Astronomy Chapter 16 Homework

12/4/2019 Ch 16 HW Ch 16 HW Due: 11:59pm on Tuesday, November 26, 2019 You will receive no credit for items you complete after the assignment is due. Grading Policy Prelecture Overview: Galaxies and the Foundation of Modern Cosmology First, launch the videobelow. Then, close the video window and answer the questions at right. You can watch the video again at any point. Part A Match the words at the left to the correct blanks in the sentences at right.Refer to the labeled images shown below. ANSWER: Reset  Help Image A is shows a(n) irregular galaxy .  Image E is shows a(n) spiral galaxy . star cluster   Image C is shows a(n) elliptical galaxy . Image B is shows a(n) cluster of galaxies .  Image D is shows a(n) set of galaxies at many different distances from Earth . set of galaxies that are all less than 1 billion years old Answer Requested Be sure to review the video if you are unsure of what any of these images represent. Part B Match the words at the left to the correct blanks in the sentences at right. Hint 1. What happens to an inflated balloon when you heat it? If you heat an inflated balloon, what happens to it? ANSWER: . 1/46 12/4/2019 Ch 16 HW he gas particles inside it move faster, causing it to expand until it stabilizes at a new, larger size. he gas particles inside it move slower, causing it to contract until it stabilizes at a new, smaller size. he rate of nuclear fusion skyrockets inside the balloon. ANSWER: Reset prominent disk Help On a graph of galaxy colors and luminosities, spiral galaxies are usually found in the blue cloud . bulge The central region of a spiral galaxy that is yellowish in color is called the bulge . halo blue cloud On a graph of galaxy colors and luminosities, elliptical galaxies are usually found in the red sequence . red sequence Spiral galaxies have a prominent disk and elliptical galaxies do not. The roundish region of a spiral galaxy that surrounds its entire disk is called the halo . Correct Part C Each item below can beused to measure the distances of astronomical objects. Rank them according to how far away each method works, from nearest to farthest. (That is, put the method that works only in our solar system at the far left, and the method that can be used for the most distant galaxies at the far right.) ANSWER: Reset  radar ranging  parallax  Cepheids Help  white dwarf supernovae 2/46 12/4/2019 Ch 16 HW Correct As discussed in the video, these are the major links in the chain of methods we use to measure cosmic distances. Part D Imagine that you lived in a dot on the surface of an expanding balloon, and as you looked to more distant dots, you observed the following: A dot at a distance of5 centimeters from you is moving away from you at a speed of 1 centimeter per hour (1 cm/hr). A dotat a distance of 10 centimeters from you is moving away from you at a speed of 2 centimeters per hour (2 cm/hr). A dot at a distance of 15 centimeters from you is moving away from you at a speed of 3 centimeters per hour (3 cm/hr). Which equation below correctly states the relationship between the dot distances and speeds as observed by you? Hint 1. Where to look in the video. As you watch the video, be sure you understand where the formula speed = distance/(3 seconds) comes from, then use the same reasoning to answer Part D. ANSWER: large{{\rm{dot \ speed = }}\frac{{{\rm{dot \ distance}}}}{{{\rm{ 5 \ hr}}}}} large{{\rm{dot \ speed = }}\frac{{{\rm{dot \ distance}}}}{{{\rm{ 1 \ hr}}}}} large{{\rm{dot \ speed = }}\frac{{{\rm{dot \ distance}}}}{{{\rm{ 5 \ cm}}}}} Correct Notice that this formula gives the correct values listed above: For the dot that is 5 centimeters from you, the formula gives a speed of \large{\frac{{{\rm{5 \ cm}}}}{{{\rm{ 5 \ hr}}}}}, which is 1 cm/hr. For the dot that is 10 centimeters from you, the formula gives a speed of \large{\frac{{{\rm{10 \ cm}}}}{{{\rm{ 5 \ hr}}}}}, which is 2 cm/hr. For the dot that is 15 centimeters from you, the formula gives a speed of \large{\frac{{{\rm{15 \ cm}}}}{{{\rm{ 5 \ hr}}}}}, which is 3 cm/hr. Part E Consider the same balloon as in Part D. Based on what you learned in the video and your answer to Part D, if you assume that your balloon began very tiny and has been expanding at a constant rate,what can you conclude about how long your balloon has been expanding? ANSWER: our balloon has been expanding for 1 hour. our balloon has been expanding for 5 hours. our balloon has been expanding for 3 seconds. ou cannot draw a conclusion with the information given. Correct As discussed in the video, we use the same basic idea to determine the age of the universe from the speed and distance observations that lead to Hubble's law. Measuring Cosmic Distances Tutorial NOTE: These activities use Flash, and are therefore not screen-reader accessible and may not work on a mobile device. If the browser you’re using no longer supports Flash, try a different browser and download the Flash plug-in for this content. Th al distances are measured. 