PHYS 20083 - Spring 2000 Complete Study Guide

Physics 20083 - Complete Study Guide

(1): Explain with the help of a simple diagram how we can use the angular size method to determine the distance to an object. What two pieces of information do we have to know to deduce the distance? Show why the distance is inversely proportional to the angular size.
(2): Explain with the help of a simple diagram how we can use parallax to determine the distance to an object. What two pieces of information do we have to know to deduce the distance. Show why the parallax angle is directly proportional to the baseline and inversely proportional to the distance.
(3): Briefly explain how parallax angles (or the lack of measured angles) encouraged scientists to remain extremely skeptical of the Copernican model of the solar system after it was first introduced. Why were skeptics at the time unconvinced? Were they doing something wrong?
(4): Why is the parallax method ultimately limited? In other words, why can we only use it to measure relatively short distances, like to other planets or nearby stars but not galaxies?
(5): Given the equation of the inverse square law, explain how we can use the inverse square law to determine the distance to an object. Also, explain why the inverse square law can be used to find some distances we are unable to measure with parallax.
(6): Why is the inverse square law method less reliable than the parallax method?
(7): Given the inverse square law equation, be able to solve simple mathematical problems, such as "Star A has twice the absolute luminosity as star B. Star A is twice as far away as star B. How does the apparent luminosity of A compare to that of B?"
(8): Explain the concept of frequency. How and why is frequency related to wavelength?
(9): Given the relationship between energy and wavelength for light, be able to identify on a typical spectrum where the high and low energy regions are. Use this information to help explain why hot objects tend to radiate most of their energy in the relatively short wavelength region of the spectrum
(10): 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.
(11): Given the law of scattering (shorter wavelengths tend to be scattered more effectively), explain why the sky is blue and why the sun appears red at sunset.
(12): Explain how we use our knowledge of continuous radiation to estimate the temperatures of stars. Starlight is always being scattered by our atmosphere. If we don't correct for this effect, our estimate of the star's temperature will be (too high, the same, too low). Explain your answer.
(13): Explain what happens when an atom absorbs or emits light.
(14): Why do different atoms absorb or emit unique sets of photon wavelengths? Explain how we use this knowledge to deduce the composition of the Sun and other stars.
(15): Describe the circumstances by which you can see an absorption line spectrum. Do the same for an emission line spectrum.
(16): Given a simplified energy level diagram, be able to state which energies can and cannot be absorbed by the atom represented in the diagram.
(17): Understand and explain the difference between line width and line strength.
(18): If a spectral line pattern for say, Hydrogen, is not present in a star's spectrum, does that necessarily mean that there is no Hydrogen along our line of sight to the star? Explain.
(19): Explain why spectral lines of elements like Hydrogen will often not appear in the spectra of stars that are extremely hot and stars that are extremely cool.
(20): 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.
(21): Know the difference between radial and transverse velocity (and components of velocity). Be able to apply this knowledge to Doppler shifts and real examples of spectra.
(22): Does distance necessarily have anything to do with Doppler shift? In other words, 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. (TQ #1)
(23): Explain why spectral line widths for a star are directly proportional to the temperature of the absorbing gas in the star's atmosphere.
(24): Explain why spectral line widths for a star are directly proportional to the rotation velocity of the star.
(25): Suppose two stars have all the same properties (size, temperature, composition, etc). Star A is rotating and is seen in the equatorial plane of the rotation. Star B is rotating with the same speed and seen pole-on. Will the line widths of these two stars be different at all? Explain your answer. (TQ #2)
(26): Explain why spectral line widths for a star are directly proportional to the density of the star.
(27): Star A and B have no rotation and the same temperature. But Star A's absorption lines are somewhat broader that star B's absorption lines. What can we probably conclude about the relative sizes of A and B? Explain your answer.
(28): Given the proportionality that relates density to mass and size, be able to answer questions like: "Star X is four times more massive than our Sun. Star X has a radius that is twice as big as our Sun. How does the density of star X compare to the density of our Sun? Show your work."
(29): Explain why a shrinking Sun would generate heat energy. Why is this method of energy generation for the Sun not thought to be the actual method of energy generation? As part of your answer, explain how to calculate the lifetime of the Sun.
(30): Explain how nuclear fusion generates energy in the Sun. Is all of the mass in the core going to be converted into energy?
(31): Why does nuclear fusion only occur in the core? What two conditions must be met in order for nuclear fusion to occur? Explain each one.
