PHY 101 - Astronomy Exam 2 Study Guide
visible light telescope
Telescopes that collect visible radiation use a lens or mirror to gather the light. - reflecting and refracting telescopes - Visible-light detectors include the human eye, photographic film, and charge-coupled devices (CCDs)
Frequency
We can also characterize different waves by their frequency, the number of wave cycles that pass by per second. If you count 10 crests moving by each second, for example, then the frequency is 10 cycles per second (cps).
Redshift
When light moves away from the observer, the wavelength increases, leading the viable light to shift towards the red end of the spectrum.
Blueshift
When light moves towards the observer, the wavelength decreases, leading the viable light to shift towards theblue end of the spectrum.
comets
a celestial object consisting of a nucleus of ice and dust and, when near the sun, a "tail" of gas and dust particles pointing away from the sun. Comets also are remnants from the formation of the solar system, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions—stored in a sort of cosmic deep freeze. This is also the realm of the larger icy worlds, called dwarf planets.
Emission spectrum
a series of bright lines of particular wavelengths produced by a hot gas under low pressure - Discrete, non-continuous spectra are an observable result of the physics of atoms. Unlike a continuous spectrum source, which can radiate at an arbitrary frequency (just change the effective temperature), the electron clouds surrounding the nuclei of atoms have very specific energies dictated by quantum mechanics. Each element on the periodic table has its own set of possible energy levels. Electrons tend to settle to the ground state, so an excited atom with an electron in a higher energy level will emit a wave of light with that exact energy to allow the electron to fall into the ground state. *This energy corresponds to a specific color, or wavelength, of light, so we see a bright line at that exact wavelength.* We can observe emission lines in spectra from comets, nebula and certain types of stars.
Wavelength (λ) and frequency (f) are related because
all electromagnetic waves travel at the same speed. - To see how this works, imagine a parade in which everyone is forced by prevailing traffic conditions to move at exactly the same speed. You stand on a corner and watch the waves of marchers come by. First you see row after row of miniature ponies. Because they are not very large and, therefore, have a shorter wavelength, a good number of the ponies can move past you each minute; we can say they have a high frequency. Next, however, come several rows of circus elephants. The elephants are large and marching at the same speed as the ponies, so far fewer of them can march past you per minute: Because they have a wider spacing (longer wavelength), they represent a lower frequency.
space telescope
an astronomical telescope that operates in space by remote control, to avoid interference by the earth's atmosphere.
Doppler effect
an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.
Waves
any disturbance that transmits energy through matter or space. - pattern of disturbance repeats in a cyclical way. - any wave motion can be characterized by a series of crests and troughs
Meteor Falls and Finds
"Finds" are meteorites which were found on the ground unrelated to any sighting, due to the finder recognizing them to be clearly identifyable as being of nonterrestrial origin. "Falls" are meteorites which were seen to fall from the sky and which were tracked down successfully.
active optics
(control for sag) Active optics provides a way of deforming a mirror to compensate for its inherent lack of structural rigidity
Meteoroids, Meteors, Meteorites
*1. Meteoroids are formed from the collision of asteroids* - Boulder-sized and smaller pieces of rock and metal in space. *2. Falling to earth* - When a meteoroid enters Earth's atmosphere, it produces a fiery trail, and it is then called a meteor. *3. Make it to earth* - If parts of the object survive the fall, the fragments that reach Earth's surface are called meteorites.
equatorial mount
A telescope mount that rotates around the polar axis and tilts around the declination axis.
electromagnetic waves
- electromagnetic waves do not require water or air as a medium of travel: the fields generate each other and so can move through a vacuum (such as outer space) - all electromagnetic waves move at the same speed in empty space (the speed of light)
blackbody radiation curve
- represents the intensity of light per wavelength - blackbody radiation can be emitted by anything that emits light. - 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).
For visible light, our eyes perceive different wavelengths...
