PHYS 20083 - Summer 2001 Complete Study Guide

Physics 20083 - Complete Study Guide

(1): Be able to define wavelength, frequency and photon. Given the relationship between frequency, speed and wavelength (the wave equation) and the relationship between photon energy and wavelength, be able to identify regions of long and short wavelength, regions of high and low frequency, and regions of high and low photon energy on a spectrum (a graph of intensity vs wavelength).
(2): As an object heats up, two things happen to the continuous radiation it emits: First, the overall intensity of radiation increases. Second, the peak wavelength of the radiation gets shorter. Explain qualitatively why both of these occur. Given a continuous radiation spectrum for an object of a certain temperature, be able to sketch the spectrum of an object that is slightly warmer or cooler based on these principles.
(3): How do we use the information from question 2 to estimate the temperatures of stars? Why do objects appear more red, then more yellow, then more blue as they get hotter?
(4): Use a graph of continuous radiation to explain why it is easier to see the photons emitted by the human body in the infrared region of the spectrum as opposed to the visible region of the spectrum.
(5): Explain why atoms can only absorb or emit certain, specific photon energies (wavelengths). Explain what happens when an atom absorbs or emits a photon.
(6): Given a simplified energy level diagram, be able to tell which energies can (or cannot) be absorbed/emitted by an electron in the atom in question.
(7): Explain how we use the principles of atomic emission and absorption to deduce the composition and abundance of different elements in clouds of gas, stars, etc.
(8): Be able to draw an intensity vs wavelength graph of a typical continuous spectrum, an emission line spectrum and an absorption line spectrum. Understand the relationship between graphical spectra (plots of intensity vs wavelength) and pictures of spectra (like the three spectral pictures in figure 5-14 on page 115 of the text).
(9): Under what conditions do we see emission line spectra? Under what conditions to we see absorption line spectra? Describe exactly how an absorption line spectrum is generated.
(10): Explain the process of ionization. Given a simple energy level diagram (e.g. "E=0,5,7,13"), be able to answer questions like "List the energies that an electron in this atom can absorb from its position in the lowest energy level." Explain why ionization typically only happens to atoms in high temperature environments (why is the ionization fraction higher when temperatures are higher?).
(11): Explain the difference between line width and line strength. Explain why abundances depend on line strength. How can temperature and ionization affect the relationship between abundance and line strength? Why does line strength tend to decline for most atoms at extremely high temperatures?
(12): Explain how atoms like Hydrogen tend to have weaker absorption line strengths at very low temperatures. How do we use this information and the relationships from question 11 to estimate the temperatures of stars?
(13): Know the two rules associated with Doppler shift (redshift/blueshift and radial velocity proportional to shift) and be able to apply them to real examples of spectra. Know the difference between radial and transverse velocity. If I show you a "rest" spectrum and a couple of other comparison spectra, be able to state whether the comparison objects are moving toward or away from us and which one is moving faster.
(14): If star A is 100 light years away and star B is 200 light years away (neither star moving relative to us), will the light from one star be shifted relative to the other (and if so, will the light be blueshifted or redshifted)? Explain your answer.
(15): Explain why spectral line widths are proportional to the temperature of a gas.
(16): What is the photosphere of the Sun? Explain the concept of limb darkening. What does limb darkening tell us about the temperature structure of the Sun's photosphere?
(17): Suppose the temperature of the photosphere were constant throughout (instead of the current situation in which it is slightly cooler as you look closer to the surface). Would we still experience the limb darkening phenomenon when we look at the Sun? Explain. (TQ #1)
(18): What is the chromosphere? Why does it cause absorption lines to appear in the Sun's spectrum?
(19): If we were to observe the Sun's corona during a total solar eclipse (the photosphere is completely blocked by the Moon), what sort of spectrum would we see? (emission, absorption, continuous) Explain. (TQ #2)
(20): Define ionization species. Explain how different species of ions are used to estimate the temperature of clouds of gas. How do the ionization species of atoms change as one looks further away from the Sun's surface in the corona? What does this tell us about the temperature structure of the corona?
(21): How do the spectral emission line widths change as one looks further away from the Sun's surface in the corona? What does this tell us about the temperature structure of the corona?
(22): Explain why, although coronal gas has a very high temperature in the vicinity of the Earth, the Earth actually absorbs very little heat energy from this gas.
(23): Explain how the principle of "conservation of energy" predicts that temperatures in the low-density gas of the corona will be much higher than in the high-density gas of the photosphere/chromosphere. As one looks further away from the surface of the Sun, the temperature of the corona (the average energy of each gas particle) increases. How would you expect the density of gas changes as you look further away from the Sun? Explain.