3/46 12/4/2019 Ch 16 HW Part A Your work within the launched tutorial will automatically save, but your score will not be recorded until you click the Submit button below. Clicking within the tutorial will bring you back to this screen. You must click Submit (below) to record your score to the MasteringAstronomy gradebook. It is not possible to do any more work toward your score once you have clicked Submit.Your score is based 50% on working through the “Lessons” portion of the launched tutorial and 50% on your first answer to each question in the “Exercises” section of the launched tutorial. ANSWER: launchtutorial Your final score on this tutorial is 100% Score Submitted You have now submitted your score and completed this tutorial. Hubble's Law Tutorial NOTE: These activities use Flash, and are therefore not screen-reader accessible and may not work on a mobile device. If the browser you’re using no longer supports Flash, try a different browser and download the Flash plug-in for this content. This tutorial will help you understand Hubble's Law and its implications for the age and rate of expansion of the universe. 4/46 12/4/2019 Ch 16 HW Part A Your work within the launched tutorial will automatically save, but your score will not be recorded until you click the Submit button below. Clicking within the tutorial will bring you back to this screen. You must click Submit (below) to record your score to the MasteringAstronomy gradebook. It is not possible to do any more work toward your score once you have clicked Submit. Your score is based 50% on working through the “Lessons” portion of the launched tutorial and 50% on your first answer to each question in the “Exercises” section of the launched tutorial. ANSWER: launchtutorial Your final score on this tutorial is 100% Score Submitted You have now submitted your score and completed this tutorial. Prelecture Narrated Figure: Visual Activity: A Graph of Hubble’s Law First,launch the videobelow. Then, close the video window and answer the questions at right. You can watch the video again at any point. 5/46 12/4/2019 Ch 16 HW Part A The graph of Hubble's law shows that galaxies with high speeds as measured from Earth are __________. Hint 1. How do you read galaxy speeds on the graph? On the graph in the interactive figure, galaxies with higher speeds away from Earth are found _____. ANSWER: igher up ower down urther right. urther left Hint 2. How do you read galaxy distances on the graph? On the graph in the interactive figure, galaxies with greater distances from Earth are found _____. ANSWER: igher up ower down urther right urther left ANSWER: moving away from Earth and are farther from Earth than galaxies with lower speeds moving toward Earth and are farther from Earth than galaxies with lower speeds moving away from Earth and are closer to Earth than galaxies with lower speeds moving toward Earth and are closer to Earth than galaxies with lower speeds Correct This is the essence of Hubble’s law: More distant galaxies are moving away from us faster. Part B By looking at the graph of Hubble's law, what can we say about the galaxies that have the lowest speeds? ANSWER: hey are moving away from Earth and are farther from Earth than galaxies with high speeds. hey are moving toward Earth and are farther from Earth than galaxies with high speeds. hey are moving away from Earth and are closer to Earth than galaxies with high speeds. hey are moving toward Earth and are closer to Earth than galaxies with high speeds. Correct This fact also follows from Hubble’s law. Note that this applies to galaxies outside the Local Group. Within the Local Group, some galaxies move[ t Milky Way. 6/46 12/4/2019 Ch 16 HW Part C Suppose that galaxy B is twice as far from Earth as galaxy A. Hubble’s law predicts that galaxy B will be moving away from Earth with approximately _____. Hint 1. Can you see Hubble’s law on the graph? On the graph in the interactive figure, Hubble’s law is _____. ANSWER: epresented by all the red dots epresented by the straight line that best fits the data (red dots) ot possible to see ANSWER: he same velocity as galaxy A our times the velocity of galaxy A alf the velocity of galaxy A wice the velocity of galaxy A Correct As the graph shows, Hubble’s law (the straight-line fit to the data) predicts that a galaxy’s recession velocity is proportional to its distance from Earth. Hubble’s law therefore predicts that a galaxy twice as far away moves at twice the speed, a galaxy three times as far away moves at three times the speed, and so on. Part D Imagine that when we looked out into the universe we found that the light from all galaxies was blueshifted (rather than redshifted) and that the light from the most distant galaxies was blueshifted by the greatest amount. Which statement best describes what we would conclude about the motions of galaxies in this case? Hint 1. What does a blueshift tell us about the object's direction of motion? If an object’s spectrum shows a blueshift, it tells us that the object _____. ANSWER: s moving toward us s moving away from us s moving across our line-of-sight ANSWER: ll are moving toward Earth, with nearby galaxies moving faster than distant galaxies. ll are moving toward Earth, with distant galaxies moving faster than nearby galaxies. ll are moving away from Earth, with nearby galaxies moving faster than distant galaxies. ll are moving away from Earth, with distant galaxies moving faster than nearby galaxies. 