(32): Why is energy transported via radiation (light) in the radiative zone of the Sun? Why does the radiative zone have an outer edge?
(33): Describe how energy is transported mechanically in the convective zone of the Sun. Why is energy transported in this way in the convective zone rather than via radiation?
(34): What is the photosphere of the Sun? How is energy transported outward from the photosphere and why is this method of transport different from that of the convective zone?
(35): Explain the concept of limb darkening. What does limb darkening tell us about the temperature structure of the Sun's photosphere?
(36): What is the chromosphere? Why does it cause absorption lines to appear in the Sun's spectrum?
(37): 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.
(38): Name and explain two pieces of evidence that tell us the further one looks above the surface of the Sun, the higher the temperature.
(39): Explain why the temperature is hotter the higher above the surface one looks. In other words, what is the theory to explain the observations discussed in 38?
(40): What is the solar neutrino problem? Although the observation of "too few neutrinos" contradicts the prediction of nuclear fusion theory, scientists still accept nuclear fusion as the Sun's energy source with only a small modification of the theory. Explain why.
(41): The solar corona has temperatures equal to or in some cases larger than the temperatures in the core. Explain why nuclear fusion doesn't occur in the corona. (TQ #3)
(42): Given the equation of absolute luminosity (relating absolute luminosity, size and temperature) and the inverse square law, summarize how we can determine the distance to a star using information from its spectrum.
(43): Give the equation of absolute luminosity, be able to answer questions such as: "Star X has a surface temperature of 3000K. The Sun's surface temperature is 6000K. Star X is twice as large as the Sun. How does the absolute luminosity of star X compare to that of the Sun?" Be able to answer these kinds of questions quantitatively and graphically (by plotting the position of X on an H-R diagram relative to the Sun).
(44): Suppose we are using Doppler shifts to estimate the mass of the central star of a binary system that we think is being seen edge-on. In fact, though, the system is not being seen edge-on. Instead, the system is tilted with respect to our line of sight. Our estimated mass will be (larger, smaller, the same) compared to the true mass of the central star. Explain your answer. (TQ #4)
(45): What is the Copernican Principle? Use this principle to help explain why a sample of 1000 or so stars closest to the Sun is probably representative of the stars in our galaxy. Would a sample of the 1000 or so stars nearest to, say, the star Rigel be representative? Explain.
(46): Use the inverse square law relationship to help explain why a sample of the 1000 or so stars that have the highest apparent luminosity is not likely to be representative.
(47): Given the equation of orbital velocity and the period equation, explain how we can use the orbital distance between Earth and Sun along with the orbital period of the Earth to deduce the mass of the Sun.
(48): Explain how we use Doppler shifts to determine the orbital velocity of the companion star in an eclipsing binary system.
(49): Describe two different ways Astronomers can determine the orbital distance between a companion star and central star in a binary system.
(50): Be able to answer basic proportionality questions about the equation of orbital velocity such as: "System A's companion star is orbiting 10 times faster than system B's companion. We know that the orbital distance separating the companion and central star is the same for both system A and system B. Which system's central star is more massive? Explain."
(51): Describe how we determine the orbital distance and orbital period for a face-on binary system, then explain how we use this knowledge (given the relevant equations) to deduce the central mass in the system.
(52): Given the mass-luminosity relation, use this to show that stellar lifetimes are shorter than the Sun's 10 billion years for stars with higher masses than the Sun. Explain the theory of why this relationship holds.
(53): Given the mass-luminosity relation, be able to answer simple mathematical questions such as: "Star X is twice as massive as our Sun. How does the absolute luminosity of star X compare to that of the Sun? How does the lifetime of star X compare to our Sun?"
(54): Explain how we can deduce the existence of dark interstellar clouds by simply looking at the distribution of visible stars on the night sky (see, for example, p415 photos).
(55): Explain what interstellar reddening does to our estimates of the distances to stars. Explain what interstellar extinction does to our estimates of the distances to stars. Treat each case independently for simplicity.
(56): How and why do ISM absorption lines differ from stellar absorption lines. Given a spectrum with both kinds of lines, be able to identify which set belongs to the ISM and which belongs to the star (as well as relative Doppler shifts due to bulk motion of either or both).
(57): How can we determine the temperature, thickness and composition of an interstellar cloud by examining the spectrum of a star whose light must pass through that cloud to reach the Earth? Explain.
(58): What is a forbidden line? Explain why forbidden lines are not seen in our labs but often seen in interstellar space.
