Question 1. Question 2. Correct. Chapter 16 Homework. Part A

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1 Chapter 16 Homework Due: 11:59pm on Thursday, November 17, 2016 To understand how points are awarded, read the Grading Policy for this assignment. Question 1 Following are a number of distinguishing characteristics of spiral and elliptical galaxies. Match each characteristic to the appropriate galaxy type. Hint 1. What is the most fundamental difference between a spiral and an elliptical galaxy? Which statement best describes the fundamental difference between spiral and elliptical galaxies? Spiral galaxies are more massive than elliptical galaxies. The oldest stars in spiral galaxies are younger than the oldest stars in elliptical galaxies. Elliptical galaxies lack a disk. The most fundamental difference between spiral and elliptical galaxies is that spiral galaxies have a disk and elliptical galaxies do not. In other words, spiral galaxies have both a disk and halo while elliptical galaxies are all halo. Because star formation occurs in a disk, a lack of disk means very little star formation and hence very few (if any) young, hot stars in elliptical galaxies. Spiral galaxies are rare in the central regions of clusters because galaxies in those regions tend to have suffered collisions, which either strip out the cool gas and dust needed to make a disk, or induce a burst of star formation that uses up the remaining cool gas. Question 2 Which of these galaxies would you most likely find at the center of a large cluster of galaxies? 1/9

2 a large spiral galaxy a large elliptical galaxy a small irregular galaxy Question 3 Which of the following is NOT one of the three major categories of galaxies? spiral galaxies globular galaxies irregular galaxies elliptical galaxies Question 4 We determine the distance of a Cepheid by measuring its parallax. knowing that all Cepheids have about the same luminosity and then applying the inverse square law for light. determining its luminosity from the period luminosity relation and then applying the inverse square law for light. Question 5 A standard candle is. a 7 cm long wax candle a light source of known luminosity another name for a barred spiral galaxy another name for a main sequence star 2/9

3 Question 6 What does Hubble's law tell us? The more distant a galaxy, the faster it is moving away from us. The faster a spiral galaxy's rotation speed, the more luminous it is. For every force, there is an equal and opposite reaction force. The longer the period of a Cepheid variable, the greater its luminosity. Question 7 If we say that a galaxy has a lookback time of 1 billion years, we mean that. it was 1 billion light years away when the light left the galaxy it is 400 million years old it is now 1 billion light years away its light traveled through space for 1 billion years to reach us Question 8 Learning Goal: To understand how observational evidence is used to test the idea that we live in an expanding universe. Introduction. Today, overwhelming evidence indicates that we live in an expanding universe evidence that must therefore be accounted for in any theory of how the universe came to exist. In this activity, you will consider a set of observations, some that are real and some that are not, and consider which of them support the idea that we live in an expanding universe and which of them, if real, would force us to modify or abandon this idea. Consider the following hypothetical observations, some of which are real and some of which are fictional. For each observation, your job is to answer this question: If the observation were real, would it provide evidence for or against the idea that the universe is expanding? Sort each observation into the appropriate bin as follows: Place an observation in the Supports the expanding universe bin if it would provide evidence that the universe is expanding. Place an observation in the Contradicts the expanding universe bin if it would provide evidence that would force us to reconsider the idea of an expanding universe. Place an observation in the Neither supports nor contradicts bin if it does not allow us to distinguish between a universe that is expanding and a universe that is not expanding. Hint 1. What is Hubble s law? Real observations of the type first made by Edwin Hubble show that, on average, galaxy speeds obey Hubble s law. Mathematically, this law is v = H0 d, where v is a galaxy s speed (velocity), d is its distance, and H0 is Hubble s constant. In words, Hubble s law states that 3/9

4 . all galaxies are moving away from us at the same high speed all galaxies travel at the speed of Hubble s constant a galaxy s speed is proportional to its distance from us Hint 2. What does a redshift tell us? A redshifted spectrum tell us that an object. is very far away is moving away from us emits more strongly in red light than in blue light Hint 3. Definitions of the Local Group and the Coma cluster The Local Group is the group of approximately 40 galaxies of which our own Milky Way is a member; these galaxies are gravitationally bound together. The Coma cluster, which gets its name because it is observed in the constellation of Coma Berenices, contains more than 1000 galaxies and is located about 300 million light years away. Hint 4. An example of an observation that would go in the "Neither supports nor contradicts" bin The category of "Neither supports nor contradicts" the expanding universe would include observations that are irrelevant to whether the universe is expanding or not. For example, an observation such as "Stars in our own stellar neighborhood are moving in all directions relative to us" would go in this category, because our own galaxy is held together by gravity and therefore is not expanding. All attempts used; correct answer displayed Note that the three observations in the middle bin are all fictional; no known, real observations contradict the idea that we live in an expanding universe. 4/9