(24): State and briefly explain the equation we use to estimate the lifetime of the Sun (based on the total fuel available and the luminosity). Be able to describe a similar example, such as a car ("tank holds 20 gallons, fuel burns at a rate of 4 gallons/hr, how many hours does the fuel last?")
(25): Explain why chemical and gravitational energy were not accepted as viable methods for energy generation in the Sun's core.
(26): Explain briefly how fission and fusion reactions generate energy. Where exactly does the energy come from in these reactions?
(27): Explain why both high temperature and high density are needed in order for fusion reactions to take place. While the Sun's corona has temperatures of millions of degrees, similar to temperatures in the core, no fusion occurs there. Why do you think this is so?
(28): Based on your reading of Philosophy and the Scientific Method, answer the following: What is the most significant concept that separates scientific and non-scientific theories? The fact that non-scientific ideas can't be disproven means they aren't useful as a basis for a system of knowledge. Why not? What is the problem with relying on irrefutable beliefs? Also, why is relying on refutable (scientific) reasoning so useful? After all, the theory might be wrong. A total of 4-5 sentences should be more than sufficient to answer these questions, if you are concise. (TQ #3)
(29): What is the "solar neutrino problem"? Despite this inconsistency with theory, most scientists still believe that nuclear fusion is the source of energy for the Sun's core. Explain why this theory hasn't been abandoned, as the scientific method suggests it ideally should.
(30): Read the following web site detailing the solar neutrino problem (SNP): http://www.maths.qmw.ac.uk/~lms/research/neutrino.html, and summarize the "physical" and "astrophysical" solutions to the SNP in 1-2 sentences each. (TQ #4)
(31): Based on the above website (there's a link to this information embedded in the text), explain the "Helium-3 Instability" in your own words (3-4 sentences, you don't have detail every single step, just a broad outline) and explain how this might be affecting climate on Earth over time. (TQ #5)
(32): How is the core of the Sun defined? What is the envelope of the Sun?
(33): Explain what happens in the radiative zone of the Sun. Explain what happens in the convective zone of the Sun. Why is energy transported differently in the convective zone? Why does energy ultimately leave the Sun in the form of light/radiation?
(34): If we were to increase the core temperature of the Sun by 10-20%, what would happen to the boundary between the radiative and convective zones? Would you expect it to remain in the same general place, move inward or move outward? Explain your answer.
(35): Read the following website from Scientific American about the concept of "cold fusion" and answer the questions that follow (there are three different essays about this ... it will help give you a good perspective if you read all three): http://www.scientificamerican.com/askexpert/physics/physics6.html. In 2-3 sentences, describe what "cold fusion" is and how it works. There have been dozens of claims since the initial claim by Pons and Flesichmann that experiments have shown "cold fusion" processes are releasing energy from Hydrogen atoms. What are two reasons these claims are not believed by the majority of the scientific community? (TQ #6)
(36): What are the four main functions of a telescope we discussed in class. Briefly define each with a simple sentence, and then state how the aperture of the telescope affects each one.
(37): What causes stellar images to appear blurred when we look at them through ground-based telescopes? Briefly explain how adaptive optics works to correct this problem.
(38): What is the difference between a reflecting and a refracting telescope? Why are reflecting telescopes more popular among professional astronomers, at least one reason? Why do astronomers use instruments attached to telescopes to gather light instead of looking through an eyepiece with their eye, at least two reasons?
(39): Given the resolution equation, explain why it is that even though radio telescopes have much larger aperture diameters than optical telescopes, the typical resolution achieved when observing with radio telescopes is very poor.
(40): Explain qualitatively how interferometry works to improve the resolution of radio telescopes. Why isn't interferometry practical for most optical telescopes yet?
(41): Explain how to use parallax to determine the distance to a nearby star. Given the parallax equation, be able to answer questions like "For a star at a given distance, if we observe using a longer baseline, explain what will happen to the observed parallax angle. What if the baseline doubles and the star's distance is doubled?"
(42): Given the parallax equation, be able to draw simple diagrams that justify it. For example, be able to show why the parallax angle should increase as the baseline increases or why the parallax angle should decrease as the distance increases. Also, be able to explain why the parallax method is generally limited to the nearest stars.
(43): Know the definitions of apparent and absolute luminosity. Given the inverse square law equation, explain briefly how we use it to estimate the distances to stars.