...as different colors! red, for example, is the longest visible wavelength, and violet is the shortest. The main colors of visible light from longest to shortest wavelength can be remembered using the mnemonic ROY G BIV—for Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
Limitations of a refracting telescope
1. Bubbles in glass 2. chromatic aberration 3. large lens sag One problem with a refracting telescope 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. Also, optical properties of transparent materials change a little bit with the wavelengths (or colors) of light, so there is some additional distortion, known as chromatic aberration. Each wavelength focuses at a slightly different spot, causing the image to appear blurry. In addition, 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. Finally, 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 gathering power
A measure of how much light a telescope gathers and brings to a focus.
reflecting telescope
A telescope that uses a curved mirror to collect and focus light - 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
refracting telescope
A telescope that uses convex lenses to gather and focus light
infrared telescope
A telescope, similar in operation to an optical telescope that is designed to detect infrared radiation. Because infrared radiation is emitted by warm objects, infrared telescopes need to be shielded from local heat sources, as by chilling them with liquid nitrogen or locating them in polar regions. Many are placed on high mountains or are mounted on balloons or satellites in order to place them above the lower atmosphere, where water vapor absorbs much of the incoming infrared radiation.
seeing
A term used to describe the ease with which good telescopic observations can be made from Earth's surface, given the blurring effects of atmospheric turbulence. - *bad seeing:* When seeing is bad, images of celestial objects are distorted by the constant twisting and bending of light rays by turbulent air.
electromagnetic radiation as particles (photons) rather than as waves
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." - Thus, each photon carries a certain amount of energy that is proportional to the frequency (f) of the wave it represents. The value of its energy (E) is given by the formula: *E=hf* where the constant of proportionality, h, is called Planck's constant. - Higher-energy photons correspond to higher-frequency waves (which have a shorter wavelength); lower-energy photons are waves of lower frequency.
how light from a source moves through space.
As waves expand, they travel away from the source, not just toward your eyes but in all directions. They must therefore cover an ever-widening space. Yet the total amount of light available can't change once the light has left the source. This means that, 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. - As light (and all other electromagnetic radiation) leaves its source, its covering distance will double while its intensity will decrease
Angular size of objects
Astronomers use angular measure to describe the apparent size of an object in the night sky. An angle is the opening between two lines that meet at a point and angular measure describes the size of an angle in degrees
Limitations of a reflecting telescope
Because they are normally open, the mirrors have to be cleaned. Also, unless the mirrors and other optics are kept at the same temperature as the outside air, there will be air currents inside the telescope that will cause images to be fuzzy.
Five planets known since ancient times those discovered since the telescope
Beside Earth, five other planets were known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—and two were discovered after the invention of the telescope: Uranus and Neptune
Radio telescopes
Devices used to detect radio waves from objects in space - Radio telescopes have to be much larger than optical telescopes because the wavelengths of radio waves are so much larger than the wavelengths of visible light.
Emission and Absorption of Photons
Emission is the process of elements releasing different photons of color as their atoms return to their lower energy levels. Atoms emit light when they are heated or excited at high energy levels. The color of light that is emitted by an atom depends on how much energy the electron releases as it moves down different energy levels. When the electrons return to lower energy levels, they release extra energy and that can be in the form of light causing the emission of light. On the other hand, absorbed light is light that isn't seen. Absorption occurs when electrons absorb photons which causes them to gain energy and jump to higher energy levels.
Visable light spectrum
ROY G BIV - defined as having wavelengths in the range of 700-400 nanometers (nm) or one billionth of a meter. 1. *Red*: 620-700 nm 2. *Orange*: 590-620 nm 3. *Yellow*: 570-590 nm 4 *Green*: 495-570 nm 5. *Blue*: 450-495 nm 6. *Violet*: 400-450 nm
Meteor Showers
Forms when Earth passes through the tail of a comet or a cloud of dust left behind by a broken-up asteroid. Meteor showers occur every time Earth's orbit intersects that of a specific comet, and those intersections occur at fixed intervals.
Resolution
In addition to gathering as much light as they can, astronomers also want to have the sharpest images possible. Resolution refers to the precision of detail present in an image: that is, the smallest features that can be distinguished. One factor that determines how good the resolution will be is the size of the telescope. Larger apertures produce sharper images.
Structure of the Atom
In the nucleus is the protons and the neutrons, while on the outer rings are the electrons
Light as an electromagnetic wave - Maxwell
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. - Something similar happens when a raindrop strikes the surface of water or a frog jumps into a pond. The disturbance moves outward and creates a pattern we call a wave in the water (Figure 5.3). - Maxwell concluded that light is an electromagnetic wave having such wavelengths that it can be detected by the eye.
Light as Electromagnetic Radiation
Maxwell was able to calculate the speed at which an electromagnetic disturbance moves through space; he found that it is equal to the speed of light, which had been measured experimentally. - On that basis, he speculated that light was one form of a family of possible electromagnetic disturbances called electromagnetic radiation, a conclusion that was again confirmed in laboratory experiments. - When light (reflected from the pages of an astronomy textbook, for example) enters a human eye, its changing electric and magnetic fields stimulate nerve endings, which then transmit the information contained in these changing fields to the brain.
