AST Quiz 3
Formula
- c = λ f -For any wave motion, the speed at which a wave moves equals the frequency times the wavelength. -Waves with longer wavelengths have lower frequencies. - c=speed of light
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.
Brown Dwarf
-A failed star. -A star is defined as an object that during some part of its lifetime derives 100% of its energy from the fusion of hydrogen nuclei (protons) into helium. -Extremely difficult to observe b/c are extremely faint & cool and put out most of their light in the infrared part of the spectrum.
Photon
-A packet of electromagnetic energy. -Sometimes light behaves more like a "particle"—or at least a self-contained packet of energy—than like a wave. -How much energy a photon has depends on its frequency when you think about it as a wave.
Wave Properties
-A repeating phenomenon: moving up and down cyclically. -Moving from one crest through a trough to the next crest completes one cycle. -The horizontal length covered by one cycle is called the wavelength. -Can characterize different waves by their frequency: number of wave cycles that pass by per second. -A cycle per second is called a hertz (Hz). -Wavelength (λ) and frequency (f) are related because all electromagnetic waves travel at the same speed.
Supernova
-A star's explosion. -Thought to be the source of many of the high-energy cosmic ray particles.
Giants or supergiants
-Above the main sequence in the upper-right region. -Low temp. and high luminosity. -Only possible b/c the star is enormous.
Radio Waves
-All electromagnetic waves that are longer than microwaves. -So broad a category that we generally divide it into several subsections. -Like radar waves, which are used in radar guns by traffic officers to determine vehicle speeds. -And AM radio waves, which were the first to be developed for broadcasting. -The wavelengths of these different categories range from over a meter to hundreds of meters, and other radio radiation can have wavelengths as long as several kilometers. -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.
Good rule Pg. 810 & 811
-All smaller nuclei want to "grow up" to be like iron, and they are willing to pay (produce energy) to move toward that goal. -But iron is a mature nucleus with good self-esteem, 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.
Black hole
-An object from which light cannot escape. -If the Sun is compressed, its mass will remain the same, but the distance between a point on the Sun's surface and the center will get smaller and smaller. -Thus, as we compress the star, the pull of gravity for an object on the shrinking surface will get stronger and stronger. -Name popularized by John Wheeler. -The concentration of matter has curved spacetime, and light is "doing its best" to go in a straight line. -All the mass really is concentrated at a point in the center. -What happens to the collapsing star-core that made the black hole? Our best calculations predict that the material will continue to collapse under its own weight, forming an infinitely squozen point—a place of zero volume and infinite density—to which we give the name singularity: where spacetime ceases to exist. The laws of physics as we know them break down. We do not yet have the physical understanding or the mathematical tools to describe the singularity itself, or even if singularities actually occur. -All matter falling into a black hole will also appear to an outside observer to stop at the event horizon, frozen in place and taking an infinite time to fall through it.
Spectral Class
-Astronomers use the patterns of lines observed in stellar spectra to sort stars into this. -B/c a star's temp. determines which absorption lines are present in its spectrum, these spectral classes are a measure of its surface temp. -7 standard spectral classes (hottest to coldest): O, B, A, F, G, K, and M. -3 more added for even cooler objects: L, T, & Y. -Annie Jump Cannon is responsible for his classification in the 1890's -Williamina Fleming was the forefather of this, classifying stars based on the strength of hydrogen absorption lines in the 1880's. -Each of these spectral classes is subdivided into 10 subclasses (0 [hottest] through 9 [coolest]).
From main sequence to red giants
-At main-sequence stage, stars derives their energy almost entirely from the conversion of hydrogen (the most abundant element in stars) to helium via the process of nuclear fusion. -Zero-age main sequence: the time when a star stops contracting and fuses hydrogen in its core. -Fusion causes hydrogen to deplete and helium to accumulate, changing the luminosity, size, temp., and interior structure of the star. -Lifetime of a star depends on how much nucleear fuel it has and on how quickly iy uses up that fuel. -The rate of fusion depends very strongly on the star's core temp. -Higher mass requires higher pressure to balance it, and higher pressure is produced by higher temp., and the higher the temp., the faster the star races through its stored hydrogen. -Heat flows outward to where it is cooler, it raises the temp. of the layer of hydrogen and begins hydrogen fusion. -Hydrogen fusion hears up outer layers of the star, causing it to expand. -Most stars generate more energy when they fuse hydrogen in the shell, increasing luminosity. -Expansion of a star's outer layers causes surface temp. to decrease, and the star becomes redder.