(44): Describe how you could determine the distance to a star thousands of parsecs away, assuming that star has spectral characteristics identical to our Sun.
(45): Be able to solve simple proportionality problems with the inverse square law, such as "Star B has an absolute luminosity one-half that of star A, and star B is only one-half the distance of star A. How does the apparent luminosity of star B compare to that of star A?"
(46): Given the equation of absolute luminosity, be able to solve simple proportionality problems such as "Star B has a Radius (size) twice as large as star A. Star B has a surface temperature one-half that of star A. How does the absolute luminosity of star B compare with that of star A?"
(47): Visit the Hipparcos mission web site at http://astro.estec.esa.nl/Hipparcos/ and answer: What was the purpose of the Hipparcos mission? Check out http://astro.estec.esa.nl/Hipparcos/poster.html to help you explain two useful scientific results that came from this mission. 1-2 sentences each.(TQ #7)
(48): Visit the GAIA mission web site at http://astro.estec.esa.nl/GAIA/Science/Science.html and answer: What is the purpose of the planned GAIA mission? Explain two useful scientific results expected to come from this mission (look on the menu under "Scientific Topics" for details. 1-2 sentences each. (TQ #8)
(49): Given the equation relation density, mass and size for a star, be able to solve simple proportionalities, such as: "Star B is 4 times more massive than star A. Star B has a radius twice the size of star A. How does the density of star A compare to that of star B?"
(50): Explain why spectral line widths should depend on the density of a gas (whether a gas cloud or a stellar atmosphere).
(51): Given two spectral emission lines from two gas clouds of the same temperature and mass, be able to state which one probably has a smaller size based on the line width, and be able to explain your reasoning.
(52): If a given spectral line is very broad, how can we tell whether that width is due to a high temperature or a high density (or both)?
(53): Be able to put together different concepts regarding stellar properties. For example, "Explain how one could use apparent luminosity, line width and peak wavelength for a given star to estimate the distance to that star." Or "Suppose two stars have the same apparent luminosity and size (radius), but star A has higher ionization species in its spectrum compared to star B. Which one is probably further away? Explain." For more examples, look through old exams.
(54): Given an H-R diagram, be able to explain how we know stars in the "giant" region really are big, simply by using information about the stars' absolute luminosities and temperatures. Do the same for the "dwarf" region of the diagram.
(55): Given an H-R diagram, be able to plot roughly the positions of stars based on properties like size, temperature and absolute luminosity. For example, "Star X has 1/2 the surface temperature of the Sun but 20 times the radius. Plot (roughly) where this star would fall on the H-R diagram relative to the Sun's position."
(56): Imagine a sample of the 1,000 stars with the highest surface temperature. Would this sample be representative? Explain. Imagine a sample of the 1,000 stars with the highest apparent luminosity as seen from the Earth. Why would this sample not be representative of the stars in our galaxy?
(57): Imagine a sample of the 1,000 nearest stars to Earth. Would this sample be representative? Explain. What is the Copernican Principle, and how does it apply here?
(58): A currently important political topic is the issue of sampling in the 2000 U. S. Census. Should the census be an actual enumeration of people, or should statistically undercounted people be included? In 2-3 sentences each, summarize each side of this position. Several good documents on this issue can be found at http://www.louisville.edu/library/ekstrom/govpubs/goodsources/census/csample.html. (TQ #9)
(59): Given the equation of orbital velocity and the period equation, explain how we can use any two of the three basic quantities associated with an orbit (orbital distance, orbital velocity, orbital period) to determine the central mass of the object that is being orbited around by another object.
(60): When astronomers observe binary star systems edge-on, the spectral line fingerprints of the system sometimes seem to split, then merge, then split, then merge, etc. Explain (with the help of a simple diagram) what is going on in the system that causes this behavior in the spectrum.
(61): How do we use the spectral line shifts in a binary system to determine the orbital velocity? How does the orbital velocity of the companion relate to the shift of its spectral lines?
(62): Explain how we determine the period for the orbit of the companion star. Given the equation of orbital velocity and the period equation, explain how we use the period and the spectral line shifts to estimate the mass of the central star in an edge-on binary system.
(63): Given the two orbit equations, be able to answer simple proportionality questions such as: "Star A and Star B have the same orbital distances to their companion stars, but star A's companion shows large Doppler shifts compared to star B's companion. Which star is probably more massive, star A or star B?" or "Star A and star B have the same mass, but star A's companion shows a smaller Doppler shift compared to star B's companion. Which star's companion is probably at a larger orbital distance, star A's companion or star B's companion?"
