool because strong magnetic fields block the outward flow of energy. Figure 16.18 shows how gas moves around underneath a sunspot. Cool material from the sunspot flows downward, and material surrounding the sunspot is pulled inward, carrying magnetic field with it and thus maintaining the strong field that is necessary to form a sunspot. As the new material enters the sunspot region, it too cools, becomes denser, and sinks, thus setting up a self-perpetuating cycle that can last for weeks.

giant molecular clouds

have densities of hundreds to thousands of atoms per cm3, much denser than interstellar space is on average. As a result, though they account for a very small fraction of the volume of interstellar space, they contain a significant fraction—20-30%—of the total mass of the Milky Way's gas. Because of their high density, molecular clouds block ultraviolet starlight, the main agent for heating most interstellar gas. As a result, they tend to be extremely cold, with typical temperatures near 10 K (−263 °C). Giant molecular clouds are also the sites where new stars form.

why is the discovery of complex molecules surprising?

he discovery of complex molecules in space came as a surprise because most of interstellar space is filled with ultraviolet light from stars, and this light is capable of dissociating molecules (breaking them apart into individual atoms). In retrospect, however, the presence of molecules is not surprising.

brown dwarf spectral class

he hottest brown dwarfs are given types L0-L9 (temperatures in the range 2400-1300 K), whereas still cooler (1300-700 K) objects are given types T0-T9

doppler effect: when the light source is at rest

he light waves spread out evenly in all directions, like the ripples from a splash in a pond. The crests are separated by a distance, λ, where λ is the wavelength. The observer, who happens to be located in the direction of the bottom of the image, sees the light waves coming nice and evenly, one wavelength apart. Observers located anywhere else would see the same thing.

how is the sun resisted collapse for so long?

he mutual gravitational attraction between the masses of various regions within the Sun produces tremendous forces that tend to collapse the Sun toward its center. Yet we know from the history of Earth that the Sun has been emitting roughly the same amount of energy for billions of years, so clearly it has managed to resist collapse for a very long time. The gravitational forces must therefore be counterbalanced by some other force. That force is due to the pressure of gases within the Sun (Figure 16.11). Calculations show that, in order to exert enough pressure to prevent the Sun from collapsing due to the force of gravity, the gases at its center must be maintained at a temperature of 15 million K. Think about what this tells us. Just from the fact that the Sun is not contracting, we can conclude that its temperature must indeed be high enough at the center for protons to undergo fusion.

What did Bohr suggest?

he suggested that the spectrum of hydrogen can be understood if we assume that orbits of only certain sizes are possible for the electron. Bohr further assumed that as long as the electron moves in only one of these allowed orbits, it radiates no energy: its energy would change only if it moved from one orbit to another.--> became the foundation of quantum mechanics

how are objects 1/100 mass of sun planets?

hey may radiate energy produced by the radioactive elements that they contain, and they may also radiate heat generated by slowly compressing under their own weight (a process called gravitational contraction). However, their interiors will never reach temperatures high enough for any nuclear reactions, to take place. Jupiter, whose mass is about 1/1000 the mass of the Sun, is unquestionably a planet, for example.

Figure 19.8 H-R Diagram of Stars Measured by Gaia and Hipparcos

his plot includes 16,631 stars for which the parallaxes have an accuracy of 10% or better. The colors indicate the numbers of stars at each point of the diagram, with red corresponding to the largest number and blue to the lowest. Luminosity is plotted along the vertical axis, with luminosity increasing upward. An infrared color is plotted as a proxy for temperature, with temperature decreasing to the right. Most of the data points are distributed along the diagonal running from the top left corner (high luminosity, high temperature) to the bottom right (low temperature, low luminosity). These are main sequence stars. The large clump of data points above the main sequence on the right side of the diagram is composed of red giant stars

why does the length of the time that photons require to reach the surface depends on ?

how far a photon on average travels between collisions, and the travel time depends on what model of the complicated solar interior we accept. Estimates are somewhat uncertain but indicate that the emission of energy from the surface of the Sun can lag its production in the interior by 100,000 years to as much as 1,000,000 years.

If all stars were the same luminosity—if they were like standard bulbs with the same light output—we could use the difference in their apparent brightnesses to tell us something we very much want to know:

how far away they are. if all the stars had the same luminosity, we could immediately infer that the brightest-appearing stars were close by and the dimmest-appearing ones were far away.

Why isnt this true: if earth orbited sun we would observe the parallax of the nearer stars against the background of more distant objects as we viewed the sky from different parts of Earth's orbit

how truly distant the stars were and how small the change in their positions therefore was, even with the entire orbit of Earth as a baseline. The problem was that they did not have tools to measure parallax shifts too small to be seen with the human eye. By the eighteenth century, when there was no longer serious doubt about Earth's revolution, it became clear that the stars must be extremely distant. Astronomers equipped with telescopes began to devise instruments capable of measuring the tiny shifts of nearby stars relative to the background of more distant (and thus unshifting) celestial objects.

A planet will transit its star only

if Earth lies in the plane of the planet's orbit. If the planets in other systems do not have orbits in the same plane, we are unlikely to see multiple transiting objects. Also, as we have noted before, Kepler was sensitive only to planets with orbital periods less than about 4 years. What we expect from Kepler data, then, is evidence of coplanar planetary systems confined to what would be the realm of the terrestrial planets in our solar system.

dust grain scarcity

if all the interstellar gas within the Galaxy were spread out smoothly, there would be only about one atom of gas per cm3 in interstellar space. The dust grains are even scarcer. A km3 of space would contain only a few hundred to a few thousand tiny grains, each typically less than one ten-thousandth of a millimeter in diameter. These numbers are just averages, however, because the gas and dust are distributed in a patchy and irregular way, much as water vapor in Earth's atmosphere is often concentrated into clouds.

The Bohr Model

if the electron moves from one orbit to another closer to the atomic nucleus, it must give up some energy in the form of electromagnetic radiation. If the electron goes from an inner orbit to one farther from the nucleus, however, it requires some additional energy. One way to obtain the necessary energy is to absorb electromagnetic radiation that may be streaming past the atom from an outside source.

hotter objects radiate more power at all wavelengths in a mathematical form

if we sum up the contributions from all parts of the electromagnetic spectrum, we obtain the total energy emitted by a blackbody. What we usually measure from a large object like a star is the energy flux, the power emitted per square meter. It turns out that the energy flux from a blackbody at temperature T is proportional to the fourth power of its absolute temperature--> Stefan Boltzmann Laws

measuring proper motion

in arcseconds (1/3600 of a degree) per year. That is, the measurement of proper motion tells us only by how much of an angle a star has changed its position on the celestial sphere. If two stars at different distances are moving at the same velocity perpendicular to our line of sight, the closer one will show a larger shift in its position on the celestial sphere in a year's time.

the sun is stable

in equilibrium = neither expanding nor contracting All the forces within it are balanced, so that at each point within the star, the temperature, pressure, density, and so on are maintained at constant values.

The best observatory sites are those that are

in high, dark dry places far from habitation Andes Mountains of Chile, the desert peaks of Arizona, the Canary Islands in the Atlantic Ocean, and Mauna Kea in Hawaii, a dormant volcano with an altitude of 13,700 feet (4200 meters).

the final meter

in terms of the velocity of light. Light in a vacuum can travel a distance of one meter in 1/299,792,458.6 second. Today, therefore, light travel time provides our basic unit of length. Put another way, a distance of one light-second (the amount of space light covers in one second) is defined to be 299,792,458.6 meters. That's almost 300 million meters that light covers in just one second; light really is very fast! we have defined the meter as a small fraction of the light-second

what deviates from H-R diagram rule

in the upper-right region, where stars have low temperature and high luminosity. How can a star be at once cool, meaning each square meter on the star does not put out all that much energy, and yet very luminous? The only way is for the star to be enormous—to have so many square meters on its surface that the total energy output is still large. These stars must be giants or supergiants, the stars of huge diameter we discussed earlier. There are also some stars in the lower-left corner of the diagram, which have high temperature and low luminosity. If they have high surface temperatures, each square meter on that star puts out a lot of energy. How then can the overall star be dim? It must be that it has a very small total surface area; such stars are known as white dwarfs (white because, at these high temperatures, the colors of the electromagnetic radiation that they emit blend together to make them look bluish-white). We will say more about these puzzling objects in a moment.

