PHYS 20083 - Spring 2002 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 its spectrum. First, the overall intensity increases as more photons are emitted by the objects. Second, the peak wavelength of the spectrum shifts to a shorter wavelength. Explain why the peak wavelength shifts. 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): 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." or "List the energies that an electron in this atom can emit from its position in a certain energy level."
(7): Under what conditions do we see emission line spectra? Under what conditions do we see absorption line spectra? Describe exactly how an absorption line spectrum is generated.
(8): Explain how we use the principles of atomic emission and absorption to deduce the composition of different elements in clouds of gas, stars, etc.
(9): 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.
(10): 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. What if star A is moving away from us at 100 meters/sec, and star B is moving away from us at 50 meters/sec? Will both stars appears shifted? One more than the other? Explain.
(11): 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?")
(12): Based on the reading Cargo Cult Science, imagine your task is to explain to a South Sea islander why planes don't land there anymore even though the islanders are going through the proper motions that have made planes land there in the past. Come up with your own honest explanation of why this is true in three or four sentences (which, not coincidentally, is about the length of an average exam answer).
(13): Based on the reading Cargo Cult Science, explain in your own words (2-3 sentences) why Mr. Young's rat-running experiment was a perfect example of the scientific method compared to most other rat-running experiments. (TQ #1)
(14): Explain why chemical and gravitational energy were not accepted as viable methods for energy generation in the Sun's core.
(15): Explain how fusion reactions work and how they generate energy. Why is fusion considered to be a viable source for energy generation in the Sun?
(16): 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.
(17): 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 #2)
(18): 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 #3)
(19): 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? 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 #4)
(20): Explain why both high temperature and high density are needed in order for fusion reactions to take place.
(21): How is the core of the Sun defined? What is the envelope of the Sun?
(22): 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?
(23): 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. (TQ #5)
(24): 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)
(25): 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?
(26): 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 #7)
(27): What is the chromosphere? Why does it cause absorption lines to appear in the Sun's spectrum?
(28): 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.
(29): 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?
(30): Why are spectral line widths proportional to the temperature of a gas? 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?
(31): 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.
(32): 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?
(33): 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 #8)
(34): 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?"
(35): 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.
(36): 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.
(37): Describe how you could determine the distance to a star thousands of parsecs away, assuming that star has spectral characteristics identical to our Sun. Suppose you want to find the distance to Rigel (too far for a parallax measurement), and you know there is a star similar to Rigel very close to us (close enough for a parallax measurement). How could you use the available information to find the distance to Rigel?
(38): 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?"
(39): 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?"
(40): Visit the GAIA mission web site at http://astro.estec.esa.nl/SA-general/Projects/GAIA/gaia.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 #9)
(41): 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).
(42): 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?"
(43): Explain why spectral line widths should depend on the density of a gas (whether a gas cloud or a stellar atmosphere).
(44): 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.
(45): 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)?
(46): 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.
(47): Explain the difference between line width and line strength.
(48): Explain how atoms like Hydrogen tend to have weaker absorption line strengths at very low temperatures and very high temperatures. If a star has a very weak Hydrogen line, how could we determine whether that is due to a high or a low temperature or just a low abundance of Hydrogen?
(49): 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.
(50): 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."
(51): Given the equation of orbital velocity and the period equation, explain how we can use the orbital distance and orbital period to determine the central mass of the object that is being orbited around by another object.
(52): What is the difference between the terms "larger" and "more massive"? Are larger stars necessarily more massive? Explain.
(53): 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.
(54): 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?
(55): 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.
(56): 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?"
(57): 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."
(58): 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?
(59): 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.
(60): 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
(61): 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.
(62): 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)
(63): In a binary system that is not edge-on, describe how we use the angular size method to determine the orbital distance. Given the angular size equation, explain what we need to know to find the orbital distance. What are a couple of ways we can find the distance from Earth to the face-on system? How do we combine the period and orbital distance information to determine the mass of the central star?
(64): 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?
(65): 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?
(66): A recent 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 #11)
(67): What is the difference between gas and dust? What sort of light does each emit and why?
(68): 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. Explain the effect that extinction has on our estimate of the distances to stars.
(69): 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.
(70): 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 108 in chapter 5.
(71): 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 #12)
(72): 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?).
(73): 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 #13)
(74): 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.
(75): 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.
(76): 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.
(77): 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?
(78): What is hydrostatic equilibrium (HSE)? Explain what happens to a star's outward-pushing pressure, self-gravity, density, size and temperature as it expands and contracts. For example, suppose a star has a very large outward pushing pressure. What changes would occur with these other quantities and how would the star find equilibrium again? What if the star's outward pushing pressure were very small compared to the self-gravity?
(79): 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).
(80): 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.
(81): 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 #14)
(82): 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?
(83): What causes variable stars to pulsate? Why do larger stars tend to have longer pulsation periods?
(84): What are Cepheids? Explain step-by-step how to use the Cepheid Period-Absolute Luminosity, or P-L relation to find the distance to a star cluster or distant galaxy.