Important Notes
-Avg. surface temp. on the sun: 5800 K. -Water freezes at 273 K. -Water boils at: 373 K. -All molecular motion ceases at 0K -Formula [Wien's law]: Max Wavelength = (3*10^6)/T -^^measured in nanometers. -What we measure from a large object like a star is the energy flux (flow), the power emitted per square meter. -Light can be reflected from a surface. And bent (or refracted) when it passes from one kind of transparent material into another.
White dwarfs
-Below the main sequence in the lower-left region. -High temp. & low luminosity. -Only possible b/c the star is very small. -Are dying stars. -Stable, compact objects with electron-degenerate cores that cannot contract any further. -Its radius shrinks as its mass increases. -A white dwarf with a mass of about 1.4 Msun or larger would have a radius of zero. -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
Infrared (Heat Radiation)
-Between visible light and radio waves are these wavelengths. -Astronomer William Herschel first discovered infrared in 1800 while trying to measure the temperatures of different colors of sunlight spread out into a spectrum. -Infrared waves are absorbed by water and carbon dioxide molecules, which are more concentrated low in Earth's atmosphere. -For this reason, infrared astronomy is best done from high mountaintops, high-flying airplanes, and spacecraft.
Maxwell's Theory
-Deals with electric charges in atoms and their effects, especially when they are moving. -In the vicinity of an electron charge, another charge feels a force of attraction or repulsion: opposite charges attract; like charges repel. -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.
Sir Issac Newton
-Discovered that sunlight is actually made up of a mixture of all the colors of the rainbow. -His prism method shows that light is refracted through it, and the bending of the beam depends on the wavelength of the light and properties of the material. -This phenomenon is called dispersion. -Violet light is bent more than the red. -This spectrum of light (array of colors) is measured with a spectrometer.
Henry Norris Russell & Ejnar Hertzsprung
-Discovered that the temp. & luminosity of stars are related. -Hertzsprung-Russell diagram (H-R diagram) plots the temp. (spectral class) of a selected group of nearby stars against their luminosity.
Electromagnetic Waves
-Do not require water or air: the fields generate each other and so can move through a vacuum (such as outer space or empty space). -Move at the speed of light no matter what. -All electromagnetic waves move at the same speed in empty space. -Just like water, the disturbance travels rapidly outward from the point of origin and can use its energy to disturb other things farther away.
Blackbody
-Does not reflect or scatter any radiation, but absorbs all the electromagnetic energy that falls onto 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. -The curves show that, at each temp., the blackbody emits radiation (photons) at all wavelengths (colors). -More energy is emitted at the avg. vibration or motion rate (highest part of each curve). -An object at a higher temp. emits more power at all wavelengths than a cooler one. -The higher the temp., the shorter the wavelength at which the max. power is emitted. -Shorter wavelength = higher frequency & energy; so, hot objects give off a larger fraction of their energy at shorter wavelengths.
Neutrino
-Each time an electron and a proton in the star's core merge to make a neutron. -Ghostly subatomic particles that carry away some of the nuclear energy. -Its presence launches the final disastrous explosion of the star. -10^46 Watts.
Electromagnetic Radiation
-Electromagnetic disturbances. -Sometimes behaves like a wave, sometimes like a particle. -Light is one form of a family of these disturbances. -Light (and all other electromagnetic radiation) gets weaker and weaker as it gets farther from its source.
Gamma Rays
-Electromagnetic radiation with the shortest wavelengths, no longer than 0.01 nanometer (1 nanometer = 10^-9 meters). -Because gamma rays carry a lot of energy, they can be dangerous for living tissues. -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 (which is a good thing for our health); thus, they can only be studied using instruments in space.
X-Rays
-Electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers. -Being more energetic than visible light, X-rays are able to penetrate soft tissues but not bones, and so allow us to make images of the shadows of the bones inside us. -While X-rays 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. -Can only be studied by sending instruments into space.
Visible Light
-Electromagnetic radiation with wavelengths between roughly 400 and 700 nm. -These are the waves that human vision can perceive. -This is also the band of the electromagnetic spectrum that most readily reaches Earth's surface. -These two observations are not coincidental: human eyes evolved to see the kinds of waves that arrive from the Sun most effectively. -Visible light penetrates Earth's atmosphere effectively, except when it is temporarily blocked by clouds.