how the color index works

index of 0 : a star with a surface temperature of about 10,000 K, such as Vega. The B-V color indexes of stars range from −0.4 for the bluest stars, with temperatures of about 40,000 K, to +2.0 for the reddest stars, with temperatures of about 2000 K. The B-V index for the Sun is about +0.65. the B-V index is always the "bluer" minus the "redder" color.

interstellar material

interstellar material is so extremely sparse = best vacuum Dust in space builds up over light-years can block light of more distant stars --> some can penetrate the smog Unidentifiable galactical matter = dark matter Some stars are born in clusters and influence each other-->> stars explode and material is recycled into Galaxy as "star dust."

why is the discovery of complex molecules NOT surprising?

interstellar space also contains significant amounts of dust capable of blocking out starlight. When this dust accumulates in a single location, the result is a dark cloud where ultraviolet starlight is blocked and molecules can survive. The largest of these structures are created where gravity pulls interstellar gas together to form giant molecular clouds, structures as massive as a million times the mass of the Sun. Within these, most of the interstellar hydrogen has formed the molecule H2 (molecular hydrogen). Other, more complex molecules are also present in much smaller quantities.

polaris : cepheid variable

is a cepheid variable that, for a long time, varied by one tenth of a magnitude, or by about 10% in visual luminosity, in a period of just under 4 days. Recent measurements indicate that the amount by which the brightness of Polaris changes is decreasing and that, sometime in the future, this star will no longer be a pulsating variable. This is just one more piece of evidence that stars really do evolve and change in fundamental ways as they age, and that being a cepheid variable represents a stage in the life of the star.

association

is a group of extremely young stars, typically containing 5 to 50 hot, bright O and B stars scattered over a region of space some 100-500 light-years in diameter. As an example, most of the stars in the constellation Orion form one of the nearest stellar associations. Associations also contain hundreds to thousands of low-mass stars, but these are much fainter and less conspicuous. The presence of really hot, luminous stars indicates that star formation in the association has occurred in the last million years or so. Since O stars go through their entire lives in only about a million years, they would not still be around unless star formation has occurred recently. It is therefore not surprising that associations are found in regions rich in the gas and dust required to form new stars. It's like a brand new building still surrounded by some of the construction materials used to build it and with the landscape still showing signs of construction. On the other hand, because associations, like ordinary open clusters, lie in regions occupied by dusty interstellar matter, many are hidden from our view.

Interference

is a technical term for the way that multiple waves interact with each other when they arrive in our instruments, and this interaction allows us to coax more detail out of our observations.

lens

is a transparent piece of material that bends the rays of light passing through it. If the light rays are parallel as they enter, the lens brings them together in one place to form an image (Figure 6.4). If the curvatures of the lens surfaces are just right, all parallel rays of light (say, from a star) are bent, or refracted, in such a way that they converge toward a point, called the focus of the lens.

total mass of the gas and dust in Milky Way Galaxy

is equal to about 15% of the mass contained in stars. This means that the mass of the interstellar matter in our Galaxy amounts to about 10 billion times the mass of the Sun. There is plenty of raw material in the Galaxy to make generations of new stars and planets

Suppose the planet is like Jupiter and has a mass about one-thousandth that of its star; in this case, the size of the star's orbit

is one-thousandth the size of the planet's

the velocity of one of the oscillating regions on the Sun.....

is only a few hundred meters per second, and it takes about 5 minutes to complete a full cycle from maximum to minimum velocity and back again. The change in the size of the Sun measured at any given point is no more than a few kilometers.

motion at the microscopic level....

is responsible for much of the electromagnetic radiation on Earth and in the universe. As atoms and molecules move about and collide, or vibrate in place, their electrons give off electromagnetic radiation. The characteristics of this radiation are determined by the temperature of those atoms and molecules. In a hot material, for example, the individual particles vibrate in place or move rapidly from collisions, so the emitted waves are, on average, more energetic. And recall that higher energy waves have a higher frequency.

problems with refractor telescopes

is that the light must pass through the lens of a refractor. That means the glass must be perfect all the way through, and it has proven very difficult to make large pieces of glass without flaws and bubbles in them optical properties of transparent materials change a little bit with the wavelengths (or colors) of light, so there is some additional distortion= chromatic aberration since the light must pass through the lens, the lens can only be supported around its edges (just like the frames of our eyeglasses). The force of gravity will cause a large lens to sag and distort the path of the light rays as they pass through it. because the light passes through it, both sides of the lens must be manufactured to precisely the right shape in order to produce a sharp image.

light year

is the distance that light (the fastest signal we know) travels in 1 year. Since light covers an astounding 300,000 kilometers per second, and since there are a lot of seconds in 1 year, a light-year is a very large quantity: 9.5 trillion (9.5 × 1012) kilometers to be exact.

The interval between successive transits

is the length of the year for that planet, which can be used (again using Kepler's laws) to find its distance from the star. Larger planets like Jupiter block out more starlight than small earthlike planets, making transits by giant planets easier to detect, even from ground-based observatories. But by going into space, above the distorting effects of Earth's atmosphere, the transit technique has been extended to exoplanets as small as Mars.

The strongest line in the visible region of the hydrogen spectrum

is the red line in the Balmer series this emission line accounts for the characteristic red glow

Radar

is the technique of transmitting radio waves to an object in our solar system and then detecting the radio radiation that the object reflects back. we can measure time with precision and because we know the speed at which radio waves travel (the speed of light), we can determine the distance to the object or a particular feature on its surface (such as a mountain)

Another way to overcome the blurring effect of Earth's atmosphere

is to observe from space. Infrared may be the optimal wavelength range in which to observe because planets get brighter in the infrared while stars like our Sun get fainter, thereby making it easier to detect a planet against the glare of its star. Special optical techniques can be used to suppress the light from the central star and make it easier to see the planet itself. However, even if we go into space, it will be difficult to obtain images of Earth-size planets.

prism refracting light

isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow The bending of the beam depends on the wavelength of the light as well as the properties of the material, and as a result, different wavelengths (or colors of light) are bent by different amounts and therefore follow slightly different paths through the prism. The violet light is bent more than the red.--> dispersion

If a white dwarf accumulates matter from a companion star at a much faster rate,

it can be pushed over the Chandrasekhar limit. The evolution of such a binary system is shown in Figure 23.18. When its mass approaches the Chandrasekhar mass limit (exceeds 1.4 MSun), such an object can no longer support itself as a white dwarf, and it begins to contract. As it does so, it heats up, and new nuclear reactions can begin in the degenerate core. The star "simmers" for the next century or so, building up internal temperature. This simmering phase ends in less than a second, when an enormous amount of fusion (especially of carbon) takes place all at once, resulting in an explosion. The fusion energy produced during the final explosion is so great that it completely destroys the white dwarf. Gases are blown out into space at velocities of about 10,000 kilometers per second, and afterward, no trace of the white dwarf remains.

what is the correct way to discuss binary systems ?

it is not correct to describe the motion of a binary star system by saying that one star orbits the other. Gravity is a mutual attraction. Each star exerts a gravitational force on the other, with the result that both stars orbit a point between them called the center of mass. Imagine that the two stars are seated at either end of a seesaw. The point at which the fulcrum would have to be located in order for the seesaw to balance is the center of mass, and it is always closer to the more massive star

cepheid variables

large, yellow, pulsating stars named for the first-known star of the group, Delta Cephei (a whole class of stars is named after the constellation in which the first one happened to be found.) The star rises rather rapidly to maximum light and then falls more slowly to minimum light, taking a total of 5.4 days for one cycle. Most cepheids have periods in the range of 3 to 50 days and luminosities that are about 1000 to 10,000 times greater than that of the Sun. Their variations in luminosity range from a few percent to a factor of 10.

Celestial equator

lies between celestial poles Motion of the celestial sphere depends on your latitude (positions north/south of the equator) Earth's axis is pointing at the celestial poles, so these two points in the sky do not appear to turn Only half of the sky north of the celestial equator is ever visible to an observer at the North Pole. Similarly, an observer at the South Pole would see only the southern half of the sky It appears above the northern horizon at an angular height, or altitude, equal to the observer's latitude. In San Francisco, for example, where the latitude is 38° N, the north celestial pole is 38° above the northern horizon &he south celestial pole is 38° below the southern horizon and, thus, never visible

how can excited atom lose excess energy?

lose its excess energy either by colliding with another particle or by giving off a radio wave with a wavelength of 21 centimeters. If there are no collisions, an excited hydrogen atom will wait an average of about 10 million years before emitting a photon and returning to its state of lowest energy. Even though the probability that any single atom will emit a photon is low, there are so many hydrogen atoms in a typical gas cloud that collectively they will produce an observable line at 21 centimeters.