(85): 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.
(86): Explain why, in a representative sample of stars, we find about 90% of them on the main sequence at any given point in time.
(87): 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?
(88): 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? Why do elements heavier than iron get formed during a supernova and at no other time?
(89): Virtually all of the elements in your body that have higher atomic number than Hydrogen were formed inside the core of a star. Virtually every atom of gold on the Earth (including in jewelry) was originated inside a supernova billions of years ago. Explain how we know these things, and explain roughly how the gold got from inside the star to the Earth.
(90): 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 #15)
(91): What is a pulsar? How is it related to a neutron star? Why do some neutron stars appear to be pulsars on Earth while most do not?
(92): What is a black hole? How do we use the concept of escape velocity to define the boundary (event horizon) of a black hole?
(93): Explain how astronomers use observations of binary star systems and accretion disks to "prove" the existence of black holes.
(94): What is a nova? How do novae produce energy?
(95): Explain how the force of gravity of a black hole is similar to and different from a normal object like the Sun. Be able to answer questions like, "The Sun's radius is 400,000 miles. Who would feel stronger gravity? An astronaut at the surface of the Sun or an astronaut 400,000 miles from a one-solar-mass black hole? Explain. What if the distance from the center of both were 100,000 miles? Explain."
(96): Explain why tidal forces from black holes can be so much stronger than tidal forces from more ordinary objects like stars and planets.
(97): How do we know that there is a supermassive black hole (20 million times the mass of our Sun) at the center of our galaxy? Will this black hole eventually suck up all the material in our galaxy? Why or why not?
(98): How do we find the distances to globular clusters? How and why can we use these distances to estimate the distance to the center of the galaxy?
(99): Astronomers use the Cepheid P-L relation often in distance determination. Suppose we try to find the distance to cluster X and cluster Y. Each one has a Cepheid. 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? Explain. (TQ #16)
(100): Describe the three main parts of the Milky Way galaxy. Why does most of the star formation in the galaxy occur in the disk?
(101): Explain why regions of young stars or regions with lots of star formation (like the disk) tend to have a blue color. Explain why regions with mostly older stars and no recent star formation tend to appear red.
(102): How do colors and ages correlate for individual stars? Do stars change colors during their main sequence lifetimes? In other words, are all young stars blue? Explain.
(103): Define metal and metallicity. How and why does the metallicity of the galaxy change over time?
(104): 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?
(105): 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)?"
(106): 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.
(107): 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.
(108): Given the equation of orbital velocity, be able to explain the basic shape of the Keplerian rotation curve. Why does our solar system follow the Keplerian curve while Astronomers initially anticipated the rotation curve of the Milky Way galaxy would not follow this curve (but instead have orbital velocities slightly higher than the Keplerian prediction)?
(109): 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. A recent article by Vera Rubin, one of the pioneers in the discovery of dark matter, can be found in the Scientific American website at http://www.sciam.com/specialissues/0398cosmos/0398rubin.html, and it contains an excellent summary of all of the issues surrounding dark matter. I will not require you to read this article, nor ask any thought questions about it, but I do strongly recommend that you read it when you have a chance to gain better insight into this topic.
(110): What are WIMPs? How is it possible that such a small thing as a neutrino might constitute the dark matter, ten times more massive than the total amount of visible matter in the galaxy? Why do we believe neutrinos might have mass?
(111): What are MACHO's? Explain gravitational lensing and how it is used to detect MACHO's. Why does a MACHO intervening along the line of sight to star cause the star to brighten rather than dim?
(112): Read the following website about an ongoing experiment to detect WIMPs: http://hepwww.rl.ac.uk/ukdmc/dark_matter/wimp.html. How does the UKDMC (United Kingdom Dark Matter Collaboration) propose to detect WIMPs? Why is the WIMP detector located far underground? Even underground, the detectors cannot escape a major source of noise. Describe it. (TQ #17)
(113): How can Astronomers tell whether a star's variation in brightness is due to a gravitational lensing event rather than some other kind of variation? As part of your answer, explain why lensing variations are different than other variations (e.g. variable stars, binaries, novae).
(114): How do we estimate the amount of MACHO's in our galaxy? What conclusion have Astronomers reached about the fraction of the dark matter that consists of MACHO's?
(115): Explain how Astronomers used globular cluster observations to eliminate very low mass (VLM) stars as a dark matter candidate. Why couldn't these observations have been done from the ground? Why couldn't we look for nearby VLM stars instead of stars in a distant cluster?
(116): How would we detect solitary black holes if they consitituted a significant fraction of the dark matter? Why does it look like we will be unlikely to test this hypothesis in the near future?
(117): 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?
(118): 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.
(119): Explain why the vast majority of the stars in the halo have red colors. (hint: when is the last time star formation occured there, the last time any significant gas/dust was present there?)
(120): 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.
(121): Elliptical galaxies tend to be found near the centers of galaxy clusters. Explain how/why this fact is consistent with the merger hypothesis.
(122): 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?