(64): Be able to answer simple proportionality questions having to do with both the period equation and the equation of orbital velocity, such as: "Suppose the companions star A and star B show the same Doppler shifts, but star A's companion has a period twice as long as star B. Which of the two central stars is more massive? Explain or show your work."
(65): How do we use light curves to help determine whether or not a binary system we are observing is truly being seen edge-on or instead at some inclination angle? What causes the apparent luminosity of an eclipsing binary to change? What would a light curve look like for some Astronomer in another solar system looking at the Sun?
(66): Suppose we are assuming that a binary system is being seen edge-on. We measure the maximum Doppler shift and from that estimate the orbital velocity, then we find the orbital distance using the angular size method (as opposed to using the period equation). We use this information to estimate the mass of the central star. Later, another Astronomer double-checks our data, discovering that while our orbital distance measurement is correct, the system is not eclipsing! The fact that this system is tilted away from being edge-on means that our estimate of the mass is incorrect. Is our estimate higher than the true mass or lower than the true mass? Explain.
(67): What is the difference between the terms "larger" and "more massive"? Are larger stars necessarily more massive? Explain.
(68): Astronomers have learned that there is a close relationship between the masses and luminosities of typical (main sequence) stars. Explain why we believe this relationship exists.
(69): Given the mass-luminosity relation, be able to relate this to the method we use for estimating the lifetimes of stars. Use this method to help explain why more massive stars (despite the fact that they have more "fuel" to burn compared to the Sun) live shorter lifetimes.
(70): What is the difference between gas and dust? What sort of light does each emit and why?
(71): The Interstellar Medium (ISM) has two effects on stars, extinction and reddening. Explain why stars appear redder when seen through a cloud of gas and dust.
(72): Explain why the Sun appears red when near the horizon. Also, explain why the sky is blue. For supporting material in the book, you might want to check out page 352 in chapter 15.
(73): Suppose we don't take into account the reddening of starlight by the ISM. How will our estimate of the surface temperature of the star differ from the true surface temperature? How will out estimate of the absolute luminosity differ? What about our distance estimate? You may answer either quantiatively (using the equation of absolute luminosity and the inverse square law) or qualitatively.
(74): Given the modified form of the inverse square law (with the ISM correction term "X" included), explain how we find X. Explaining how we determine the type and number of gas atoms along our line of sight (how and why do ISM spectral fingerprints different from the fingerprints of gas in a stellar atmosphere?).
(75): This question regards the fictional element "nebulium" that some scientists once thought might be real. This is not discussed in your book, so you'll have to do some research on your own on the web (and I'm not providing links this time...you're on your own). Answer the following questions: What led scientists to think nebulium was a previously unknown element? What is nebulium, and why weren't we able to originally identify it correctly? What are forbidden lines, and why do we not see forbidden lines in typical laboratory spectra while we see them all the time in interstellar gas clouds (and in our upper atmosphere during aurorae). (TQ #10)
(76): Read the following article about organic molecules in interstellar clouds, http://www.sciam.com/1999/0799issue/0799bernstein.html, then answer the following: How do we know that roughly 30 tons of complex organic molecules are "raining" down into our atmosphere? How do we think these molecules originated in molecular clouds, and what does star formation (and ultraviolet light from new stars) have to do with it? What role might these molecules play in an origin of life scenario? What implications does this research have on the possibility that life might exist elsewhere outside our solar system? (TQ #11)
(77): Read the following article about Brown Dwarf stars, http://www.sciam.com/2000/0400issue/0400basri.html, then answer the following: What is a brown dwarf? How and why do the spectra of brown dwarfs differ from other stars? What is the brown dwarf desert? Perhaps the solution to the "brown dwarf desert" problem has to do with biased vs representative samples. Explain how previous studies may have been biased against finding brown dwarfs (especially since broad random surveys seem to turn up brown dwarfs just as often as other kinds of low mass stars). (TQ #12)
(78): Read the following brief article about the Faint Sun Paradox, http://earthsky.com/2000/es000226.html and then state the basic paradox and how the Earth has "resolved" this paradox through changes in its atmosphere. (TQ #13)
(79): In a luminosity function, the number of stars in a representative sample seems to decrease as one looks for higher and higher luminosities. Explain why this happens.
(80): What is pressure equilibrium. Given the pressure equation, explain why the condition of pressure equilibrium leads to the inverse relationship between density and temperature in the ISM.