Planets in order from the sun
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Wavelength
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
Trans-Neptunian Objects (TNOs)
Objects orbiting the Sun beyond the orbit of Neptune - The first to be found, in 1930, was Pluto, but others have been discovered during the twenty- first century. One of them, Eris, is about the same size as Pluto and has at least one moon (Pluto has five known moons.) The largest TNOs are also classed as dwarf planets, as is the largest asteroid, Ceres. To date, more than 1750 of these TNOs have been discovered.
Kuiper Belt and Oort cloud
Origin of comets The Kuiper Belt is a belt-like ring of asteroids and dwarf planets just beyond the 8 planets that we normally think of as our solar system, and roughly in the same plane. It is similar to the Asteroid Belt found between the orbits of Mars and Jupiter. Pluto's orbit extends into the Kuiper Belt. The Oort Cloud is a huge spherical region way beyond that, extending halfway to the nearest stars. It is where most long-period comets come from. - Most of the bright comets that have been studied in the past (such as Hyakutake and Hale-Bopp) are Oort cloud comets
Doppler effect and its use to astronomers
The Doppler effect does not change the pattern of lines from a given element—it only shifts the whole pattern slightly toward redder or bluer wavelengths. The shifted pattern is still quite easy to recognize. Best of all, when we do recognize a familiar element's pattern, we get a bonus: *the amount the pattern is shifted can enable us to determine the speed of the objects in our line of sight.*
Best locations to place telescopes
The best observatory sites are therefore high, dark, and dry
Comets tails
The brightest part of the tail is called the dust tail, to differentiate it from a fainter, straight tail made of ionized gas, called the ion tail. The ion tail is carried outward by streams of ions (charged particles) emitted by the Sun. The smoother dust tail curves a bit, as individual dust particles spread out along the comet's orbit, whereas the straight ion is tail pushed more directly outward from the Sun by our star's wind of charged particles
Wien's Law
The higher the temperature of a body the shorter the wavelength of it's maximum radiation. (hotter = shorter wavelength)
terrestrial planets
The name given to the four inner planets: Mercury, Venus, Earth, and Mars. The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters, mountains, and volcanoes.
gas giants
The name given to the four outer planets: Jupiter, Saturn, Uranus, and Neptune. These jovian planets much larger and are composed primarily of lighter ices, liquids, and gases.
radiant (of a meteor shower)
The point in the sky from which the meteors of a shower appear to radiate.
Absorption spectrum
The range of a pigment's ability to absorb various wavelengths of light. - If light from a stellar core with a continuous spectrum encounters an atom, the wavelengths corresponding to possible energy transitions within the atom will be absorbed. The light may be re-emitted later, but as it will be re-emitted in a random direction the spectrum along the line of sight will be preferentially lacking in flux at the wavelength which corresponds to the energy transitions within the atom. We can observe absorption features in spectra from regions in space where a cooler gas lies between us and a hotter source, from stars, from planets with atmospheres, and from galaxies.
Stefan-Boltzmann law
The relationship stating that a blackbody object emits energy at a rate proportional to the fourth power of its temperature, in Kelvins. F = σT ^4 - F stands for the energy flux and σ (Greek letter sigma) is a constant number (5.67 × 10-8). - Increasing the temperature of a star would have a tremendous effect on the power it radiates. If the Sun, for example, were twice as hot—that is, if it had a temperature of 11,600 K—it would radiate 24, or 16 times more power than it does now. Tripling the temperature would raise the power output 81 times.
altazimuth mount
The simplest telescope mount; has a base that spins horizontally and tilts vertically.
Spectra
The spectra observed from galaxies are formed from a combination of stars, molecular clouds, and star-forming regions. We can examine them to determine many properties, among them the radial velocity of the galaxy, the star-formation rate, and the average age and metallicity of the stellar populations, and the kinematics (mass) of the galaxy. A spectrum has three components: the continuum, absorption lines, and emission lines.
spectral signatures
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 When the gas was pure hydrogen, it would emit one pattern of colors; when it was pure sodium, it would emit a different pattern. A mixture of hydrogen and sodium emitted both sets of spectral lines. The colors the gases emitted when they were heated were the very same colors as those they had absorbed when a continuous source of light was behind them. From such experiments, scientists began to see that different substances showed distinctive spectral signatures by which their presence could be detected. Just as your signature allows the bank to identify you, the unique pattern of colors for each type of atom (its spectrum) can help us identify which element or elements are in a gas.