7/46 12/4/2019 Ch 16 HW Correct In this hypothetical case, all the galaxies would be rushing toward Earth, with the more distant galaxies approaching at the fastest pace. This would tell us we lived in a collapsing universe rather than an expanding one. Part E The age of the universe is related to the slope of the graph of Hubble’s law, and current data put the age of the universe at about 14 billion years. Suppose that future observations showed that the slope of Hubble’s law on the graph is actually steeper than that shown. In that case, the age of the universe would be _________ than 14 billion years because the universe is expanding ______ than current data suggest. (Each choice gives words to fill in the two blanks, separated by a slash.) Hint 1. How is the rate of expansion related to the age of the universe? The greater the expansion rate, the ______ the age of the universe. ANSWER: lder ounger ANSWER: ounger / more rapidly ounger / more slowly lder / more rapidly lder / more slowly Correct A more steeply sloped line for Hubble’s law would indicate faster speeds for galaxies at all distances, which would mean a faster rate of expansion. And a faster rate of expansion means it has taken less time since the Big Bang for the universe to reach its present size. In other words, a steeper slope (which means a larger value of Hubble’s constant) means a younger and more rapidly expanding universe. Ranking Task: Understanding Hubble’s Law Part A The following figures give the approximate distances of five galaxies from Earth. Rank the galaxies based on the speed with which each should be moving away from Earth due to the expansion of the universe, from fastest to slowest. Hint 1. What is Hubble’s law? Which of the following correctly captures the meaning of Hubble’s law? ANSWER: xpansion of the universe means that more distant galaxies move away from us at faster speeds. xpansion of the universe means that more distant galaxies move toward us at faster speeds. he speeds of distant galaxies depend both on their distances and on their masses. ANSW 8/46 12/4/2019 Ch 16 HW Reset     Help  Correct Hubble’s law tells us that the more distant a galaxy is from Earth, the faster it is moving away from us. Part B The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on their distance from Earth, from farthest to closest. Hint 1. What is Hubble’s law? Which of the following correctly captures the meaning of Hubble’s law? ANSWER: xpansion of the universe means that more distant galaxies move away from us at faster speeds. xpansion of the universe means that more distant galaxies move toward us at faster speeds. he speeds of distant galaxies depend both on their distances and on their masses. ANSWER: 9/46 12/4/2019 Ch 16 HW Reset     Help  Correct Again, we see Hubble’s law in action: more distant galaxies move at higher speeds. Part C The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on the amount of redshift that would be observed in each galaxy’s spectrum, from largest to smallest. Hint 1. What does a Doppler shift tell us? If an object’s spectrum shows a redshift, which is a Doppler shift toward longer wavelengths, we know that ___________. ANSWER: he object is moving toward us, and the larger the redshift the faster it is moving he object is moving away from us, and the larger the redshift the faster it is moving he object is old, and the larger the redshift the older the object must be ANSWER: 10/46 12/4/2019 Ch 16 HW Reset     Help  Correct Recall that a redshift tells us that an object is moving away from us, and the larger the redshift, the higher the speed. Note that expansion applies to the whole universe, so astronomers tend to think of galaxies being carried along with the expansion rather than moving “through” the universe. For this reason, redshifts due to expansion are often called “cosmological redshifts.” Part D Hubble’s law is a relationship between galaxy speeds and galaxy distances. This relationship can be shown as a line on a graph of speed versus distance. Each of the following four graphs shows a possible relationship expressing Hubble’s law. Rank the graphs based on their predictions of the speed, from fastest to slowest, for a galaxy located 400 million light-years away from Earth. Hint 1. How do you read speeds from the graphs? On the graph with the steepest slope (closest to being a vertical line) in Part D, what is the approximate speed predicted for a galaxy located 400 million light-years away? ANSWER: 0,000 km/s 0,000 km/s 0,000 km/s ANSWER: 11/46 12/4/2019 Ch 16 HW Reset    Help  Correct If you look at the speed corresponding to a distance of 400 million light-years on each graph, you will see that the speed declines in the order shown. Note also that this ranking puts the graphs in order of declining steepness. In other words, a steeper slope for Hubble’s law would predict faster speeds for galaxies at particular distances. Part E The following four graphs are the same as those from Part D, with each showing a possible relationship for Hubble’s law. Rank the graphs based on the prediction they each would make for the current age of the universe, from oldest to youngest. Hint 1. How do galaxy speeds tell us the expansion rate of the universe? From Part D, you already know that a steeper graph (of galaxy speeds versus distances) implies that galaxies at any particular distance are moving away from us at higher speeds. This, in turn, implies that a steeper graph means a _____. ANSWER: more rapid rate of expansion lower rate of expansion more uniform rate of expansion Hint 2. What does a more rapid expansion rate imply about the age of the universe? A more rapid rate of expansion implies _____. ANSWER: younger age for the universe n older age for the universe more massive universe ANSWER: 12/46 12/4/2019 Ch 16 HW Reset    Help  Correct From Part D, you know that a steeper graph of Hubble’s law predicts