Magnitude equation

m1-m2 = -2.5log(b1/b2) b2/b1 = 2.5 ^( m1-m2) 2.5 vs 100^.2

Very Long Baseline Array (VLBA)

made up of 10 individual telescopes stretching from the Virgin Islands to Hawaii (Figure 6.21). The VLBA, completed in 1993, can form astronomical images with a resolution of 0.0001 arcseconds, permitting features as small as 10 astronomical units (AU) to be distinguished at the center of our Galaxy.

Kepler Discoveries include:

many rocky, Earth-size planets, far more than Jupiter- size gas planets. This immediately tells us that the initial Doppler discovery of many hot Jupiters was a biased sample, in effect, finding the odd planetary systems because they were the easiest to detect. However, there is one huge difference between this observed size distribution and that of planets in our solar system. The most common planets have radii between 1.4 and 2.8 that of Earth, sizes for which we have no examples in the solar system. These have been nicknamed super-Earths, while the other large group with sizes between 2.8 and 4 that of Earth are often called mini-Neptunes.

color thermometer for stars

many stars give off most of their energy in visible light, the color of light that dominates a star's appearance is a rough indicator of its temperature. If one star looks red and another looks blue, which one has the higher temperature? Because blue is the shorter-wavelength color, it is the sign of a hotter star.

Measurements of the widths of spectral lines show that

many stars rotate faster than the Sun, some with periods of less than a day! These rapid rotators spin so fast that their shapes are "flattened" into what we call oblate spheroids. An example of this is the star Vega, which rotates once every 12.5 hours. Vega's rotation flattens its shape so much that its diameter at the equator is 23% wider than its diameter at the poles The Sun, with its rotation period of about a month, rotates rather slowly. Studies have shown that stars decrease their rotational speed as they age. Young stars rotate very quickly, with rotational periods of days or less. Very old stars can have rotation periods of several months.

A Star in Crisis

mass like the Sun's just after it had climbed up to the red-giant region of the H-R diagram for a second time and had shed some of its outer layers to form a planetary nebula. Recall that during this time, the core of the star was undergoing an "energy crisis." Earlier in its life, during a brief stable period, helium in the core had gotten hot enough to fuse into carbon (and oxygen). But after this helium was exhausted, the star's core had once more found itself without a source of pressure to balance gravity and so had begun to contract.

T Tauri stars

may actually be stars in a middle stage between protostars and hydrogen-fusing stars such as the Sun. High-resolution infrared images have revealed jets of material as well as stellar winds coming from some T Tauri stars, proof of interaction with their environment.

how do more precisely measure star temperature

measuring how much energy a star gives off at each wavelength and by constructing diagrams like (pic) The location of the peak (or maximum) in the power curve of each star can tell us its temperature. The average temperature at the surface of the Sun, which is where the radiation that we see is emitted, turns out to be 5800 K.

Newton's Third Law

momentum is conserved in an isolated system--> change in momentum is system is balanced by another change Forces occur in pairs and are equal and opposite in nature Things fall toward earth, but the acceleration of our planet is far too small to be measured

So the star becomes simultaneously

more luminous and cooler. On the H-R diagram, the star therefore leaves the main-sequence band and moves upward (brighter) and to the right (cooler surface temperature). Over time, massive stars become red supergiants, and lower-mass stars like the Sun become red giants. (We first discussed such giant stars in The Stars: A Celestial Census; here we see how such "swollen" stars originate.) You might also say that these stars have "split personalities": their cores are contracting while their outer layers are expanding. (Note that red giant stars do not actually look deep red; their colors are more like orange or orange-red.)

stellar diameters

most nearby stars are roughly the size of the Sun, with typical diameters of a million kilometers or so. Faint stars, as we might have expected, are generally smaller than more luminous stars. However, there are some dramatic exceptions to this simple generalization. A few of the very luminous stars, those that are also red (indicating relatively low surface temperatures), turn out to be truly enormous. These stars are called, appropriately enough, giant stars or supergiant stars. An example is Betelgeuse, the second brightest star in the constellation of Orion and one of the dozen brightest stars in our sky. Its diameter, remarkably, is greater than 10 AU (1.5 billion kilometers!), large enough to fill the entire inner solar system almost as far out as Jupiter

what happens at the temperature inside the stars with masses smaller than about 1.2 times the mass of our Sun?

most of the energy is produced by the reactions we have just described, and this set of reactions is called the proton-proton chain (or sometimes, the p-p chain). In the proton-proton chain, protons collide directly with other protons to form helium nuclei.

what happens when the ultraviolet protons reach the sun's surface

most of the photons have given up enough energy to be ordinary light—and they are the sunlight we see coming from our star. (To be precise, each gamma-ray photon is ultimately converted into many separate lower-energy photons of sunlight.) So, the sunlight given off by the Sun today had its origin as a gamma ray produced by nuclear reactions deep in the Sun's core.

composition of complex molecules

mostly combinations of hydrogen, oxygen, carbon, nitrogen, and sulfur atoms. Many of these molecules are organic (those that contain carbon and are associated with the carbon chemistry of life on Earth.) They include formaldehyde (used to preserve living tissues), alcohol, and antifreeze. astronomers discovered acetic acid (the prime ingredient of vinegar) in a cloud lying in the direction of the constellation of Sagittarius. To balance the sour with the sweet, a simple sugar (glycolaldehyde) has also been found.

individual grains

must be just slightly smaller than the wavelength of visible light. If the grains were a lot smaller, they would not block the light efficiently, as Figure 20.13 and other images in this chapter show that it does. On the other hand, if the dust grains were much larger than the wavelength of light, then starlight would not be reddened. Things that are much larger than the wavelength of light would block both blue and red light with equal efficiency. In this way we can deduce that a characteristic interstellar dust grain contains 106 to 109 atoms and has a diameter of 10-8 to 10-7 meters (10 to 100 nanometers)

Stefan-Boltzmann law for the relationship between energy radiated and temperature

n this method, the energy flux (energy emitted per second per square meter by a blackbody, like the Sun) is given by F = σT ^4 where σ is a constant and T is the temperature. The surface area of a sphere (like a star) is given by A = 4πR^2 The luminosity (L) of a star is then given by its surface area in square meters times the energy flux: L = (A × F)

each time an electron and a proton in the star's core merge to make a neutron, merger releases....

neutrino. These ghostly subatomic particles, introduced in The Sun: A Nuclear Powerhouse, carry away some of the nuclear energy. It is their presence that launches the final disastrous explosion of the star. The total energy contained in the neutrinos is huge. In the initial second of the star's explosion, the power carried by the neutrinos (1046 watts) is greater than the power put out by all the stars in over a billion galaxies. While neutrinos ordinarily do not interact very much with ordinary matter (we earlier accused them of being downright antisocial), matter near the center of a collapsing star is so dense that the neutrinos do interact with it to some degree. They deposit some of this energy in the layers of the star just outside the core. This huge, sudden input of energy reverses the infall of these layers and drives them explosively outward. Most of the mass of the star (apart from that which went into the neutron star in the core) is then ejected outward into space. As we saw earlier, such an explosion requires a star of at least 8 MSun, and the neutron star can have a mass of at most 3 MSun. Consequently, at least five times the mass of our Sun is ejected into space in each such explosive event!

gamma rays

no longer than 0.01 nanometer--> shortest wavelength--> carry a lot of energy Gamma radiation is generated deep in the interior of stars, as well as by some of the most violent phenomena in the universe, such as the deaths of stars and the merging of stellar corpses. **Gamma rays coming to Earth are absorbed by our atmosphere before they reach the ground; thus, they can only be studied using instruments in space.

where are the nearest supernova

no massive stars that promise to become supernovae within 50 light- years of the Sun. (This is in part because the kinds of massive stars that become supernovae are overall quite rare.) The massive star closest to us, Spica (in the constellation of Virgo), is about 260 light-years away, probably a safe distance, even if it were to explode as a supernova in the near future.

Pauli exclusion principle

no two electrons can be in the same place at the same time doing the same thing. We specify the place of an electron by its position in space, and we specify what it is doing by its motion and the way it is spinning.