(123): Explain how the standard ruler method of distance determination works. Why is this method unreliable?
(124): 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.
(125): 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.
(126): 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?
(127): 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. Explain why individual galaxies are not useful as standard candles.
(128): Although star clusters and planetary nebula (PN) have varying absolute luminosities, it is possible to use them as standard candles if one is careful about selecting them (by taking advantage of their natural range of properties). How is it possible to pick two objects and have some notion that they have similar absolute luminosities? What is the reasoning behind this technique?
(129): Explain why the absolute luminosities of the brightest planetary nebula (PN) in a distribution of PN's is predictable.
(130): How and why do type I and type II supernovae differ (see pp 515-6 in your book for more on this)? Why do type Ia supernovae have such predictable brightnesses? Why do they make better standard candles than any of the other possible objects we've mentioned?
(131): How do we know that the TF method and the various standard candle techniques are accurate and reliable? How do we test the results?
(132): Describe how we use masers to determine the distances to some nearby galaxies.
(133): Explain why the maser technique is distance-limited. Why will this technique improve in effectiveness over time?
(134): Why is the maser technique so important with respect to other distance determination methods, despite its limitations?
(135): What is Hubble's Law? How could you use the Hubble relation to determine the distance to a galaxy with a known redshift?
(136): Explain how we know that quasars (QSO's) are hundreds of times more luminous than a typical galaxy.
(137): How do we know that QSO light does not come from typical stars or galaxies?
(138): How do we know that QSO's have small sizes compared to galaxies? Explain fully.
(139): 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. (Hint: We talked in class about the sign conventions used for radial velocity of galaxies) (TQ #18)
(140): Read the relevant section in chapter 27 in the text and the following article: http://www.sciam.com/0797issue/0797fishman.html. Based on your reading, answer the following questions: Describe one possible model of how a gamma-ray burst (GRB) can occur. Why do we think GRB's are not located in our own galaxy but are instead at very large (cosmological) distances (just describe one line of evidence)? (TQ #19)
(141): Describe how lookback time is related to distance. How do we know that quasars only existed in large numbers back when the Universe was about 2-4 billion years old?
(142): Why do we think QSO's are powered by massive black holes? How do we think QSO's are related to galaxies? Explain why we make this link.
(143): If you were to draw a graph of galaxy metallicity vs distance from Earth, taking lookback time into account, how would it look? Would galaxy metallicity seem to increase at greater distances? Decrease? Or remain the same? Explain.
(144): If we were to draw a graph of the ratio of spirals to ellipticals in clusters vs distance from Earth, how would it look? Would the ratio seem to increase at greater distances? Decrease? Explain.
(145): 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 questions 143 and 144? Would the abundance of Hydrogen increase with increasing redshift? Decrease? Stay the same? Explain. (TQ #20)
(146): 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.
(147): 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.
(148): Explain why the Hubble relation, which indicates that all galaxies seem to be moving away from our location, is not a violation of Copernican Principle.
(149): 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 #21)
(150): Explain how galaxy clusters create scatter in the Hubble relation. Why is it that some galaxies actually have negative radial velocity components and that these galaxies are all found relatively close to us?
(151): 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.
(152): Explain how an expanding Universe contributes to the fact that the night sky is dark.
(153): What is the visible horizon? Why can't we see anything outside of our visible horizon?
(154): What is the Cosmic Background Radiation (CBR)? Where does it come from? Why does it originate just inside our visible horizon?
(155): Explain why the current structure of the Universe led Astronomers to believe that the CBR must have a lumpy nature, with measurable fluctuations.
(156): Explain how the existence of the CBR, 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 CBR really seen as the key to confirming the Big Bang instead of just relying on Hubble's Law?
(157): Explain why the CBR looks redder (cooler) in one direction and warmer (bluer) in the opposite direction, not taking into consideration the much smaller fluctuations discussed in 155.
(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): 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 #22)
(161): 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?
(162): Explain how and why Astronomers hoped to use Hubble's Law as an independent method for estimating the density of the Universe. How would a high vs a low density Universe compare? Explain.
(163): 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.
(164): 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?
(165): What prediction does the Hubble relation make about the ages of the oldest stars in the Universe? How do we estimate the ages of stars? Are these estimates consistent with the Big Bang?
(166): 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.
(167): 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.
(168): Explain the horizon problem. How does the theory of inflation "solve" the horizon problem?
(169): 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)
(170): On the same web site (http://www.skypub.com/news/special/seti_toc.html), 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)
(171): 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?
(172): 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.
(173): Explain the implications to SETI if scientists verify the existence of life on Mars and/or Jupiter's moon Europa.
(174): Why is "Lifetime" an important factor in the Drake equation? Explain how it affects the "detectability" of a civilization.
(175): When looking for extraterrestrial signals, how do we decide what general region of the spectrum to search? A graph would help here.
(176): 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?).
(177): Explain why we must use very narrow channels in order to search for ET signals. Why would other civilizations likely broadcast signals in very narrow channels instead of broad channels (like, say, radio stations)?