Electromagnetic radiation's wave-like characteristics
The wavelength (λ) is the distance between crests, the frequency (f) is the number of cycles per second, and the speed (c) is the distance the wave covers during a specified period of time (e.g., kilometers per second).
spectral lines
The wavelengths where a specific element can absorb or emit light - 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.
Meteors and Meteorites
There are countless grains of broken rock, which we call cosmic dust, scattered throughout the solar system. When these particles enter Earth's atmosphere (as millions do each day) they burn up, producing a brief flash of light in the night sky known as a meteor (meteors are often referred to as shooting stars). Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is known as a meteorite.
Similarities between terrestrial planets and gas giants
They were all formed at roughly the same time 4.5 billion years ago. Both Jovian and Terrestrial planets orbit the sun. Both groups have magnetic fields. *Planet definition:* 1. A celestial body in orbit around the sun 2. Have sufficient mass for self gravity 3. Has cleared the neighborhood around its orbit
Eyepieces and magnification
To view the image formed by the lens in a telescope, we use an additional lens called an eyepiece. The eyepiece 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.
High-Energy Telescope
Ultraviolet, X-ray, and direct gamma-ray (high-energy electromagnetic wave) observations can be made only from space.
charge-coupled device (CCD)
array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum - In a CCD, 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 - 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. Among these are many small moons around the outer planets, icy dwarf planets beyond Pluto, and dwarf galaxies of stars. CCDs 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.
The formula for this relationship between Wavelength (λ) and frequency (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=λf
dust tail of a comet
forms when sunlight strikes the dust particles in the coma and the pressure exerted pushes the dust from the coma outward
Comets orbits are
highly elliptical
Continuum spectrum
light at all wavelengths - Continuous spectra (also called thermal or blackbody spectra) arise from dense gases or solid objects which radiate heat. They emit radiation over a broad range of wavelengths, thus the spectra appear smooth and continuous. Stellar cores emit light in a predominantly continuous spectrum, as do incandescent light bulbs, electric cooking stove burners, flames, fire embers, and ... you.
energy flux
power emitted per square meter
Asteroids
rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter. Most asteroids are remnants of the initial population of the solar system that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids. Tens of thousands of people witnessed directly the explosion of a smaller (20-meter) projectile over the Russian city of Chelyabinsk on an early winter morning in 2013. It exploded at a height of 21 kilometers in a burst of light brighter than the Sun, and the shockwave of the 0.5-megaton explosion broke tens of thousands of windows and sent hundreds of people to the hospital. Rock fragments (meteorites) were easily collected by people in the area after the blast because they landed on fresh snow.
Spectroscopes
spreads light into different wavelengths - Spectroscopy is one of the astronomer's most powerful tools, providing information about the composition, temperature, motion, and other characteristics of celestial objects.
Radio Interferometry
the largest radio dishes on Earth, operating alone, cannot make out as much detail as the typical small visible- light telescope used in a college astronomy lab. To overcome this difficulty, radio astronomers have learned to sharpen their images by linking two or more radio telescopes together electronically. Two or more telescopes linked together in this way are called an interferometer.
aperture
the opening through which light travels or reflects. - The amount of light a telescope can collect increases with the size of the aperture
adaptive optics
the optical elements of the telescope are instantaneously and continually adjusted to compensate for—in effect, to cancel out—the blurring effect of the Earth's atmosphere.
The Electromagnetic Spectrum
the range of wavelengths or frequencies over which electromagnetic radiation extends. *From shortest to longest waves:* *1. Gamma rays* - Less than 0.01 nm - Produced in nuclear reactions; require very high-energy processes *2. X-rays* - 0.01-20 nm - Gas in clusters of galaxies, supernova remnants, solar corona *3. Ultraviolet* - 20-400 nm - Supernova remnants, very hot stars *4. Visible* - 400-700 - From stars *5. Infrared* - 10^3-10^6 nm - Cool clouds of dust and gas, planets, moons *6. Microwave* - 10^6-10^9 - Active galaxies, pulsars, cosmic background radiation *7. Radio* - More than 10^9 - Supernova remnants, pulsars, cold gas
Doppler shift
the shift to a different wavelength
We can use the Doppler effect equation to calculate the radial velocity of an object if we know three things:
the speed of light, the original (unshifted) wavelength of the light emitted, and the difference between the wavelength of the emitted light and the wavelength we observe
What determines the type of electromagnetic radiation emitted by the Sun, stars, and other dense astronomical objects?
their temperature. - The temperature of something is thus a measure of the average motion energy of the particles that make it up.
