Melissa Beauvais Final Astronomy 406 exam review on Chap 14-16 Chapter 14 1. The circular but relatively flat portion of the galaxy is the DISK. 2. The first portion of the galaxy to form was the HALO 3. A tightly packed group of a few hundred thousand very old stars is a GLOBULAR CLUSTER 4. A SPIRAL ARM stands out in a photo of a galaxy because it shines brightly with lights from massive young stars and glowing clouds of gas and dust. 5. Our Milky Way galaxy is a SPIRAL GALAXY. 6. Stars orbiting in the BULGE near the galaxy’s center can have orbits highly inclined to the galactic plane. 7. Why do disk stars bob up and down as they orbit the galaxy? -Because the gravity of other disk stars always pulls them toward the disk 8. Which part of the galaxy has gas with the hottest average temperature? -The Halo 9. Where are large dust clouds predominantly located in the galaxy M51? -Within or on the edges of the spiral arms -As is generally true for spiral galaxies, large, star-forming dust clouds are located primarily within or on the edges of spiral arms. 10. Where are the ionization nebulae predominantly located in the galaxy M51? -Within or on the edge of the spiral arms of the galaxy. -Like the dust clouds, ionization nebulae are located primarily within or on the edges of spiral arms. This is because spiral arms are regions of active star formation. Stars are born within dust clouds, and ionization nebulae are ionized by the intense light from massive, young stars. 11. Why are the ionization nebulae so bright? -They are regions where gas is ionized by hot, young stars. 12. Which of these forms of radiation passes most easily through the disk of the Milky Way? -Infrared Light. 13. The following figures show several stars found in the disk and halo of the Milky Way Galaxy. Rank the stars based on their current age, from oldest to youngest. If two (or more) stars have approximately the same age (that is, ages within a few million years), rank them as equal by dragging one on top of the other(s). Oldest Youngest Red Giant in globular cluster M13, The Sun, Hot blue main- Red main-sequence star in globular cluster M13 sequence star in disk -The two globular cluster stars have the same age, because all stars in a cluster form at about the same time. These are also the oldest stars, because globular clusters usually contain the oldest stars in the universe. The Sun is next, with its age of about 4 1/2 billion years. The hot, blue main-sequence star must be no more than a few million years old, because these stars do not live much longer than that. 14. Listed following are several stars found in the disk and halo of the Milky Way Galaxy. Assume that both the blue and yellow disk stars are members of the same open cluster. Rank the stars based on the abundance of elements heavier than carbon that you would expect to find in each of the stars, from highest to lowest. If you expect two (or more) stars to have approximately the same abundance, rank them as equal by dragging one on top of the other(s). High Abundance Low Abundance Yellow main-sequence star in open cluster in disk Red giant in globular cluster M13 Hot blue main-sequence star in open cluster in disk Red main-sequence star in globular cluster M13 15. Disk Stars (Yellow) Halo Stars (Green) -Youngest stars -Globular clusters -The sun -Oldest stars -Stars that all orbit in nearly -Stars whose orbits can be inclined the same plane. at any angle. -High-mass stars -Stars with the smallest abundance of heavy elements. 16. Where are most of the Milky Way's globular clusters found? -In the halo 17. How do we determine the Milky Way's mass outside the Sun's orbit? -from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun 18. We measure the mass of the black hole at the galactic center from: -the orbits of stars in the galactic center. 19. Compare your answer with the mass of Saturn listed in the table below. -The answer is in agreement with the experimental data. 20. 21. 22. 23. Which of the following statements correctly summarize key differences between the disk and the halo? Check all that apply. 24. The image sequence zooms in on the center of the Milky Way Galaxy. Consider the first image in the sequence, which is the image that is zoomed-out as far as possible. Approximately what fraction of the Milky Way's total diameter is spanned by this image? 1/200 25. The sequence of photos in the Interactive Figure consists of infrared and radio images. Why doesn't the sequence include any visible-light images? -Interstellar dust in the galactic disk prevents us from seeing the galactic center with visible light. 26. Now zoom through the image sequence, looking carefully at each image. Which of the following are reasonable conclusions? -There are strong magnetic fields in the central region of the galaxy. -Gas orbits the radio source called Sgr A*. -Stars near the galactic center are much closer together than stars around our Sun. 27. The last image in the sequence shows two arrows pointing to the precise location of Sgr A*, yet the image shows nothing located at that position. Based on your study of the entire sequence, which of the following types of image would most likely allow us to see something at this location? -A very high-resolution radio image. 28. Astronomers have concluded that Sgr A* is actually a very small but extremely massive object, which is presumed to be a massive black hole. What additional observations or measurements (not shown in this sequence) must have been made for astronomers to reach this conclusion? -Measurements of the orbital speeds and distances of gas clouds and stars around Sgr A*. 