5 Now, use the following questions to help make sure you understand all the answers from. Part B Consider the hypothetical observation "Irregular galaxies outside the Local Group are moving toward us." From, this observation would contradict the idea of an expanding universe. Why? Hint 1. Does expansion affect different galaxy types in different ways? For any particular distance, which galaxy type moves away from us at the highest speed due to the expansion of the universe? irregular galaxies only spiral galaxies only elliptical galaxies only all galaxies Because the idea that the universe is expanding should be observable only within our Local Group. Because while irregular galaxies are expected to move with irregular speeds, they should still be moving away from us. Because Hubble s law predicts that all galaxies outside our Local Group should be moving away from us. Because irregular galaxies are expected to show a different pattern of expansion than other galaxy types. Hubble s law describes the expansion of the universe, which is an expansion of space itself. Therefore, it must affect all galaxies outside the Local Group in the same way, causing them to move away from us, regardless of their type. Part C Consider the observation "The Andromeda Galaxy, a member of our Local Group, is moving toward us." Why doesn t this observation contradict the idea that the universe is expanding? Hint 1. Does Hubble s Law apply to all galaxies? Hubble s law predicts that a galaxy s speed away from us should be proportional to its distance from us. Do all galaxies obey this law precisely? No, because galaxy speeds are also affected by the gravity of other galaxies near them. No, because the Big Bang shot some galaxies out at higher speeds than others. Yes. Because the galaxies of the Local Group are gravitationally bound together. Because the Local Group is located at the center of the expansion. Because the Andromeda Galaxy is a spiral galaxy. Because expansion is expected to cause some galaxies to move toward us. The universe as a whole is expanding, but groups and clusters of galaxies (as well as individual galaxies) represent places where gravity has won out over the expansion. Therefore, in a gravitationally bound group, such as the Local Group, we do not expect the galaxy motions to follow Hubble s law. 5/9

6 Part D We can in principle measure the expansion rate by studying galaxies in many different directions in space and at different times of year. If we compare such observations, we would find that the expansion rate is. Hint 1. Phenomena that vary seasonally Phenomena that vary seasonally, such as the weather or the path of the Sun through the sky, must be caused in some way by the properties of Earth itself as it orbits the Sun. Phenomena that are not intrinsic to Earth should not show seasonal variations. the same at all times of year, but varies in different directions the same no matter when or in which direction we measure it the same in all directions, but varies with the time of year varies with both the time of year and the direction in which we look The expansion rate is a property of the universe as a whole, and therefore it should not vary with either direction from Earth or time of year. That is why the statement in suggesting that the expansion rate is the same in all directions supports the idea of an expanding universe, while the statement about a seasonal variation would contradict the idea of an expanding universe. Question 9 Which of these items is a key assumption in our most successful models for galaxy formation? The distribution of matter was perfectly uniform early in time. Some regions of the universe were slightly denser than others. Galaxies formed around supermassive black holes. Question 10 Current understanding holds that a galaxy's type (spiral, elliptical, or irregular). may either be a result of the mass of the protogalactic cloud that formed it or the result of the heavy element abundance in that cloud may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies is always determined by the angular momentum of the protogalactic cloud that formed it is determined by whether the galaxy is located in a cluster where collisions are likely or outside a cluster where collisions are less likely Question /9

7 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. Galaxies were more active in the past and therefore would have collided with each other more frequently. Galaxies were much bigger in the past since they had not contracted completely. Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars. Question 12 Match the words in the left hand column to the appropriate blank in the sentences in the right hand column. Use each word only once. Hint 1. What the galactic wind is The galactic wind is a wind of low density but extremely hot gas flowing out from a starburst galaxy, created by the combined energy of many supernovae. Hint 2. What quasars are Quasars are the brightest types of active galactic nuclei. Hint 3. What starburst galaxies are Starburst galaxies are galaxies in which stars are forming at an unusually high rate. Hint 4. What radio galaxies are Radio galaxies are galaxies that emit unusually large quantities of radio waves. They are thought to contain an active galactic nucleus powered by a supermassive black hole. Hint 5. What central dominant galaxies are Central dominant galaxies are giant elliptical galaxies found at the center of a dense cluster of galaxies, apparently formed by the merger of several individual galaxies. Hint 6. What supermassive black holes are Supermassive black holes are giant black holes, with masses of millions to billions times that of our Sun, thought to reside in the centers of many galaxies and to power active galactic nuclei. 7/9

8 Reset Help 1. Starburst galaxies have the highest rates of star formation. 2. The largest individual galaxies in the universe are known as central dominant galaxies. 3. The collective activity of many supernova events in a relatively small volume of a galaxy can create galactic winds. 4. The most luminous objects in the universe are quasars. 5. The energy for all active galactic nuclei is thought to comes from in fall of matter into supermassive black holes. 6. Radio galaxies are often characterized by sources of immense energy located hundreds of thousands of light years away on either side of their centers. Question 13 The luminosity of a quasar is generated in a region the size of the Milky Way. a star cluster. the solar system. Question 14 The primary source of a quasar's energy is chemical energy. nuclear energy. gravitational potential energy. 8/9

9 Question 15 Which of the following phenomena is probably NOT related to the presence of a supermassive black hole? quasars the radio emission from radio galaxies the presence of globular clusters in the halos of galaxies the huge jets seen emerging from the centers of some galaxies Question 16 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. intense radiation emitted by the black hole itself radio waves emitted from radio lobes found on either side of the galaxy we see in visible light light emitted by hot gas in an accretion disk that swirls around the black hole the combined light of thousands of young, high mass stars that orbit the black hole Score Summary: Your score on this assignment is 95.9%. You received out of a possible total of 41 points. 9/9

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