(59): Hydrogen's 21 cm line is a forbidden line. Where does the "21 cm" name come from? Why is this line so important, allowing us to detect so much matter that would otherwise be invisible?
(60): Explain the concept of pressure equilibrium in the ISM. Understand what the heliosphere is, and explain how this boundary represents an example of pressure equilibrium.
(61): As a star shrinks, explain what happens to the star's self-gravity (given the relevant equation) and the star's density, temperature and outward-pushing pressure.
(62): Explain why the existence of radioactive Al-26 decay traces in meteorites is seen as evidence that a supernova explosion was probably responsible for the initial collapse and subsequent formation of our Sun and the solar system.
(63): What do stars have a minimum mass? For objects that don't meet this mass requirement, what happens to them? Why don't they collapse due to their self-gravity?
(64): What is hydrostatic equilibrium (HSE)? Explain how it is stable by imagining subsequent changes in a star after the gravitational force changes (increase or decrease) and after the outward-pushing pressure changes (increase or decrease).
(65): What is differentiation? What happens to a star's core during the main-sequence (hydrogen burning) portion of its life? Why does this differentiation generate heat and thus additional outward pressure?
(66): Assume that the outward-pushing pressure of a star is proportional to the absolute luminosity. Use the mass-luminosity relation and the equation of self-gravity to explain why stars have a maximum possible mass (hint: what happens to both quantities as mass increases...do they change at the same rate?). (TQ #5)
(67): Use HSE arguments to explain why Helium differentiation in the Sun's core has caused the Sun to gradually increase in size during its main sequence lifetime.
(68): The fact that the inert Helium ash at the center of a star's core is supported by its own solid strength leads to a gradual expansion of the star over its main sequence lifetime. Use HSE arguments to help explain why this is true.
(69): Use HSE to explain what happens to the star's size, temperature and density after Hydrogen fusion ends and before Helium fusion begins.
(70): Explain why Helium fusion in the core requires a much higher temperature and density relative to Hydrogen fusion. Use this fact to help explain why stars with masses much smaller than the Sun will probably never enter undergo Helium fusion in their cores.
(71): Once Helium fusion begins, it starts explosively, in a "Helium flash". Explain what causes this flash.
(72): After the onset of Helium fusion, Hydrogen begins fusing into Helium in a thin shell surrounding the core. Why does this hydrogen now become part of the core? In other words, what is different now about this region of the star that wasn't true during the main sequence?
(73): Use HSE arguments to explain why stars in the Helium-burning phase of their lives grow to such enormous sizes. Also, explain why these red giants have their red color, despite the fact that their cores are much hotter and more luminous.
(74): Suppose we're looking at a planetary nebula as shown in figure 20.11 in your book (p 457). We can measure the angular size of the outer edge of the nebula with an angle-measuring device. We know from historical records the time the planetary nebula first went off (so we know the age of the expansion). We also can measure the cloud's velocity via Doppler shifts. Explain how we can use these three pieces of information to deduce the distance from Earth to this nebula. (TQ #6)
(75): Explain what a planetary nebula is. Why do stars with masses similar to our Sun undergo this phase of stellar evolution while very massive stars (greater than about six solar masses) do not?
(76): Be able to look at the H-R diagrams of two different clusters of stars and determine which cluster is older, and be able to explain your answer.
(77): Very massive stars come to the end of their lifetime when fusion of Iron is attempted in the core. What is the important difference between Iron fusion and fusion of lighter elements, like Hydrogen or Helium? Why does this difference signal the end of the life of the star?
(78): Given the equation for absolute luminosity, be able to explain why white dwarf stars, with surface temperatures 5-10x higher than the Sun, are found below the Sun (lower absolute luminosity) on the H-R diagram.
(79): In a binary star system, a red giant star is found next to a yellow main sequence star. Assuming both stars have the same age, which of the two stars began its lifetime as a more massive star? Explain
(80): Explain the phenomenon of recurrent novae. What causes a nova explosion, and why do they recur after a certain period of time has passed?
(81): Every atom in your body with an atomic number higher than that of Helium comes from the cycle of stellar evolution, and every atom with an atomic number higher than that of iron comes from a supernova explosion. Explain why we can make these statements with confidence.
(82): Use conservation of angular momentum (mass*size*rotation speed) to explain why neutron stars spin so quickly.