(81): What is self-gravity? Given the equation for self-gravity, explain why the self-gravity of a typical molecular cloud (typically thousands of times more massive than a star like our Sun) is so small. Explain how a triggering event (such as a nearby supernova explosion) can change an object's self-gravity, initiating star formation. Given two clouds of the same mass but different sizes (or same size but different mass) but able to tell which has stronger self-gravity.
(82): During the collapse of a molecular cloud, name and explain what is happening to the self-gravity, the density, the temperature and the outward-pushing pressure of the cloud.
(83): Why do stars have a minimum mass? In other words, why is it that objects with masses smaller than about 8% of the mass of the Sun never form into stars? What stops the collapse of these objects? What stops the collapse of clouds with masses greater than about 8% of the mass of the Sun?
(84): What is hydrostatic equilibrium (HSE)? During the main sequence lifetime of the star, the temperature of the core slowly increases. Explain why this occurs and how this affects the absolute luminosity of the star and the overall size of the star (via HSE and increased outward pushing pressure).
(85): During the main sequence lifetime of the star, what happens to the Helium in the core? Explain how the self-gravity of the star is affected in terms of HSE by the behavior of the Helium in the core, and explain what effect this in turn has on the star's size.
(86): In the Sun's core, Helium fusion will begin eventually but only after the density and temperature reach a much higher value than they needed to for Hydrogen fusion. Explain in detail two reasons why Helium fusion requires higher density and higher temperature compared to Hydrogen fusion.
(87): At the end of the sun's main sequence (Hydrogen-burning) phase, it will have grown to such an enormous size that it will turn red in color instead of its characteristic yellow. Explain why the Sun will change color like this.
(88): The Hydrogen-burning phase of the Sun's lifetime will last for about 10 billion years, and the Helium-burning phase will go much faster, lasting for perhaps only about 1 billion years. The same ratio generally holds true for all stars. Use this fact to explain why, in a representative sample of stars, we find about 90% of them on the main sequence at any given point in time.
(89): Explain how the H-R diagram of a cluster evolves over time. How do these changes confirm our ideas about the relationship between stellar mass and main sequence lifetime?
(90): What is the "turnoff point" on a cluster H-R diagram? Assuming we know the mass of a star at the turnoff point, how and why can we use that information to estimate the age of the cluster? For example, we know that the Sun has a main sequence lifetime of 10 billion years. Suppose a cluster H-R diagram has a turnoff point with a star of about 0.9 solar masses. Is this cluster younger or older than 10 billion years? Explain.
(91): Explain what a planetary nebula is and why (generally) it occurs. Why is this fate suffered mainly only by low-mass stars instead of high mass stars?
(92): What is different about Iron fusion compared to the fusion of other, lighter elements? Why does the onset of iron fusion signal the end of the lifetime of even the most massive stars?
(93): With a simple diagram and 2-3 sentences, briefly explain how a pulsar works. What distinguishes neutron stars from pulsars? In other words, why do some neutron stars appear to pulse as seen from Earth while others do not? You do not need to describe in any detail the physics behind the "jet" of radiation at the magnetic poles, just know that it is there (though it is similar in many respects to the physics that direct solar wind particles to points near the Earth's magnetic poles, generating aurorae at high northern and southern latitudes).
(94): Virtually every atom of gold on the Earth (including in jewelry) was originated inside a supernova billions of years ago. Explain how we know this, and explain roughly how the gold got from inside the star to the Earth.
(95): Given the definition of angular momentum (and given that it is conserved), why do neutron stars tend to rotate so quickly?
(96): What is a black hole? How do we use the concept of escape velocity to define the boundary (event horizon) of a black hole?
(97): Define metal and metallicity. How and why does the metallicity of the ISM change over time?
(98): Does the metallicity of a main sequence star change over time? Why or why not? How does the metallicity of an old main sequence star compare to the metallicity of a young main sequence star, and why would you expect them to be different?
(99): Be able to answer (with explanations) questions comparing the metallicity of two stars, such as "Star X is the same size, mass and temperature as the sun, but it is about 3 billion years older. Which has a higher metallicity, X or the sun?", "Star X is a main sequence star with an unknown age that has a mass ten times that of the Sun. Which has a higher metallicity, X or the sun (or can you not tell)?" and "Star X is a main sequence star with an unknown age that has a mass one half that of the Sun. Which has a higher metallicity, X or the sun (or can you not tell)?"
(100): Explain step-by-step how to use the Cepheid Period-Luminosity (Absolute), or P-L relation to find the distance to a star cluster or distant galaxy.