29. To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________. -if they observed for many more years, the dots would trace out ellipses 30. To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. Let's consider the star with the smallest orbit, which is represented by red and pink dots. What is the approximate orbital period of this star? -13 years 31. Look again at the orbit of the star with the smallest orbit, represented by red and pink dots. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____. -800 AU 32. Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest. Farthest Closest A globular cluster a cloud of gas & dust our solar system the edge of the In the outskirts of in the outskirts of the central bulge the halo disk 33. If we could see our own galaxy from 2 million light-years away, it would appear -as a flattened disk with a central bulge and spiral arms 34. How does the interstellar medium affect our view of most of the galaxy? -It prevents us from seeing most of the galactic disk with visible and ultraviolet light 35. About what percentage of the mass of a molecular cloud is in the form of dust? -1% 36. How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk? -It should have a higher fraction of elements heavier than hydrogen and helium. 37. Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not? -The 10% figure is too high because there are not enough heavy elements to make that much dust. 38. The most common form of gas in the disk of the Milky Way galaxy is? -Atomic hydrogen gas 39. How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today? -The total amount of gas will be much less than it is today. 40. Over time, the star-gas-star cycle leads the gas in the Milky Way to -have a greater abundance of heavy elements 41. Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful? -the Chandra X-ray Observatory 42. If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see? -Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. 43. All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________ -globular clusters 44. Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found -only in or near star-forming clouds 45. Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed? -It is probably a halo star that is currently passing through the disk. 46. What evidence suggests that most of the mass of the Milky Way is in the form of dark matter? -The orbital speeds of stars far from the galactic center are surprisingly high. 47. Spiral arms appear bright because -they contain more hot young stars than other parts of the disk 48. How did star formation likely proceed in the protogalactic cloud that formed the Milky Way? -The stars that formed first could orbit the center of the galaxy in any direction at any inclination. 49. If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen? -Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out. 50. What is the best evidence for an extremely massive black hole in the center of the Milky Way? 51. Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy? -It is by far the brightest source of visible light lying in the direction of the galactic center. CHAPTER 15 1. 2. Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy? -A galaxy cannot have a disk if it does not have gas. 3. High density tends to lead to more rapid star formation in a protogalactic cloud. Why does this rapid star formation tend to lead to an elliptical galaxy, rather than a spiral galaxy? -Rapid star formation means that there may not be enough gas left to make a disk. 4. High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy? -Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud. 5. 6. Which of these galaxies would you most likely find at the center of a large cluster of galaxies? -A large elliptical galaxy 7. To get started, click on the blue Cepheids link on the main screen of the Interactive Figure. The Interactive Figure (in red) shows a graph of the Cepheid period-luminosity relation. This graph indicates that -Cepheids with longer periods have higher luminosities 8. A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun. (You will have to click on the blue Cepheids link on the main screen of the Interactive Figure to get back to the relevant plot.) -10,000 9. Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax? -the earth-sun distance 10. Which cosmic distance measurement techniques are considered standard candle techniques? -main-sequence fitting -white dwarf supernovae (distant standards) -Cepheids 11. Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true? -We would not be able to measure the distances of distant galaxies. 12. Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases? -We have not observed white dwarf supernovae in most galaxies. 13. Imagine that radar had never been invented and that we instead had to rely on a less reliable method of measuring distances in our solar system. If that method led us to underestimate the Earth-Sun distance by 10%, how would it affect other measurements in the distance chain? -They would all be off by the same 10% 14. Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away? -Cepheids 15. If all the stars on the main sequence of a star cluster are typically only one-hundredth as bright as their main-sequence counterparts in the Hyades Cluster, then that cluster's distance is -10 times as far as the Hyades's distance. 16. CHAPTER 16: 1. Dark matter is inferred to exist because: -we can observe its gravitational influence on visible matter. 