Latitude

north-south location is the number of degrees of arc you are away from the equator along the meridian Measured north or south of equator (0 to 90 degrees)--> latitude of equator = 0 degrees and north pole = 90 degrees

what happens in stars hotter than the sun?

nother set of reactions, called the carbon-nitrogen-oxygen (CNO) cycle, accomplishes the same net result. In the CNO cycle, carbon and hydrogen nuclei collide to initiate a series of reactions that form nitrogen, oxygen, and ultimately, helium. The nitrogen and oxygen nuclei do not survive but interact to form carbon again. Therefore, the outcome is the same as in the proton-proton chain: four hydrogen atoms disappear, and in their place, a single helium atom is created. The CNO cycle plays only a minor role in the Sun but is the main source of energy for stars with masses greater than about the mass of the Sun.

objects

objects even cooler than M9-type stars. We use the word object because many of the new discoveries are not true stars. A star is defined as an object that during some part of its lifetime derives 100% of its energy from the same process that makes the Sun shine—the fusion of hydrogen nuclei (protons) into helium. Objects with masses less than about 7.5% of the mass of our Sun (about 0.075 MSun) do not become hot enough for hydrogen fusion to take place.

diameter of stars blocked by the Moon

observe the dimming of light that occurs when the Moon passes in front of a star. What astronomers measure (with great precision) is the time required for the star's brightness to drop to zero as the edge of the Moon moves across the star's disk. Since we know how rapidly the Moon moves in its orbit around Earth, it is possible to calculate the angular diameter of the star. If the distance to the star is also known, we can calculate its diameter in kilometers. This method works only for fairly bright stars that happen to lie along the zodiac, where the Moon (or, much more rarely, a planet) can pass in front of them as seen from Earth.

Most exoplanet detections are made using techniques where we

observe the effect that the planet exerts on the host star. For example, the gravitational tug of an unseen planet will cause a small wobble in the host star. Or, if its orbit is properly aligned, a planet will periodically cross in front of the star, causing the brightness of the star to dim.

most common planet sizes

of are those with radii from 1 to 3 times that of Earth—what we have called "Earths" and "super-Earths." Each group occurs in about one-third to one-quarter of stars. In other words, if we group these sizes together, we can conclude there is nearly one such planet per star! And remember, this census includes primarily planets with orbital periods less than 2 years. We do not yet know how many undiscovered planets might exist at larger distances from their star.

How many years a star remains in the main-sequence band depends

on its mass. You might think that a more massive star, having more fuel, would last longer, but it's not that simple. The lifetime of a star in a particular stage of evolution depends on how much nuclear fuel it has and on how quickly it uses up that fuel. (In the same way, how long people can keep spending money depends not only on how much money they have but also on how quickly they spend it. This is why many lottery winners who go on spending sprees quickly wind up poor again.) In the case of stars, more massive ones use up their fuel much more quickly than stars of low mass.

Constellation

one of the 88 sectors into which we divide the sky--> imaginary lines separate the sky between constellations Ancient civilization groups stars into familiar geometric patterns to make it resemble something that they knew Some stars are not a part of a distinctive pattern at all --> formal organization system --> constellation

supernova explosions occurrences

one supernova explodes roughly every 100 years somewhere in the Galaxy. On average, shocks launched by supernovae sweep through any given point in the Galaxy about once every few million years. These shocks keep some interstellar space filled with gas at temperatures of millions of degrees, and they continually disturb the colder gas, keeping it in constant, turbulent motion.

which stars appear brightest in the sky?

only six of the 20 stars that appear brightest in our sky— Sirius, Vega, Altair, Alpha Centauri, Fomalhaut, and Procyon—are found within 26 light-years of the Sun

Magellanic Clouds

opportunity to study the behavior of variable stars independent of their distance. For all practical purposes, the Magellanic Clouds are so far away that astronomers can assume that all the stars in them are at roughly the same distance from us. If all the variable stars in the Magellanic Clouds are at roughly the same distance, then any difference in their apparent brightnesses must be caused by differences in their intrinsic luminosities.

What is helioseismology important tool for?

or predicting solar storms that might impact Earth. Active regions can appear and grow large in only a few days. The solar rotation period is about 28 days. Therefore, regions capable of producing solar flares and coronal mass ejections can develop on the far side of the Sun, where, for a long time, we couldn't see them directly. Fortunately, we now have space telescopes monitoring the Sun from all angles, so we know if there are sunspots forming on the opposite side of the Sun. Moreover, sound waves travel slightly faster in regions of high magnetic field, and waves generated in active regions traverse the Sun about 6 seconds faster than waves generated in quiet regions. By detecting this subtle difference, scientists can provide warnings of a week or more to operators of electric utilities and satellites about when a potentially dangerous active region might rotate into view. With this warning, it is possible to plan for disruptions, put key instruments into safe mode, or reschedule spacewalks in order to protect astronauts.

Light as a wave and a particle

our common sense says that waves and particles are opposite concepts. On one hand, a wave is a repeating disturbance that, by its very nature, is not in only one place, but spreads out. A particle, on the other hand, is something that can be in only one place at any given time.

what would happen if the outer layer of Sun starts falling inward

outer layer - gas made pf individual atoms, all moving about in random directions--> if layer falls inward, atoms move fasting b/c of falling motion --> if outside fall in = contracts moving atoms closer together = collisions more likely = extra speed of falling motion transferred to other atoms --> increases the speed of atoms -->The temperature of a gas is a measure of the kinetic energy (motion) of the atoms within it; hence, the temperature of this layer of the Sun increases. Collisions also excite electrons within the atoms to higher-energy orbits. When these electrons return to their normal orbits, they emit photons, which can then escape from the Sun (

why is the problem with measure even the nearest stars?

parallax angles are usually only a fraction of a second of arc. Recall that one second of arc (arcsec) is an angle of only 1/3600 of a degree. A coin the size of a US quarter would appear to have a diameter of 1 arcsecond if you were viewing it from a distance of about 5 kilometers (3 miles). Think about how small an angle that is.

Sidereal month

period of its revolution about Earth measure with respect to the star is little over 27 days The time interval in which the phases repeat (full to full) is the solar month(little over 29.5 days) The difference results from Earth's motion around the Sun. The Moon must make more than a complete turnaround the moving Earth to get back to the same phase with respect to the Sun. the Moon changes its position on the celestial sphere rather rapidly: even during a single evening, the Moon creeps visibly eastward among the stars, traveling its own width in a little less than 1 hour. The delay in moonrise from one day to the next caused by this eastward motion averages about 50 minutes The Moon rotates on its axis in exactly the same time that it takes to revolve about Earth. As a consequence, the Moon always keeps the same face turned toward Earth --> synchronous rotation

photographic & electronic Detectors

photographic film or glass plates served as the prime astronomical detectors, whether for photographing spectra or direct images of celestial objects.

light as a photon

physicists had to accept that sometimes light behaves more like a "particle"—or at least a self-contained packet of energy—than a wave. We call such a packet of electromagnetic energy a photon.

evaporating gas globules

pillars shows some very dense globules, many of which harbor embryonic stars. It is possible that because these EGGs are exposed to the relentless action of the radiation from nearby hot stars, some may not yet have collected enough material to form a star

why do visible-light and infrared telescopes on Earth's surface not produce images as sharp as the theory of light said they should?

planet's atmosphere is turbulent contains many small-scale blobs or cells of gas that range in size from inches to several feet. Each cell has a slightly different temperature from its neighbor, and each cell acts like a lens, bending (refracting) the path of the light by a small amount. This bending slightly changes the position where each light ray finally reaches the detector in a telescope. The cells of air are in motion, constantly being blown through the light path of the telescope by winds, often in different directions at different altitudes. As a result, the path followed by the light is constantly changing.

what are the solid particles made of

primarily composed of elements that are abundant in the universe (and in interstellar matter). After hydrogen and helium, the most abundant elements are oxygen, carbon, and nitrogen. These three elements, along with magnesium, silicon, iron—and perhaps hydrogen itself—turn out to be the most important components of interstellar dust. Many of the dust particles can be characterized as sootlike (rich in carbon) or sandlike (containing silicon and oxygen). Grains of interstellar dust are found in meteorites and can be identified because the abundances of certain isotopes are different from what we see in other solar system material. + graphite & diamonds

how exactly does the sun tap into energy of nuclei through fusion?

process takes four hydrogen nuclei and fuses them together to form a single helium nucleus. The helium nucleus is slightly less massive than the four hydrogen nuclei that combine to form it, and that mass is converted into energy.

how is an astronomical radio receiver like a spectrometer on visible/infrared light

providing information about how much radiation we receive at each wavelength or frequency. After computer processing, the radio signals are recorded on magnetic disks for further analysis.

spectroscopy

providing information about the composition, temperature, motion, and other characteristics of celestial objects.