(101): Be able to answer simple mathematical questions about the P-L relation, such as "Cepheids X and Y have the same apparent luminosity, but star X's period is twice as long as star Y's period. Assuming no interstellar corrections, which one is further away?" Be able to identify periods of Cepheids by looking at their light curves.
(102): How do forbidden line electron transitions differ from ordinary electron transitions? Explain in 1-2 sentences why forbidden electron transitions only tend to occur in very low density gas. (TQ #14)
(103): Look on the web for information about the Next Generation Space Telescope (NGST), the planned successor to the Hubble Space Telescope, and answer the following: Why is it useful to have telescopes above the Earth's atmosphere when we have such advanced equipment on the ground? What wavelengths of light will the NGST observe, and why are these wavelengths important? What are two major things we hope to learn about with the NGST, and why will the NGST be better equipped to answer these questions than other telescopes? (TQ #15)
(104): Describe how William Herschel discovered infrared light. What two gases are largely responsible for the absorption of infrared light in our atmosphere (making it difficult to observe infrared light coming from stars, nebulae and galaxies from the Earth's surface)? List at least two examples of objects that emit most of their light in the infrared portion of the spectrum. (TQ #16)
(105): Read the following article entitled "The Ghostliest Galaxies" at http://www.sciam.com/specialissues/0398cosmos/0398bothun.html and answer the following: How did astronomers detect these Low Surface Brightness (LSB) galaxies, since ordinary starlight from them is not bright enough for conventional techniques? How do the numbers of LSB galaxies compare to the number of ordinary galaxies, and why do we think there might be even more that we can't detect? Finally, how do we know that these galaxies haven't evolved very much since the beginning of the Universe? (TQ #17)
(106): Do some research on the World Wide Web regarding the "Anthropic Principle" and define two versions of it (Weak, Strong, etc) in your own words. Give three examples of an "Anthropic Coincidence", the kind of thing that leads people to propose the Anthropic Principle in the first place. (TQ #18)
(107): What are gravitational waves? Explain how scientists hope to detect them with a gravitational wave observatory. (TQ #19)
(108): How and why do the colors of star clusters change over time? Would you expect a cluster with high metallicity to have a blue color or red color? Explain.
(109): Explain why the disk of our galaxy contains virtually all of the new stars that are forming in the galaxy, and also explain why the disk (especially the spiral arms) tends to have a blue color.
(110): How do colors of individual stars change over the course of the main sequence lifetimes? If you see a solitary blue star, what can you say with confidence about its age? If you see a solitary red star, what can you say with confidence about its age? Explain.
(111): How do we know that there is a supermassive black hole at the center of our galaxy? Explain how tidal forces tend to tear apart stars that wander too close to the black holes. Why don't tidal forces from the Earth tear us apart? Why don't tidal forces from the Sun tear planets apart?
(112): What is 21-cm radiation? Why is it so important that Hydrogen is able to emit this radiation for our study of the galaxy? Would we be able to see Hydrogen if it couldn't emit this sort of radiation? Explain how we use observations of Hydrogen's 21 cm emission line to map the motions of the disk of our galaxy.
(113): Given the equation of orbital velocity, be able to explain the basic shape of the Keplerian rotation curve. Also, be able to explain the shape of the solid-body rotation curve.
(114): Explain why the extended nature of the galaxy's mass leads us to expect the rotation curve of the galaxy will have a slightly different shape than Keplerian (orbital velocities slightly higher).
(115): Explain how the flat rotation curve of our galaxy leads us to believe that the galaxy has a very large amount of dark matter, much more than the visible matter in stars, gas and dust that we can easily see.
(116): Suppose the amount of dark matter in the Milky Way Galaxy were 10 times more than it is currently. Would it be possible in such a case to get a *rising* rotation curve (more similar to a solid-body curve)? Explain why or why not.
(117): One possible candidate for the dark matter in our galaxy is very faint stars, about 1/10th the mass of the Sun, stars that aren't ordinarily visible to telescopes on the Earth. How do we know that these probably do not account for the dark matter?
(118): Explain gravitational lensing, or the effect in which the presence of mass alters the path of a beam of light.
(119): Explain how microlensing works. What happens to the appearance of a star during a microlensing event? How does this variation in a star differ from other kinds of variations (eclipsing binaries, Cepheid variables)?
(120): How do we use microlensing to estimate the amount of dark matter in the galaxy consists of planet-sized objects (MACHOs)? Do planets constitute any of the dark matter? How do we know?
(121): How will scientists conclude whether or not massive, solitary black holes make up a significant portion of the dark matter? Explain why these are more difficult to find than MACHOs and how we would go about finding them.