2. Dark energy has been hypothesized to exist in order to explain: -observations suggesting that the expansion of the universe is accelerating. 3. Examine the visible light image of the Bullet Cluster. What kind of cluster is it? -a galaxy cluster 4. Study the composite image of the Bullet Cluster, as well as the animations. What do the two large, red-colored regions in the composite image represent? -X-ray emission from hot gas 5. There are also two large blue-colored regions in the composite image. These blue regions are labeled as "dark matter" based on - observations of gravitational lensing by the cluster 6. From Parts A though C, you should understand the three major components shown in the composite Bullet Cluster image. Notice that the hot, X-ray-emitting gas (red) is not in the same place as most of the visible galaxies and most of the matter (blue). How do we think the hot gas came to be in a different place from the galaxies and most of the matter? -The Bullet cluster is actually two separate galaxy clusters that have collided, stripping out hot gas in the process. 7. Careful measurements show that the hot, X-ray-emitting gas in the Bullet Cluster contains about 7 times as much total mass as all the stars in the cluster's galaxies combined. Therefore, the fact that the most of the gravity is in the blue regions that surrounds the visible galaxies, rather than in the regions with the hot gas, indicates that -there is even more matter surrounding the galaxies than there is in the hot gas 8. As noted in the Introduction, some scientists have proposed that dark matter does not really exist. According to this view, all matter is ordinary (baryonic), but at large distances from matter, gravity does not precisely obey either Newton's or Einstein's theories of gravity. Is this alternative view of gravity consistent with what we observe in the Bullet Cluster? Why or why not? -No. If all matter was ordinary, then the blue region representing the location of most of the matter would line up with the red region representing the hot gas. 9. Consider a distant galaxy located directly behind a cluster of galaxies, as shown in this interactive figure. As seen from Earth, the gravitationally lensed images of the distant galaxy will appear more widely separated if the intervening cluster of galaxies has -a relatively high mass and is located relatively near Earth. 10. Which of the following best explains why a higher-mass cluster of galaxies causes light from a distant galaxy to bend more than a lower-mass cluster of galaxies? -The stronger gravity of a larger cluster curves space itself by a greater amount, and light follows the curvature of space. 11. Consider a distant galaxy located directly behind a cluster of galaxies as shown in this interactive figure. Knowing the distance to the cluster of galaxies and the angular separation of the lensed images of the distant galaxy, astronomers can estimate: -the total amount of matter in the cluster of galaxies, including both dark matter and matter in stars 12. The Rotation Curve for a Merry-Go-Round animation shows a rotating merry-go-round. Click several points on the merry-go-round, and watch the corresponding points appear on the graph below it. This graph is what we call a rotation curve, because it plots the speed at which any point rotates around the center (the orbital speed) against the distance of that point from the center. Based on the graph, you can conclude that doubling the distance of a point from the center of the merry-go-round would -double its orbital speed 13. Now, let’s turn our attention to rotation curves for orbiting masses which are not rigidly held together like the points on a merry-go-round. The Rotation Curve of the Solar System animation shows the orbits of Mercury, Venus, Earth, and Mars. Click on each of the orbits, and watch the corresponding points appear on the graph below it. If a planet (or asteroid) orbited the Sun at three times the distance of Mars, its orbital speed would be -between one-third and two-thirds the orbital speed of Mars 14. Why does the rotation curve for the solar system show speeds that become slower with increasing distance from the Sun? -Because the sun contains most of the mass of the solar system 15. Open the Rotation Curve of a Spiral Galaxy animation and click at various distances from the galactic center to create a graph of the galaxy's rotation curve; be sure to start with points very close to the galactic center and continue to well beyond the visible part of the galaxy. Which of the following statements best describes the pattern of the graph? -Starting from the center, speeds at first rise rapidly, and then become nearly constant with increasing distance. 16. To understand why the galaxy rotation curve in Part D is flat, we must first understand how gravity determines the orbital speeds of stars around the galactic center. Consider any star on a circular orbit around the center of the galaxy. The speed of the star's orbit depends on the star's distance from the galactic center and -the mass of the galaxy that resides within the stars orbit 17. From Part D, you know that the galaxy's rotation curve is nearly flat, even well beyond the point at which we see any stars in the galaxy. According to the law of gravity as we understand it today, the only explanation for these this flatness is that -substantial amounts of mass must reside at great distances from the galactic center 18. 