radio waves

radar waves, which are used in radar guns by traffic officers to determine vehicle speeds, AM radio waves, which were the first to be developed for broadcasting. (1m to 100s of meters) With such a wide range of wavelengths, not all radio waves interact with Earth's atmosphere in the same way. FM and TV waves are not absorbed and can travel easily through our atmosphere. AM radio waves are absorbed or reflected by a layer in Earth's atmosphere called the ionosphere

what does the supernova explosion lead to

recycled by space produces a flood of energetic neutrons that barrel through the expanding material. These neutrons can be absorbed by iron and other nuclei where they can turn into protons. Thus, they can build up elements that are more massive than iron, possibly including such terrestrial favorites as gold, silver and uranium. Supernovae (and, as we will shortly see, the explosive mergers of neutron stars) are the only candidates we have for places where such heavier atoms can be made. Next time you wear some gold jewelry (or give some to your sweetheart), bear in mind that those gold atoms were forged long ago in these kinds of celestial explosions! When supernovae explode, these elements (as well as the ones the star made during more stable times) are ejected into the existing gas between the stars and mixed with it. Thus, supernovae play a crucial role in enriching their galaxy with heavier elements, allowing, among other things, the chemical elements that make up earthlike planets and the building blocks of life to become more common as time goes on

Radiation Laws

refer 5.8 This graph shows in arbitrary units how many photons are given off at each wavelength for objects at four different temperatures. The wavelengths corresponding to visible light are shown by the colored bands. Note that at hotter temperatures, more energy (in the form of photons) is emitted at all wavelengths. The higher the temperature, the shorter the wavelength at which the peak amount of energy is radiated (this is known as Wien's law).

why produced these remarkable temperatures?

remarkable temperatures is the explosion of massive stars at the ends of their lives (Figure 20.7). Such explosions, called supernovae, will be discussed in detail in the chapter on The Death of Stars. For now, we'll just say that some stars, nearing the ends of their lives, become unstable and literally explode. These explosions launch gas into interstellar space at velocities of tens of thousands of kilometers per second (up to about 30% the speed of light). When this ejected gas collides with interstellar gas, it produces shocks that heat the gas to millions or tens of millions of degrees.

Tides

results from the gravitations forces exerted by the Moon at several points on Earth Forces differ slightly from one another= all parts are not equally distant from the moon or in the same direction the differences among the forces of the Moon's attraction on different parts of Earth (called differential forces) cause Earth to distort slightly. Side closest is most attracted that side opposite moon = differential forces tend to stretch Earth into prolate spheroid (a football shape) , long diameter pointed towards the moon Measurements of the actual deformation of Earth show that the solid Earth does distort, but only about one-third as much as water would, because of the greater rigidity of Earth's interior. Because the tidal distortion of the solid Earth amounts—at its greatest—to only about 20 centimeters, Earth does not distort enough to balance the Moon's differential forces with its own gravity. Hence, objects at Earth's surface experience tiny horizontal tugs, tending to make them slide about. These tide-raising forces are too insignificant to affect solid objects like astronomy students or rocks in Earth's crust, but they do affect the waters in the ocean

most widely accepted model pictures the interstellar grains

rocky cores that are either like soot (rich in carbon) or like sand (rich in silicates). In the dark clouds where molecules can form, these cores are covered by icy mantles (Figure 20.16). The most common ices in the grains are water (H2O), methane (CH4), and ammonia (NH3)—all built out of atoms that are especially abundant in the realm of the stars. The ice mantles, in turn, are sites for some of the chemical reactions that produce complex organic molecules. FIGURE 20.16

Sidereal

rotation period of Earth with respect to other stars (astronomers also use this) A solar day is slightly longer than a sidereal day because Earth not only turns but also moves along its path around the Sun in a day When Earth has completed one rotation with respect to the distant star and is at day 2, the long arrow again points to the same distant star. However, notice that because of the movement of Earth along its orbit from day 1 to 2, the Sun has not yet reached a position above the observer. To complete a solar day, Earth must rotate an additional amount, equal to 1/365 of a full turn. The time required for this extra rotation is 1/365 of a day, or about 4 minutes. So the solar day is about 4 minutes longer than the sidereal day. Because our ordinary clocks are set to solar time, stars rise 4 minutes earlier each day. Astronomers prefer sidereal time for planning their observations because in that system, a star rises at the same time every day. LOOK AT EXAMPLE 4.3

The remaining 1% of the interstellar material

s solid—frozen particles consisting of many atoms and molecules that are called interstellar grains or interstellar dust (Figure 20.2). A typical dust grain consists of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices.

nearest star visible without a telescope from most of the United States

s the brightest appearing of all the stars, Sirius, which has a distance of a little more than 8 light-years. It too is a binary system, composed of a faint white dwarf orbiting a bluish-white, main-sequence star. It is an interesting coincidence of numbers that light reaches us from the Sun in about 8 minutes and from the next brightest star in the sky in about 8 years.

scattering of sunlight

scattering of sunlight is also what causes our sky to look blue, even though the gases that make up Earth's atmosphere are transparent. As sunlight comes in, it scatters from the molecules of air. The small size of the molecules means that the blue colors scatter much more efficiently than the greens, yellows, and reds. Thus, the blue in sunlight is scattered out of the beam and all over the sky. The light from the Sun that comes to your eye, on the other hand, is missing some of its blue, so the Sun looks a bit yellower, even when it is high in the sky, than it would from space.

which stars shed enough mass to reach this limit?

search for young clusters that contain one or more white dwarf stars. Remember that more massive stars go through all stages of their evolution more rapidly than less massive ones. Suppose we find a cluster that has a white dwarf member and also contains stars on the main sequence that have 6 times the mass of the Sun. This means that only those stars with masses greater than 6 MSun have had time to exhaust their supply of nuclear energy and complete their evolution to the white dwarf stage. The star that turned into the white dwarf must therefore have had a main-sequence mass of more than 6 MSun, since stars with lower masses have not yet had time to use up their stores of nuclear energy. The star that became the white dwarf must, therefore, have gotten rid of at least 4.6 MSun so that its mass at the time nuclear energy generation ceased could be less than 1.4 MSun. Astronomers continue to search for suitable clusters to make this test, and the evidence so far suggests that stars with masses up to about 8 MSun can shed enough mass to end their lives as white dwarfs. Stars like the Sun will probably lose about 45% of their initial mass and become white dwarfs with masses less than 1.4 MSun.

how to astronomers address the blurriness of images?

search the world for locations where the amount of atmospheric blurring, or turbulence, is as small as possible. best sites are in coastal mountain ranges and on isolated volcanic peaks in the middle of an ocean. Air that has flowed long distances over water before it encounters land is especially stable.

The International Date Line

set by international agreement to run approximately along the 180° meridian of longitude. By convention, at the date line, the date of the calendar is changed by one day. Crossing the date line from west to east, thus advancing your time, you compensate by decreasing the date; crossing from east to west, you increase the date by one day. To maintain our planet on a rational system of timekeeping, we simply must accept that the date will differ in different cities at the same time

Other reasons earth is round

ship disappearing into the sea has a hull that disappear but mast remains visible, different time zones based on the location of sun in the sky in diff locations and satellite images every 90 min

how much of our stars do white dwarfs make up?

shows that about 7% of the true stars (spectral types O-M) in our local neighborhood are white dwarfs. A good example of a typical white dwarf is the nearby star 40 Eridani B. Its surface temperature is a relatively hot 12,000 K, but its luminosity is only 1/275 LSun. Calculations show that its radius is only 1.4% of the Sun's, or about the same as that of Earth, and its volume is 2.5 × 10-6 that of the Sun. Its mass, however, is 0.57 times the Sun's mass, just a little more than half. To fit such a substantial mass into so tiny a volume, the star's density must be about 210,000 times the density of the Sun, or more than 300,000 g/cm3. A teaspoonful of this material would have a mass of some 1.6 tons! At such enormous densities, matter cannot exist in its usual state; we will examine the particular behavior of this type of matter in The Death of Stars. For now, we just note that white dwarfs are dying stars, reaching the end of their productive lives and ready for their stories to be over.

Kepler has discovered....

some interesting and unusual planetary systems. For example, most astronomers expected planets to be limited to single stars. But we have found planets orbiting close double stars, so that the planet would see two suns in its sky, like those of the fictional planet Tatooine in the Star Wars films. At the opposite extreme, planets can orbit one star of a wide, double-star system without major interference from the second star.

if particles comes together under strong nuclear force and and form atomic nucleus what happens?

some of the nuclear energy is released. The energy given up in such a process is called the binding energy of the nucleus.