(83): Given the equation of escape velocity, be able to answer questions such as "Star X has a size (radius) half that of the Sun and the same mass as the Sun. Which star has a higher escape velocity?"
(84): Explain how the equation of escape velocity is used to define the boundary of a black hole.
(85): 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?"
(86): What is an accretion disk? Given the equation of orbital velocity, explain how energy is generated in an accretion disk.
(87): Explain how we can "prove" the existence of a black hole if we can measure its edge-on accretion disk's orbital velocity, angular size and distance from Earth.
(88): Explain step-by-step how Astronomers calibrate the Cepheid Period-Luminosity relation. Why are accurate parallax measurements an important part of building an accurate P-L relation?
(89): Explain step-by-step how to use the P-L relation to find the distance to a star cluster or distant galaxy.
(90): 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.
(91): Describe the three main parts of the Milky Way galaxy. Why does most of the star formation in the galaxy occur in the disk?
(92): Explain why regions of young stars or regions with lots of star formation (like the disk) tend to have a blue color.
(93): Explain why regions with mostly older stars and no recent star formation tend to appear red.
(94): 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.
(95): Explain gamma-ray bursts (see book pp 498-500). Describe in your own words how the distribution of these bursts on the sky is evidence that they do not originate in the Milky Way galaxy. Explain the evidence that leads us to believe gamma-ray bursting objects have extremely small sizes (much smaller than galaxies). (TQ #7)
(96): 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? Why did the galaxy have any angular momentum to begin with?
(97): 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.
(98): Apply the concept of question 76. For example, be able to answer questions about the age of a globular cluster such as: "The mass of a star at the turnoff point of cluster X is slightly less than the mass of the Sun. Is this cluster younger than or older than 10 billion years (the main sequence lifetime of the Sun)? Explain."
(99): How do we determine the distances to globular clusters using main sequence turnoff point stars? Why is this more accurate than estimates of the distance to an individual star?
(100): How do these turnoff point stellar distances compare with distances derived via the Cepheid P-L relation? Explain why this increases our confidence that the measurements are accurate.
(101): Given the equation of orbital velocity, be able to explain the basic shape of the Keplerian rotation curve.
(102): 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).
(103): 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.
(104): One possible dark matter candidate is MACHOs. Use a simple diagram to help explain how these are detected. How does the light curve of a gravitationally lensed star compare to a typical variable star?
(105): Explain how scientists have come to the conclusion that MACHOs constitute about 30 percent of the dark matter in our galaxy.
(106): Explain the Malmquist Bias. In other words, explain why there is a difference between the distribution of masses for a brightness-limited sample and the true distribution of masses.
(107): Explain the experiment that seeks to determine whether Very Low Mass (VLM) stars constitute the dark matter.
(108): Explain how Astronomers have determined that solitary black holes makes up an insignificant portion of the dark matter.
(109): List the major differences between spiral galaxies and elliptical galaxies. Explain why the colors of these galaxies are different.
(110): Explain the concept of lookback time. Why is lookback time only truly significant when we are studying distant galaxies rather than when we are studying stars in our own galaxy?
(111): Explain how Astronomers think the differences between spirals and ellipticals arises. Explain the observational evidence to support this theory.
(112): Explain why the Cepheid Period-Luminosity method of distance determination is only useful for galaxies relatively close to our own.
(113): Explain how the Standard Candle (SC) method of distance determination works. Why is this method unreliable when using the simplest assumptions? What are some ways to improve the accuracy of this method?
(114): Explain how the Standard Ruler (SR) method of distance determination works. Why is this method unreliable? What are some ways to improve the accuracy of this method?
(115): Explain how to use Hubble's Law to estimate the distance to an object.
(116): Be able to plot a graph of a car race at various times as we did in lecture given some basic data.
(117): Given a graph of a car race, be able to find the slope of the graph and the age of the race.
(118): 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.
(119): Is it accurate to assume that galaxy radial velocity is constant during the expansion of the Universe? Explain.
(120): Explain why Hubble's Law does not violate the Copernican Principle.
(121): Explain why globular cluster ages serve as an important independent check of Hubble's Law and the deduced age of the Universe.
(122): 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 #8)
(123): Explain how the Tully-Fisher (TF) relationship is calibrated. Explain why errors in other distance determination techniques (such as parallax, Cepheids, standard candle, spectroscopic) can lead to errors in the TF calibration.
(124): Explain how the TF relation is used to find distances to galaxies. Explain why spectral line width for galaxies is related to their rotation speed.