(122): Describe the initial collapse of the Milky Way galaxy from a roughly spherical cloud into a disk of gas and dust. Why did the galaxy collapse into a disk shape instead of into a small point?
(123): Why did the stars that formed during the collapse of the galaxy not become a part of the disk? Explain the two different types of collisions that we discussed in class and how this implies that stars and clusters that form in the halo will remain in the halo.
(124): Explain why the vast majority of the stars in the halo have red colors.
(125): Explain what a density wave is and how it contributes to star formation in the spiral arms. Even though the spiral arms contain a few percent more mass than the rest of the disk, they put out most of the light and are very easily visible compared to the rest of the disk. Explain why.
(126): Explain the merger hypothesis (that describes why ellipticals are different from spirals and how ellipticals form). List the general properties of spirals vs ellipticals and discuss how these are consistent with the merger hypothesis (structure and color...mergers tend to trigger lots of star formation, depleting the gas content of a galaxy all at once).
(127): Elliptical galaxies tend to be found near the centers of galaxy clusters. Explain how/why this fact is consistent with the merger hypothesis.
(128): Explain why the Cepheid P-L relation as a method of distance determination is distance limited. In other words, why doesn't it work beyond a certain distance from the Earth?
(129): Discuss briefly how the standard candle method of distance determination works. Explain why individual stars are not useful as standard candles for finding distances to galaxies that are very far away.
(130): Although star clusters and galaxies have varying absolute luminosities, it is possible to use them as standard candles if one is careful about selecting them (by taking advantage of the natural range of properties of galaxies, in absolute luminosity or size). How is it possible to pick two galaxies and have some notion that they have similar absolute luminosities? What is the reasoning behind this technique?
(131): Explain how the standard ruler method of distance determination works. Why is this method unreliable?
(132): Explain what the Tully-Fisher (TF) relation is. Describe briefly how one would use the TF relation to find the distance to an edge-on galaxy.
(133): Explain how the inclination angle can affect our distance estimate. For example, suppose we fail to take the inclination angle of a galaxy into account so that the radial velocity we measure for that galaxy's rotation is smaller than the true rotation speed. Will we underestimate or overestimate the distance to that galaxy? Explain.
(134): Explain why the TF relation is distance limited. In other words, why doesn't it work as a distance determination technique beyond a certain distance from the Earth?
(135): What is Hubble's Law? How can it be used to find the distances to galaxies?
(136): Be able to plot a graph of a car race at various times as we did in lecture given some basic data. Given a graph of a car race, be able to find the slope of the graph and the age of the race.
(137): What is the Hubble constant? A Hubble constant of 70 implies that the age of the Universe is 10 billion years. What if the Hubble constant were recalculated to be 50...how would our estimate of the age of the Universe change (younger or older)? Explain.
(139): Assuming gravity is slowing down the expansion of the Universe, how would you expect Hubble's Law to deviate from a straight line in the presence of a little bit of gravity or a lot of gravity? Explain why the graph changes shape, using the concept of lookback time.
(140): Explain how current observations of galaxy radial velocities and distances compare to the expectations of Hubble's Law under the influence of gravity. Explain why these observations lead us to believe that the Universe is accelerating away from us in all directions.
(141): Explain what the Cosmological Constant is (and why it is relevant in light of recent observations referenced in 154). Why did Einstein originally introduce the idea of a Cosmological Constant? Why was it later abandoned?
(142): Why do supernovae make such great standard candles? Why are they so difficult to observe (so much so that they have only been observed in large numbers in the past 10 years or so)?
(143): How and why would you expect the observed metallicity of galaxies to change as one looks further and further away from the Earth?
(144): How and why would you expect the number of spirals relative to the number of elliptical galaxies to change as one looks further and further away from the Earth?
(145): Explain how we know that quasars (QSO's) are hundreds of times more luminous than a typical galaxy (start by talking about redshift, Hubble's Law and then use the inverse square law)?
(146): How do we know that QSO light does not come from typical stars or galaxies, and how do we know that QSO's are so small compared to a typical galaxy? Explain.
(147): Why do we think QSO's are only found far away from the Earth? How do we think QSO's relate to galaxies?
(148): What is the visible horizon? Why can't we see anything outside of our visible horizon?
(149): What is the Microwave Background Radiation (MBR)? Where does it come from? Why does it originate just inside our visible horizon?
(150): Explain why the current structure of the Universe leads Astronomers to believe that the MBR must have a lumpy nature, with measurable fluctuations.