This description of a protostar surrounded by a rotating disk of gas and dust

sounds very much like what happened in our solar system when the Sun and planets formed. one of the most important discoveries from the study of star formation in the last decade of the twentieth century was that disks are an inevitable byproduct of the process of creating stars.

19th century Scientists = Sun's Energy

source of the Sun's heat might be the mechanical motion of meteorites falling into it --> calculations show in order to produce total energy emitted by Sun, energy emitted by the Sun, the mass in meteorites that would have to fall into the Sun every 100 years would equal the mass of Earth. The resulting increase in the Sun's mass would, according to Kepler's third law, change the period of Earth's orbit by 2 seconds per year. Such a change would be easily measurable and was not, in fact, occurring. Scientists could then disprove this as the source of the Sun's energy.

why does reddening mean

starlight is reddened by interstellar dust means that long-wavelength radiation is transmitted through the Galaxy more efficiently than short-wavelength radiation. Consequently, if we wish to see farther in a direction with considerable interstellar material, we should look at long wavelengths. This simple fact provides one of the motivations for the development of infrared astronomy. In the infrared region at 2 microns (2000 nanometers), for example, the obscuration is only one-sixth as great as in the visible region (500 nanometers), and we can therefore study stars that are more than twice as distant before their light is blocked by interstellar dust. This ability to see farther by observing in the infrared portion of the spectrum represents a major gain for astronomers trying to understand the structure of our Galaxy or probing its puzzling, but distant, center (

which stars can be seen with the naked eye?

stars fainter than the Sun cannot be seen with the unaided eye unless they are very nearby. For example, stars with luminosities ranging from 1/100 to 1/10,000 the luminosity of the Sun (LSun) are very common, but a star with a luminosity of 1/100 LSun would have to be within 5 light-years to be visible to the naked eye—and only three stars (all in one system) are this close to us. The nearest of these three stars, Proxima Centauri, still cannot be seen without a telescope because it has such a low luminosity.

Why are we missing most of the brightest stars when we take our census of the local neighborhood?

stars that appear brightest are not the ones closest to us. The brightest stars look the way they do because they emit a very large amount of energy—so much, in fact, that they do not have to be nearby to look brilliant. distances for the 20 stars that appear brightest from Earth. The most distant of these stars is more than 1000 light-years from us. In fact, it turns out that most of the stars visible without a telescope are hundreds of light- years away and many times more luminous than the Sun. Among the 6000 stars visible to the unaided eye, only about 50 are intrinsically fainter than the Sun most are spectral type B

astronomy

study of the objects that lie beyond our planet Earth and the processes by which these objects interact with one another. also humanity's attempt to organize what we learn into a clear history of the universe, from the instant of its birth in the Big Bang to the present Science is not merely a body of knowledge, but a method by which we attempt to understand nature and how it behaves--> models approximate nature New models or ideas = hypotheses--> much of astronomy is still just educated guesses Hypothesis must be a proposed explanation that can be tested through an experimental procedure More experiments that agree with hypothesis more likely to accept the hypotheses as useful description of nature

James Webb Space Telescope

successor the Hubble Space Telescope; it is planned to study the evolution of galaxies, the production of elements by stars, and the process of star and planet formation With the ability to measure both visible and infrared wavelengths

antimatter

such antimatter is that when a particle comes into contact with its antiparticle, the original particles are annihilated, and substantial amounts of energy in the form of photons are produced. --> cannot survive very long -->individual antiparticles are found in cosmic rays (particles that arrive at the top of Earth's atmosphere from space) and can be created in particle accelerators. -->is created in the core of the Sun and other stars.

Sun's Illumination at Specific Dates of the Year

summer solstice: the Sun shines down most directly upon the Northern Hemisphere of Earth. It appears about 23° north of the equator, and thus, on that date, it passes through the zenith of places on Earth that are at 23° N latitude To a person at 23° N (near Hawaii, for example), the Sun is directly overhead at noon. This latitude, where the Sun can appear at the zenith at noon on the first day of summer, is called the Tropic of Cancer. As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun is as far north on this date as it can get; thus, 90° - 23° (or 67° N) is the southernmost latitude where the Sun can be seen for a full 24-hour period (sometimes called the "land of the midnight Sun"). That circle of latitude is called the Arctic Circle. On June 21st all places within 23 degree of the South Pole- south of Antarctic Circle- no sun for 24h Reversed 6 months later, about December 21 (winter solstice)--> Arctic Circle that has 24 hour night and Antarctic Circle that has the midnight Sun. At latitude 23° S, called the Tropic of Capricorn, the Sun passes through the zenith at noon. Days are longer in the Southern Hemisphere and shorter in the north. In the United States and Southern Europe, there may be only 9 or 10 hours of sunshine during the day. It is winter in the Northern Hemisphere and summer in the Southern Hemisphere (44 degrees) Less tilt MIDLER SEASONAL CHANGES--> LOOK AT EXAMPLE 4.1 Halfway between the solstices, on about March 21 and September 21, the Sun is on the celestial equator. From Earth, it appears above our planet's equator and favors neither hemisphere. Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night. The points where the Sun crosses the celestial equator are called the vernal (spring) and autumnal (fall) equinoxes.

the nearest stars

tellar neighbors nearest the Sun are three stars in the constellation of Centaurus. To the unaided eye, the brightest of these three stars is Alpha Centauri, which is only 30○ from the south celestial pole and hence not visible from the mainland United States. Alpha Centauri itself is a binary star—two stars in mutual revolution—too close together to be distinguished without a telescope. These two stars are 4.4 light-years from us. Nearby is a third faint star, known as Proxima Centauri. Proxima, with a distance of 4.3 light-years, is slightly closer to us than the other two stars. If Proxima Centauri is part of a triple star system with the binary Alpha Centauri, as seems likely, then its orbital period may be longer than 500,000 years.

how to plot H-R diagrams

temperature increases toward the left and luminosity toward the top. . The great majority are aligned along a narrow sequence running from the upper left (hot, highly luminous) to the lower right (cool, less luminous). This band of points is called the main sequence.

the absence of an element's spectral lines does not necessarily mean

that the element itself is absent. As we saw, the temperature and pressure in a star's atmosphere will determine what types of atoms are able to produce absorption lines. Only if the physical conditions in a star's photosphere are such that lines of an element should (according to calculations) be there can we conclude that the absence of observable spectral lines implies low abundance of the element. If we see lines of iron in a star's spectrum, for example, then we know immediately that the star must contain iron.

Eyepiece

the additions lens used to view the image formed by the lens in a telescope focuses the image at a distance that is either directly viewable by a human or at a convenient place for a detector. Using different eyepieces, we can change the magnification (or size) of the image and also redirect the light to a more accessible location.

apparent brightness

the amount of a star's energy that reaches a given area (say, one square meter) each second here on Earth. Stars are democratic in how they produce radiation; they emit the same amount of energy in every direction in space.

The Turning Earth

the apparent rotation of the celestial sphere could be accounted for either by a daily rotation of the sky around a stationary Earth or by the rotation of Earth itself Jean Foucault: demonstration of rotation--> started swinging pendulum evenly, if earth had not been turning there would have been no alteration to the path, but the pendulum's place of motion was turning --> Earth was turning beneath it

when a cation meets an anion....

the atom emits one or more photons. Which photons are emitted depends on whether the electron is captured at once to the lowest energy level of the atom or stops at one or more intermediate levels on its way to the lowest available level.

The Proton-Proton Chain

the chain of fusion reactions, leading from hydrogen to helium, that powers main-sequence stars 1H + 1H --> 1H + e^+ + v 2H + 1H--> 3He + y 3He + 3He --> 4He+ 1H +1H (pg 573) Here, the superscripts indicate the total number of neutrons plus protons in the nucleus, e+ is the positron, v is the neutrino, and γ indicates that gamma rays are emitted. Note that the third step requires two helium-3 nuclei to start; the first two steps must happen twice before the third step can occur.

using spectrometers to measure

the changing velocity of stars with planets around them. As the star and planet orbit each other, part of their motion will be in our line of sight (toward us or away from us). Such motion can be measured using the Doppler effect and the star's spectrum. As the star moves back and forth in orbit around the system's center of mass in response to the gravitational tug of an orbiting planet, the lines in its spectrum will shift back and forth.