(125): Explain how inclination is a major source of potential error in the TF method. Use this fact to help explain why the TF method is distance-limited (only works on relatively nearby galaxies the shape and orientation of which we can resolve successfully)
(126): Explain why supernovae make excellent standard candles (better than individual stars, globular clusters or galaxies).
(127): Be able to draw and explain how the Hubble relation deviates from a straight line for the cases of a decelerating Universe and an accelerating Universe (use the concept of lookback time).
(128): Briefly explain the origin of the cosmological constant in Einstein's theory. Why is the cosmological constant relevant in the light of recent supernova standard candle observations?
(129): Explain why, with so many "flaws" in the Big Bang theory (such as possible age discrepancies or the newly discovered but not really predicted accelerating universe), scientists still accept it as the "standard model" of how the Universe works.
(130): Given the equation of orbital velocity, explain how we prove the existence of dark matter by observing the motion of a satellite galaxy orbiting another galaxy.
(131): Why do we assume that the velocity of any individual galaxy in a cluster is less than that cluster's escape velocity?
(132): Given the equation of escape velocity, explain how we prove the existence of dark matter in a galaxy cluster by observing typical galaxy doppler shifts in the cluster, knowing the distance from earth and angular size of the cluster.
(133): 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?
(134): Explain the evidence that leads us to believe quasars are located extremely far away from the Earth, billions of light years away. Also, explain how we know that quasars are not bright because of simple stellar light like galaxies.
(135): Use the inverse square law to explain how quasar distances imply that they have luminosities much brighter than typical galaxies.
(136): Explain how the short time variability of quasars implies that they have sizes very small compared to the sizes of galaxies.
(137): Explain why black holes may be the mechanism that makes quasars so small and so bright. Why does a black hole accretion disk system have so much more luminosity than a typical star?
(138): Explain how quasars are used to study the intervening dark material that lies along our line of sight to the quasar.
(139): What is a Lyman-alpha absorption line? Why do most absorbers along the line of sight show this feature? What is the Lyman-alpha forest? Given a series of Lyman-alpha absorptions lines in a quasar spectrum, be able to identify and explain which of the absorption lines comes from the most distant source.
(140): 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.
(141): Explain how an expanding Universe contributes to the fact that the night sky is dark.
(142): Explain why the density of the Universe has an effect on the ultimate fate of the Universe (whether it will expand forever or collapse).
(143): What is the critical density? If the observed density of the Universe is bigger than the critical density, explain the ultimate fate of the Universe.
(144): What is the Anthropic Principle? The observed density of all matter and energy in the Universe is approximately equal to the critical density. Explain why this is viewed as an Anthropic coincidence necessary for the existence of life.
(145): The Anthropic Principle has several different variations (weak, strong, participatory, etc). Do some research on the World Wide Web to find out more about this concept, and define two different versions of the Anthropic Principle in your own words. Justify your belief in one or the other. (TQ #9)
(146): What is the Microwave Background Radiation (MBR)? How does the observation of the MBR confirm the Big Bang theory?
(147): Explain why Astronomers expected to see lumps in the MBR in the most recent satellite observations.
(148): Explain why the MBR looks a little bit hotter in one direction and a little bit cooler in the opposite direction on the sky.
(149): Define metallicity. What was the initial composition of the Universe, and how has it changed over time?
(150): If you were to plot a graph of metallicity of galaxies vs redshift, what would it look like? Would the average metallicity of galaxies increase with increasing redshift? Decrease? Stay the same? Sketch what you think the graph would look like and explain your answer. (TQ #10)
(151): 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.
(152): 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.
(153): What is Deuterium? Based on the book's discussion of deuterium in the Universe (p 633), how would you expect the abundance of deuterium to change over time in the Universe? How would you expect a graph of typical galaxy deuterium abundance vs redshift look? Explain. (TQ #11)
(154): Explain the horizon problem. How does the theory of inflation "solve" the horizon problem?
(155): Explain how the theory of inflation resolves the "flatness problem". What important prediction does inflation make about the density of matter and energy in the Universe?
(156): In the Drake equation, explain why "N" (the number of detectable, intelligent extraterrestrial civilizations) depends on "L", the average lifetime of a typical intelligent, advanced civilization.
(157): Explain why Astronomers search for extraterrestrial life passively, by looking for radio signals, instead of actively attempting to travel around the galaxy. Why does this logic imply that it is unlikely we have been physically visited by extraterrestrial life?