(151): Explain how the existence of the MBR, the shape of its spectrum and its fluctuations are seen as convincing evidence of the Big Bang theory. What is different about this evidence in favor of the Big Bang compared to Hubble's original observations? Why are the observations of the MBR really seen as the key to confirming the Big Bang instead of just relying on Hubble's Law?
(152): Although most Astronomers are confident that the Big Bang theory is likely the correct model to explain the observational facts we studied, this theory still has some minor but important inconsistencies. Why are Astronomers so confident of the Big Bang theory and skeptical about inconsistencies in the Big Bang?
(153): Explain why the MBR looks redder (cooler) in one direction and warmer (bluer) in the opposite direction, not taking into consideration the much smaller fluctuations discussed in 151.
(154): Explain Olbers' paradox. Explain why a Universe that is either finite in space or time (or both) saves us from having a bright night sky. Explain how an expanding Universe contributes to the fact that the night sky is dark.
(155): Read http://map.gsfc.nasa.gov/m_mm.html about the Microwave Anisotropy Probe (MAP) mission and answer the following based on the links within: What exactly is MAP going to measure? Describe a fluctuation spectrum in your own words. What do we hope to learn from this spectrum (give 2 examples)? (TQ #20)
(156): How would you expect the abundance of Hydrogen relative to other elements to look if you plotted it for galaxies at various distances from Earth, as in question 143? Would the abundance of Hydrogen increase with increasing redshift? Decrease? Stay the same? Explain. (TQ #21)
(157): What would Hubble's Law look like if the Universe were not expanding? What would it look like if the Universe were contracting? Draw your best guess graph and explain it. (TQ #22)
(158): How do we know whether or not the Universe will expand forever or at some point turn around and collapse? How do we try to answer this question? How and why is the study of dark matter important?
(159): What is the critical density? How and why does the relationship between the density of matter in the Universe and the critical density determine the ultimate fate of the Universe (whether it will expand forever or collapse)?
(160): Explain why effectively no fusion occured prior to a time when the Universe was about 1 second old. Explain why nucleosynthesis ended when the Universe was about 3 minutes old.
(161): Explain why the density of matter in the Universe has a bearing on how much Helium was created during nucleosynthesis in the first three minutes of the Universe's existence. If the Universe had been more dense at the time, would more Helium have been created? Less? The same amount? Explain.
(162): Explain the horizon problem. How does the theory of inflation "solve" the horizon problem?
(163): Explain the flatness problem. How does the theory of inflation "solve" the flatness problem? What prediction does inflation make about the value of Omega, the ratio of the density of the Universe to the critical density? Explain.
(164): Explain how one can use galaxy counts at a variety of distances as a way to estimate the matter density of the Universe. Why should galaxy counts depend at all on the overall density of the Universe? If the density of the Universe is very high, will galaxy counts tend to rise faster with distance or slower with distance compared to the "flat" Universe case? Explain.
(165): Visit http://www.skypub.com/news/special/seti_toc.html and answer the following questions about SETI: What is the Fermi paradox, and what are two possible resolutions of this paradox? (TQ #23)
(166): On the same web site, find out why Astronomers are beginning to think that only searching in the radio region of the spectrum for SETI signals might not be a good idea. Describe the arguments in favor of Optical SETI searches and why other civilizations might try to use optical wavelengths instead of radio to communicate with us. (TQ #24)
(167): What is SETI? What does "N" stand for in the Drake Equation? Briefly explain the broad principles of the Drake Equation and how we attempt to use it to answer the question of how successful searching for Extraterrestrial Intelligence will be.
(168): Why do scientists tend to be very skeptical of UFO reports and the like (excluding the "economic" argument asked in the second part of this study guide question)? Why do we expect that our first contact with alien intelligence, should it ever occur, will be via some sort of light/radio communication rather than a physical visit by a spaceship of some kind?
(169): Explain the implications to SETI if scientists verify the existence of life on Mars and/or Jupiter's moon Europa.
(170): Why is "Lifetime" an important factor in the Drake equation? Explain how it affects the "detectability" of a civilization.
(171): When looking for extraterrestrial signals, how do we decide what general region of the spectrum to search? A graph would help here.
(172): Two other factors that must be considered in searching for signals are sensitivity and sky coverage. Explain the trade-off between these two (in other words, given a limited amount of time, what happens to your sensitivity as you increase sky coverage and why?).
(173): Explain why we must use very narrow channels in order to search for ET signals. What is the drawback to using lots of very narrow channels as opposed to a few large channels?