Stefan-Boltzmann Law
-Energy flux from a blackbody at temp. T is proportional to the 4th power of its absolute temp. -Equation: F=*sigma*T^4 -F=energy flux; sigma=5.67 x 10^-8
Degenerate Gas
-Extremely dense gas with electrons crowding and resisting any further crowding.
Continuous Spectrum
-Formed when a solid or very dense gas gives off radiation. -It is an array of all wavelengths or colors of the rainbow.
A star's life
-Gravity pulls inward trying to collapse the star. -Pressure pushes outward trying to force the star to expand. -Strong gravitational attraction first forms the star, which is a dense & hot ball of matter. -Star formation is not a very efficient process, about 1% of the material in the cloud has turned into stars. -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. -When stars exhaust their supply of fuel, they explode, and the energy of the explosion heats the gas around the stars (causing them to expand), and compresses the material in the cold molecular cloud (increasing density) and forming stars. -Protostar: when a dense core is contracting to become a star before the process of the fusion of protons to produce helium begins. -This collapsing core of a protostar spins, and spins more rapidly as it shrinks in size. -Material (gas and dust) falls onto the poles, instead of the equator. -Protostars emit infrared radiation. -At it's full mass, protostars are called T Tauri stars.
William Wollaston & more
-In 1802, he built an improved spectrometer that included a lens to focus the sun's spectrum on a screen. -In 1815, Joseph Fraunhofer found 600 dark lines (missing colors) disproving the idea that the colors had natural boundaries. -Later we found out that certain lines in the spectrum "go with" certain elements. -Different substances showed distinctive spectral signatures. -Each particular gas can absorb or emit only certain wavelengths of the light peculiar to that gas. -Temp. & 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 remain the same. -In 1860, Gustav Kirchhoff became the 1st person to use spectroscopy to identify an element in the sun when he found the spectral signature of sodium gas.
Sir William Huggins & Lady Margaret Huggins
-In the 1860's, they 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 & planets exist in the stars.
L~M^3.9
-Luminosity varies as the fourth power of the mass. -(~) means the 2 quantities are proportional.
Giant Molecular Clouds
-Most massive reservoirs of interstellar matter. -Some of the most massive objects in the Milky Way Galaxy. -Cold interiors (10-20 k) -Birthplace of most stars: within there are cold dense regions that are clumps, and within clumps there are cores (the embryo of stars) that are very low temp. and high density. -The Orion molecular cloud is much larger than the star pattern. -Star formation takes place at the Orion Nebula (halfway down the sword) & also has a tight cluster of stars called the Trapezium.
Color Indices
-One filter used in astronomy measures stellar brightness at 3 wavelengths: U (ultraviolet), B (blue), and V (visual, for yellow). -U: 360 nanometers; B: 420 nanometers; V: 540 nanometers. -The brightness measured through each filter is expressed in magnitudes. -The difference between any 2 of these magnitudes is called a color index.
Radiation Law
-Or Stefan-Boltzmann law. -Used to get the diameter of a star. -The energy flux is given by F=sigmaT^4. -The surface area of a sphere is given by A=4piR^2. -Luminosity is L=(AxF). -Temp. doesn't just determine luminosity, but radius.
Emission Spectrum
-Or a bright line. -Appears as a pattern or series of bright lines; it sonsists of light in which only certain discrete wavelengths are present.
Absorption Spectrum
-Or a dark line. -Consists of a series or pattern of dark lines-missing colors-superimposed upon the continuous spectrum of a source.
James Clerk Maxwell
-Physicist. Born and educated in Scotland. -Inspired by a number of ingenious experiments that showed an intimate relationship between electricity and magnetism, Maxwell developed a theory that describes both electricity and magnetism with only a small number of elegant equations. -It is this theory that gives us important insights into the nature and behavior of light.
Ultraviolet
-Radiation intermediate between X-rays and visible light with higher energy than violet. -Is sometimes called "black light" because our eyes cannot see it. -Ultraviolet radiation is 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 skin cancer in human beings. -Ultraviolet astronomy is also best done from space.
Massive Stars
-Stars with 150 Msun die with a bang. -In death, 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.
General Relativity Theory
-Tells us that gravity is really a curvature of spacetime. -As gravity increases (as in the collapsing Sun of our thought experiment), the curvature gets larger and larger.
The Inverse Square Law
-The apparent brightness of a source gets weaker with distance for light propagation. -In this respect, the propagation of light is similar to the effects of gravity. -The force of gravity between two attracting masses is also inversely proportional to the square of their separation.