19. Listed following are three possible models for the long-term expansion (and possible contraction) of the universe in the absence of dark energy. Rank each model from left to right based on the ratio of its actual mass density to the critical density, from smallest ratio (mass density much smaller than critical density) to largest ratio (mass density much greater than critical density). 20. Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank the models from left to right based on their predictions for the average distance between galaxies five billion years from now, from smallest to largest. 21. Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank each model from left to right based on its prediction for the current age of the universe, from youngest to oldest. FINAL EXAM REVIEW ONLINE QUIZ 1. How do disk stars orbit the center of the galaxy? -They all orbit in roughly the same plane and in the same direction. 2. What do we mean by the star-gas-star cycle? -It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium. 3. If we could see our own galaxy from 2 million light-years away, it would appear -as a flattened disk with a central bulge and spiral arms 4. How does the interstellar medium affect our view of most of the galaxy? - It prevents us from seeing most of the galactic disk with visible and ultraviolet light. 5. About what percentage of the mass of a molecular cloud is in the form of dust? -1% - Elements heavier than helium constitute a total of about 2% of the mass, and about half of the atoms of heavy elements in a molecular cloud are found in dust grains. 6. How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk? -it should have a higher fraction of elements heavier than hydrogen and helium. 7. Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not? - The 10% figure is too high because there are not enough heavy elements to make that much dust 8. The most common form of gas in the disk of the Milky Way galaxy is -Atomic hydrogen gas 9. How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today? -The total amount of gas will be much less than it is today. - With each subsequent generation of stars, some material is "locked away" permanently in brown dwarfs and stellar corpses. Thus, the amount of gas available for recycling gradually declines with time. 10. Over time, the star-gas-star cycle leads the gas in the Milky Way to -Have a greater abundance of heavy elements -with every cycle, new stars have died and produced heavy elements that are recycled Into space. 11. Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful? -The Chandra X-ray Observatory 12. If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see? - Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. 13. All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except -Globular clusters 14. Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found -Only in or near star-forming clouds 15. Which of the following statements comparing halo stars to our Sun is not true? - Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime. - This is not true of most of the stars, because most stars are less massive than the Sun and therefore progress through their stages of life at a much slower rate than the Sun. These very low-mass stars have more of their main-sequence lives remaining than the Sun, despite their greater ages. 16. Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed? -It is probably a halo star that is currently passing through the disk - All the disk stars in our vicinity should be orbiting the galactic center at about the same speed, which is why their speeds relative to the Sun are fairly low. Halo stars have very different orbits, so when they pass through the disk they have high speeds relative to the nearby disk stars. 17. What evidence suggests that most of the mass of the Milky Way is in the form of dark matter? - The orbital speeds of stars far from the galactic center are surprisingly high. 18. Spiral arms appear bright because -They contain more hot young stars than other parts of the disk 19. How did star formation likely proceed in the protogalactic cloud that formed the Milky Way? - The stars that formed first could orbit the center of the galaxy in any direction at any inclination. 20. If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen? - Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out. 21. What is the best evidence for an extremely massive black hole in the center of the Milky Way? - The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System. 22. Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy? - It is by far the brightest source of visible light lying in the direction of the galactic center. 23. Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in the textbook in your textbook), the estimated number of galaxies in our observable universe is about -100 Billion 24. Which of the following is not one of the three major categories of galaxies? -Globular Galaxies 25. A standard candle is? -A light source of a known luminosity 26. What is main sequence fitting? - a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram 27. What is a Cepheid variable? -A type of very luminous star that makes an excellent standard candle 28. What two observable properties of a Cepheid variable are directly related to one another? -The period between its peaks of brightness and its luminosity 29. What does Hubble’s law tell us? -The more distant a galaxy, the faster it is moving away from us. 30. Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies? -They are rare events, so we have observed them in only a tiny fraction of all galaxies. 