When the collapse of a high-mass star's core is stopped by degenerate neutrons....

the core is saved from further destruction, but it turns out that the rest of the star is literally blown apart.

precession

the direction in which Earth's axis points does indeed change slowly but regularly (axis resembles top like motion)--> 26000 years for one circle precession

what happens to the neutrino that forms in step 1

the fusion of two hydrogen atoms to form deuterium results in the emission of a neutrino. Because neutrinos interact so little with ordinary matter, those produced by fusion reactions near the center of the Sun travel directly to the Sun's surface and then out into space, in all directions. Neutrinos move at nearly the speed of light, and they escape the Sun about two seconds after they are created.

challenges in observing the universe in the infrared band of the spectrum

the infrared region extends from wavelengths near 1 micrometer (μm), which is about the long wavelength sensitivity limit of both CCDs and photography, to 100 micrometers or longer main challenge to astronomers using infrared is to distinguish between the tiny amount of heat radiation that reaches Earth from stars and galaxies, and the much greater heat radiated by the telescope itself and our planet's atmosphere. Typical temperatures on Earth's surface are near 300 K, and the atmosphere through which observations are made is only a little cooler. According to Wien's law, the telescope, the observatory, and even the sky are radiating infrared energy with a peak wavelength of about 10 micrometers. The challenge is to detect faint cosmic sources against this sea of infrared light.

what happens on the inside of white dwarfs

the matter inside them behaves in a very unusual way—unlike anything we know from everyday experience. At this high density, gravity is incredibly strong and tries to shrink the star still further, but all the electrons resist being pushed closer together and set up a powerful pressure inside the core. This pressure is the result of the fundamental rules that govern the behavior of electrons

how can small velocity variations be used to determine what the interior of the Sun is like.

the motion of the Sun's surface is caused by waves that reach it from deep in the interior. Study of the amplitude and cycle length of velocity changes provides information about the temperature, density, and composition of the layers through which the waves passed before they reached the surface. The situation is somewhat analogous to the use of seismic waves generated by earthquakes to infer the properties of Earth's interior. For this reason, studies of solar oscillations (back-and-forth motions) are referred to as helioseismology. It takes a little over an hour for waves to traverse the Sun from center to surface, so the waves, like neutrinos, provide information about what the solar interior is like at the present time. In contrast, remember that the sunlight we see today emerging from the Sun was actually generated in the core several hundred thousand years ago.

Aristotle

the progression of the Moon's phases—its apparent changing shape—results from our seeing different portions of the Moon's sunlit hemisphere as the month goes by

chromatic aberration

the property of a lens whereby light of different colors is focused at different places = causes image to appear blurry

Doppler-shift method—which relies on

the pull of a planet making its star "wiggle" back and forth around the center of mass—is most effective at finding planets that are both close to their stars and massive. These planets cause the biggest "wiggles" in the motion of their stars and the biggest Doppler shifts in the spectrum. Plus, they will be found sooner, since astronomers like to monitor the star for at least one full orbit (and perhaps more) and hot Jupiters take the shortest time to complete their orbit.

what happens after a telescope collects radiation from an astronomical source

the radiation must be detected and measured. detectors : eye --> photography

what happens when binding energy is released?

the resulting nucleus has slightly less mass than the sum of the masses of the particles that came together to form it. In other words, the energy comes from the loss of mass. This slight deficit in mass is only a small fraction of the mass of one proton. But because each bit of lost mass can provide a lot of energy (remember, E = mc2), this nuclear energy release can be quite substantial.

The reason massive stars are such spendthrifts is that, as we saw above, the rate of fusion depends very strongly on

the star's core temperature. And what determines how hot a star's central regions get? It is the mass of the star—the weight of the overlying layers determines how high the pressure in the core must be: higher mass requires higher pressure to balance it. Higher pressure, in turn, is produced by higher temperature. The higher the temperature in the central regions, the faster the star races through its storehouse of central hydrogen. Although massive stars have more fuel, they burn it so prodigiously that their lifetimes are much shorter than those of their low-mass counterparts. You can also understand now why the most massive main-sequence stars are also the most luminous. Like new rock stars with their first platinum album, they spend their resources at an astounding rate.

space velocity

the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun to know sv we must know its radial velocity, proper motion, and distance

Luminosity

the total amount of energy at all wavelengths that it emits per second. express the luminosity of other stars in terms of the Sun's luminosity.

what about rotation on stars far away?

they all appear as unresolved points. The best we can do is to analyze the light from the entire star at once. Due to the Doppler effect, the lines in the light that come from the side of the star rotating toward us are shifted to shorter wavelengths and the lines in the light from the opposite edge of the star are shifted to longer wavelengths. You can think of each spectral line that we observe as the sum or composite of spectral lines originating from different speeds with respect to us. Each point on the star has its own Doppler shift, so the absorption line we see from the whole star is actually much wider than it would be if the star were not rotating. If a star is rotating rapidly, there will be a greater spread of Doppler shifts and all its spectral lines should be quite broad. In fact, astronomers call this effect line broadening, and the amount of broadening can tell us the speed at which the star rotates (

why couldn't they detect hydrogen?

they could not yet detect hydrogen, the most common element, due to its lack of spectral features in the visible part of the spectrum. The Balmer line of hydrogen is in the visible range, but only excited hydrogen atoms produce it. In the cold interstellar medium, the hydrogen atoms are all in the ground state and no electrons are in the higher-energy levels required to produce either emission or absorption lines in the Balmer series.) Direct detection of hydrogen had to await the development of telescopes capable of seeing very-low-energy changes in hydrogen atoms in other parts of the spectrum. The first such observations were made using radio telescopes, and radio emission and absorption by interstellar hydrogen remains one of our main tools for studying the vast amounts of cold hydrogen in the universe to this day.

apparent solar time

time reckoned by the actual position of the Sun in the sky or below the horizon--> represented by sundials During the first half of the day, the Sun has not yet reached the meridian (the great circle in the sky that passes through our zenith). We designate those hours as before midday (ante meridiem, or a.m.), before the Sun reaches the local meridian. We customarily start numbering the hours after noon over again and designate them by p.m. (post meridiem), after the Sun reaches the local meridian Although apparent solar time seems simple, it is not really very convenient to use. The exact length of an apparent solar day varies slightly during the year. The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth's axis of rotation is not perpendicular to the plane of its revolution. --> does not advance at a uniform rate.

field

to describe the action of forces that one object exerts on other distant objects. Sun produces a gravitational field that controls Earth's orbit, even though the Sun and Earth do not come directly into contact. ***we can say that stationary electric charges produce electric fields, and moving electric charges also produce magnetic fields.

grating

to disperse the spectrum. A grating is a piece of material with thousands of grooves on its surface. While it functions completely differently, a grating, like a prism, also spreads light out into a spectrum.

The natural turbulence inside a clump tends to....

to give any portion of it some initial spinning motion (even if it is very slow). As a result, each collapsing core is expected to spin. According to the law of conservation of angular momentum (discussed in the chapter on Orbits and Gravity), a rotating body spins more rapidly as it decreases in size. In other words, if the object can turn its material around a smaller circle, it can move that material more quickly—like a figure skater spinning more rapidly as she brings her arms in tight to her body. This is exactly what happens when a core contracts to form a protostar: as it shrinks, its rate of spin increases.

The wind from a forming star will

ultimately sweep away the material that remains in the obscuring envelope of dust and gas, leaving behind the naked disk and protostar, which can then be seen with visible light. We should note that at this point, the protostar itself is still contracting slowly and has not yet reached the main- sequence stage on the H-R diagram (a concept introduced in the chapter The Stars: A Celestial Census). The disk can be detected directly when observed at infrared wavelengths or when it is seen silhouetted against a bright background

One commonly used set of filters in astronomy measures stellar brightness at three wavelengths corresponding with

ultraviolet, blue, and yellow light. The filters are named: U (ultraviolet), B (blue), and V (visual, for yellow). These filters transmit light near the wavelengths of 360 nanometers (nm), 420 nm, and 540 nm, respectively. The brightness measured through each filter is usually expressed in magnitudes

how mass to energy conversion cause sun to radiate energy

un could be produced by the complete conversion of about 4 million tons of matter into energy inside the Sun each second. Destroying 4 million tons per second sounds like a lot when compared to earthly things, but bear in mind that the Sun is a very big reservoir of matter. In fact, we will see that the Sun contains more than enough mass to destroy such huge amounts of matter and still continue shining at its present rate for billions of years.

what happens if convection doesn't occur

unless convection occurs, the only significant mode of energy transport through a star is by electromagnetic radiation. Radiation is not an efficient means of energy transport in stars because gases in stellar interiors are very opaque, that is, a photon does not go far (in the Sun, typically about 0.01 meter) before it is absorbed. (The processes by which atoms and ions can interrupt the outward flow of photons—such as becoming ionized—were discussed in the section on the Formation of Spectral Lines.) The absorbed energy is always reemitted, but it can be reemitted in any direction. A photon absorbed when traveling outward in a star has almost as good a chance of being radiated back toward the center of the star as toward its surface. A particular quantity of energy, therefore, zigzags around in an almost random manner and takes a long time to work its way from the center of a star to its surface (Figure 16.13). Estimates are somewhat uncertain, but in the Sun, as we saw, the time required is probably between 100,000 and 1,000,000 years. If the photons were not absorbed and reemitted along the way, they would travel at the speed of light and could reach the surface in a little over 2 seconds, just as neutrinos do (Figure 16.14).