Color & Temperature
-The color of a star provides a measure of its intrinsic or true surface temp. -Blue colors dominate the visible light output of very hot stars. -Cool stars emit most of their visible light energy at red wavelengths. -Hottest stars: 40,000 K -Coolest stars:2,000 K -Our Sun: 6,000 K -Our sun's peak wavelength color: slightly greenish-yellow. -Looks white in space and yellow on earth (due to nitrogen molecules). -The blue sky is sunlight (blue wavelengths) scattered by Earth's atmosphere (nitrogen).
Main sequence
-The great majority of stars are aligned along a narrow sequence running from the upper left to the lower right on the H-R diagram. -90% of all stars are found here. -A good reason why a star spends 90% of its life fusing the abundant hydrogen in its core into helium. -These stars differ on the spectrum b/c of composition and total mass. -The main sequence is a sequence of stellar masses. -Most massive of these MS stars are the most luminous.
Light Proportion
-The increase in the area that the light must cover is proportional to the square of the distance that the light has traveled. -If we stand twice as far from the source, our eyes will intercept two-squared (2 × 2), or four times less light.
Important
-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.
Formation of Stellar Spectra
-The primary reason that stellar spectra look different is because the stars have different temperatures. -Hydrogen is the most abundant element in most stars, but not seen in the spectra b/c of low energy state. But strongest in stars with intermediate temps. -They are completely ionized in the atmosphere of the hottest stars. Because the electron and the proton are separated, ionized hydrogen cannot produce absorption lines. -In the atmosphere of the coolest stars, they can have their electrons attached & switch energy levels to produce lines.
Magnetism
-The result of moving charged particles. -It is the alignment of the electrons' motion that causes the material to become magnetic.
How stars die
-The star exhausts its store of nuclear energy. -Without a source of internal pressure to balance the weight of overlying layers, every star eventually gives way to the inexorable pull of gravity and collapses under its own weight. -Mass of the star when it is ready to die determines whether it'll be a bang or whimper. -1.4 Msun before death is a crucial dividing line.
Event horizon
-The star's geometry cuts off communication with the rest of the universe at precisely the moment when the escape velocity becomes equal to the speed of light. -The size of the star at this moment defines this surface (the boundary of a black hole). -The characteristics of an event horizon were first worked out by Karl Schwarzschild.
Temperature determines...
-The type of electromagnetic radiation emitted by astronomical objects. -Temp. is a measure of the average motion energy of the particles that make it up.
Microwave
-Used in short-wave communication and microwave ovens. -Wavelengths vary from 1 millimeter to 1 meter and are absorbed by water vapor, which makes them effective in heating foods. -The "micro-" prefix refers to the fact that microwaves are small in comparison to radio waves, the next on the spectrum.
Field
-Word used to describe the action of forces that one object exerts on other distant objects. Ex: Gravitational field. -Stationary electric charges produce electric fields, and moving electric charges also produce magnetic fields. -Experiments showed that changing magnetic fields could produce electric currents (and thus changing electric fields), and changing electric currents could in turn produce changing magnetic fields.
The visible light and other radiation we receive from the stars and planets is generated by processes at...
....the atomic level—by changes in the way the parts of an atom interact and move.
Maxwell analyzed what would happen if electric charges were oscillating (moving constantly back and forth)...
...and found that the resulting pattern of electric and magnetic fields would spread out and travel rapidly through space.
Atoms and molecules (which consist of charged particles)...
...oscillate back and forth all the time.
Speed of light
300,000 km/s.
Radiation
A general term for waves (including light waves) that radiate outward from a source.
Quantum Mechanics
A more complicated theory of waves and particles that began by the confusion of the wave-particle duality of light caused in physics.
Neutron star
Crushed ball made mainly of neutrons.
Mass-Luminosity Relation
Generally, the more massive stars are the more luminous.
Heinrich Hertz
In 1887, he made invisible electromagnetic waves (what today are called radio waves) on one side of a room and detected them on the other side, it ushered in a new era that led to the modern age of telecommunications.
Mnemonic for main colors of visible light from longest to shortest wavelength
ROY G BIV—for Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
Selection effect
The contrast between two samples of stars, those that are close to us and those that can be seen with the unaided eye.
Chandrasekhar limit
The max. mass that a star can end its life with and still become a white dwarf.
The Speed of Light =
The speed at which an electromagnetic disturbance moves through space.