31. When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent? -The inside of the balloon does not represent any part of the universe. 32. If we say that a galaxy has a lookback time of 1 billion years, we mean that? -Its light traveled through space for 1 billion years to reach us 33. Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called -The cosmological horizon 34. Cosmological redshift is the result of? -The expansion of the universe 35. Current estimates place the age of the universe at about? -14 billion years 36. Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in -Infrared light 37. Which of the following is an important starting assumption in models of galaxy formation? -Some regions in the universe start out denser than others 38. Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) - may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies 39. Collisions between galaxies typically unfold over a period of -Hundreds of millions of years 40. Why are collisions between galaxies more likely than collisions between stars within a galaxy? -Relative to their sizes, galaxies are closer together than stars. 41. Which of the following features is not a feature of central dominant galaxies? -They are often spiral galaxies -Central dominant galaxies are actually elliptical galaxies 42. Why should galaxy collisions have been more common in the past than they are today? - Galaxies were closer together in the past because the universe was smaller. 43. The distinguishing feature of a starburst galaxy is - a rate of star formation that may be 100 or more times greater than that in the Milky Way 44. The unusually bright centers found in some galaxies are called -Active galactic nuclei 45. According to current understanding, what is a quasar? -An active galactic nucleus that is particularly bright 46. Which of the following phenomena is probably not related to the presence of a supermassive black hole? -The presence of globular clusters in the halos of galaxies 47. The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly -1 billion solar masses 48. According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of _ -Light emitted by hot gas in an accretion disk that swirls around the black hole 49. Current understanding that a galaxy’s type (spiral, elliptical, or irregular)… - may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies 50. Which of the following best summarizes what we mean by dark matter? - matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light 51. Which of the following best summarizes what we mean by dark energy? - It is a name given to whatever is causing the expansion of the universe to accelerate with time. 52. The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that -dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way - Observations indicate that dark matter may represent more than 90% of the Milky Way Galaxy's overall mass, and that it extends far beyond the galaxy's visible disk and the visible stars of the halo. 53. What is a rotation curve? - a graph showing how orbital velocity depends on distance from the center for a spiral galaxy 54. What is the primary way in which we determine the mass distribution of a spiral galaxy? - We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center. 55. What do we mean when we say that the rotation curve for a spiral galaxy is "flat"? - Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. 56. Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest? - Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies. 57. Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters? - measuring the temperatures of stars in the halos of the galaxies 58. When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean? - It bends or distorts the light coming from galaxies located behind it. 59. Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies? - Dark matter is the dominant form of mass in both clusters and in individual galaxies. 60. What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)? - It consists of atoms or ions with nuclei made from protons and neutrons. 61. Some dark matter may be ordinary matter that orbits in the halo of the galaxy. Which of the following would not be considered a type of ordinary dark matter? -Globular clusters in the halo of the galaxy 62. What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting? - They respond to the weak force but not to the electromagnetic force, which means they cannot emit light. 63. Which of the following best sums up current scientific thinking about the nature of dark matter? - Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified. 64. When we speak of the large-scale structure of the universe, we mean -the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe 65. The critical density of the universe is the - average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist 66. What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating? -Observations of white dwarf supernovae 67. Which of the following best sums up current scientific thinking about the nature of dark energy? - Dark energy probably exists, but we have little (if any) idea what it is. 68. What happens to gas as it freely expands? -It gets less dense and cools 69. Which of the following statements can NOT be tested by science today? -Prior to the Planck time, our Universe emerged from another universe Gas and dust are abundant in the disk but not in the halo. Clusters of young stars are found only in the disk. Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits. -The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.