The first successful use of the Doppler effect to find a planet

used this technique to find a planet orbiting a star resembling our Sun called 51 Pegasi, about 40 light-years away. (The star can be found in the sky near the great square of Pegasus, the flying horse of Greek mythology, one of the easiest-to-find star patterns.) To everyone's surprise, the planet takes a mere 4.2 days to orbit around the star. Mayor and Queloz's findings mean the planet must be very close to 51 Pegasi, circling it about 7 million kilometers away (Figure 21.18). At that distance, the energy of the star should heat the planet's surface to a temperature of a few thousand degrees Celsius (a bit hot for future tourism). From its motion, astronomers calculate that it has at least half the mass of Jupiter[1], making it clearly a jovian and not a terrestrial-type planet.

purpose of radar waves

used to determine the distances to planets and how fast things are moving in the solar system have determined the rotation periods of Venus and Mercury, probed tiny Earth-approaching asteroids, and allowed us to investigate the mountains and valleys on the surfaces of Mercury, Venus, Mars, and the large moons of Jupiter.

Edwin Hubble & Cepheids

using cepheids, when he observed them in nearby galaxies and discovered the expansion of the universe.

Distances from Spectral Types

variable stars have been for distance measurement, these stars are rare and are not found near all the objects to which we wish to measure distances--->turns out the H-R diagram can come to our rescue.

what more can we learn from a stars spectrum?

we can detect pressure differences in stars from the details of the spectrum. This knowledge is very useful because giant stars are larger (and have lower pressures) than main-sequence stars, and supergiants are still larger than giants. If we look in detail at the spectrum of a star, we can determine whether it is a main-sequence star, a giant, or a supergiant.

exoplanetary systems

we don't expect to find only one planet per star. Our solar system has eight major planets, half a dozen dwarf planets, and millions of smaller objects orbiting the Sun. The evidence we have of planetary systems in formation also suggest that they are likely to produce multi-planet systems.

To estimate the number of Earth-size planets in our Galaxy,

we need to remember that there are approximately 100 billion stars of spectral types F, G, and K. Therefore, we estimate that there are about 30 billion Earth-size planets in our Galaxy. If we include the super-Earths too, then there could be one hundred billion in the whole Galaxy. This idea—that planets of roughly Earth's size are so numerous—is surely one of the most important discoveries of modern astronomy.

Extremes of stellar luminosities, diameters & densities

we saw that the most massive main-sequence stars are the most luminous ones. We know of a few extreme stars that are a million times more luminous than the Sun, with masses that exceed 100 times the Sun's mass. These superluminous stars, which are at the upper left of the H-R diagram, are exceedingly hot, very blue stars of spectral type O. These are the stars that would be the most conspicuous at vast distances in space. The cool supergiants in the upper corner of the H-R diagram are as much as 10,000 times as luminous as the Sun. In addition, these stars have diameters very much larger than that of the Sun. As discussed above, some supergiants are so large that if the solar system could be centered in one, the star's surface would lie beyond the orbit of Mars (see Figure 18.16). We will have to ask, in coming chapters, what process can make a star swell up to such an enormous size, and how long these "swollen" stars can last in their distended state. In contrast, the very common red, cool, low-luminosity stars at the lower end of the main sequence are much smaller and more compact than the Sun. An example of such a red dwarf is Ross 614B, with a surface temperature of 2700 K and only 1/2000 of the Sun's luminosity. We call such a star a dwarf because its diameter is only 1/10 that of the Sun. A star with such a low luminosity also has a low mass (about 1/12 that of the Sun). This combination of mass and diameter means that it is so compressed that the star has an average density about 80 times that of the Sun. Its density must be higher, in fact, than that of any known solid found on the surface of Earth. (Despite this, the star is made of gas throughout because its center is so hot.) The faint, red, main-sequence stars are not the stars of the most extreme densities, however. The white dwarfs, at the lower-left corner of the H-R diagram, have densities many times greater still.

Global Cluster

were given this name because they are nearly symmetrical round systems of, typically, hundreds of thousands of stars. The most massive globular cluster in our own Galaxy is Omega Centauri, which is about 16,000 light-years away and contains several million stars (Figure 22.6). Note that the brightest stars in this cluster, which are red giants that have already completed the main-sequence phase of their evolution, are red-orange in color. These stars have typical surface temperatures around 4000 K. As we will see, globular clusters are among the oldest parts of our Milky Way Galaxy.

The basic idea of triggered star formation is this:

when a massive star is formed, it emits a large amount of ultraviolet radiation and ejects high-speed gas in the form of a stellar wind. This injection of energy heats the gas around the stars and causes it to expand. When massive stars exhaust their supply of fuel, they explode, and the energy of the explosion also heats the gas. The hot gases pile into the surrounding cold molecular cloud, compressing the material in it and increasing its density. If this increase in density is large enough, gravity will overcome pressure, and stars will begin to form in the compressed gas. Such a chain reaction—where the brightest and hottest stars of one area become the cause of star formation "next door"—seems to have occurred not only in Orion but also in many other molecular clouds.

selection effect

where our technique of discovery selects certain kinds of objects as "easy finds." As an example of a selection effect in everyday life, imagine you decide you are ready for a new romantic relationship in your life. To begin with, you only attend social events on campus, all of which require a student ID to get in. Your selection of possible partners will then be limited to students at your college. That may not give you as diverse a group to choose from as you want. In the same way, when we first used the Doppler technique, it selected massive planets close to their stars as the most likely discoveries. As we spend longer times watching target stars and as our ability to measure smaller Doppler shifts improves, this technique can reveal more distant and less massive planets too

horizon

where the dome meets the Earth (usually hidden by buildings trees etc.) Stars rising on the eastern horizon (sun and moon) moving across the dome of the sky and setting in the west

emission nebulae in the constellation of Sagittarius

where we see an H II region surrounded by a blue reflection nebula. Which type of nebula appears brighter depends on the kinds of stars that cause the gas and dust to glow. Stars cooler than about 25,000 K have so little ultraviolet radiation of wavelengths shorter than 91.2 nanometers—which is the wavelength required to ionize hydrogen—that the reflection nebulae around such stars outshine the emission nebulae. Stars hotter than 25,000 K emit enough ultraviolet energy that the emission nebulae produced around them generally outshine the reflection nebulae.

Comparing the average density of exoplanets to the density of planets in our solar system helps us understand

whether they are rocky or gaseous in nature. This has been particularly important for understanding the structure of the new categories of super-Earths and mini-Neptunes with masses between 3-10 times the mass of Earth. A key observation so far is that planets that are more than 10 times the mass of Earth have substantial gaseous envelopes (like Uranus and Neptune) whereas lower-mass planets are predominately rocky in nature (like the terrestrial planets).

Ultra-cool brown dwarfs

with temperatures of 500-600 K. These objects exhibited absorption lines due to ammonia (NH3), which are not seen in T dwarfs. A new spectral class, Y, was created for these objects. As of 2015, over two dozen brown dwarfs belonging to spectral class Y have been discovered, some with temperatures comparable to that of the human body (about 300 K). Figure 17.8

direct imaging works best for

young gas giant planets that emit infrared light and reside at large separations from their host stars. Young giant planets emit more infrared light because they have more internal energy, stored from the process of planet formation. Even then, clever techniques must be employed to subtract out the light from the host star. In 2008, three such young planets were discovered orbiting HR 8799, a star in the constellation of Pegasus (Figure 21.21). Two years later, a fourth planet was detected closer to the star. Additional planets may reside even closer to HR 8799, but if they exist, they are currently lost in the glare of the star. Since then, a number of planets around other stars have been found using direct imaging. However, one challenge is to tell whether the objects we are seeing are indeed planets or if they are brown dwarfs (failed stars) in orbit around a star.