a higher speed for galaxies at particular distances. Because galaxies all started out close together (early in the history of the universe), higher speeds would mean that they would have reached their current distances in less time — and less time means a younger universe. In other words, the steeper the graph for Hubble’s law, the more rapid the rate of expansion (a higher value of Hubble’s constant), and the more rapid the rate of expansion the younger the universe. Ranking Task: Expansion of the Universe Part A The following diagrams represent a balloon analogy for the expansion of the universe. Each diagram shows two balloons: The small pink balloon represents the universe at an early time and the large red balloon represents the universe at some later time. The black dot on each balloon represents a galaxy. Rank the diagrams based on how much the galaxy has expanded in size (due to the overall universal expansion) from the early time to the later time in each case, from most expansion to least expansion. If you think that two (or more) of the diagrams show galaxies that have expanded by the same proportion, indicate this equality by dragging one diagram on top of the other(s). Hint 1. What is growing in size as the universe expands? As the universe expands, __________. ANSWER: verything in the universe is gradually growing in size at the same rate arger galaxies expand more rapidly than smaller galaxies he average distance between galaxies is growing, but galaxies themselves remain stable in size ANSWER: 13/46 12/4/2019 Ch 16 HW Reset     Help  Correct The universe as a whole is expanding, but gravitationally bound objects such as galaxies are not expanding with time. Because no galaxy expands at all, all the diagrams are ranked equal. Part B The following diagrams are similar to those in Part A, except this time each one shows two galaxies: “your galaxy” and a second galaxy. Rank the diagrams based on how far the second galaxy has moved away from your galaxy due to expansion of the universe between the early time and the later time, starting at the left with the one that has moved the largest distance and moving to the one that has moved the smallest distance at the right. ANSWER: Reset    Help  14/46 12/4/2019 Ch 16 HW Correct Notice that the galaxies that started farther away (on the small balloon) traveled greater distances as the balloon expanded. Part C The following diagrams are similar to those from Part B, with each diagram showing the position of “your galaxy” and another galaxy at an early and later time in the history of the universe. Rank the diagrams based on the speed at which the other galaxy is moving away from your galaxy as the universe expands, from fastest to slowest. Hint 1. How do you determine the relative speeds of the galaxies? All four diagrams represent the same amount of time between the small and large balloons. Given that fact, it follows that __________. ANSWER: he galaxy that moves the greatest distance must be moving at the greatest speed he galaxy that moves the greatest distance must be moving at the slowest speed ll the galaxies must be moving at exactly the same speed ANSWER: Reset    Help  Correct Notice that the farther away a galaxy is from your galaxy, the faster it will be moving away from you. This simple statement embodies the idea behind Hubble’s law. Part D The following diagrams are similar to those from parts B and C, with each diagram showing the position of “your galaxy” and another galaxy at an early and later time in the history of the universe. (But note that this set of diagrams differs from the prior sets.) Rank the diagrams based on the speed at which the other galaxy is moving away from your galaxy as the universe expands, from fastest to slowest. If two (or more) of the diagrams show galaxies moving at the same speed, show this equality by dragging one diagram on top of the other(s). (Hint: Notice that two of the diagrams show the galaxies separated by one distance and two of the diagrams show them separated by a different distance; that is, there are only two different distances shown among the four diagrams.) 15/46 12/4/2019 Ch 16 HW Hint 1. How do speeds of galaxies compare when viewed from different locations? Suppose that observers in three different galaxies are all looking at the same “Galaxy X.” Which observer will see Galaxy X moving at the highest speed due to expansion of the universe? ANSWER: he observer located closest to the center of the universe he observer in the galaxy closest to Galaxy X he observer in the galaxy farthest from Galaxy X ANSWER: Reset    Help  Correct Notice that the location of your galaxy does not matter: The speed at which the other galaxy is moving away from you depends only on its distance, with more distant galaxies moving faster in accord with Hubble’s law. Process of Science: Testing the Expanding Universe 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. Part A 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. 16/46 12/4/2019 Ch 16 HW 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 = H_0 \times d, where \texttip{v}{v} is a galaxy’s speed (velocity), \texttip{d}{d} is its distance, and \texttip{H_{\rm 0}}{H_0} is Hubble’s constant. In words, Hubble’s law states that __________. ANSWER: ll galaxies are moving away from us at the same high speed galaxy’s speed is proportional to its distance from us ll galaxies travel at the speed of Hubble’s constant Hint 2. What does a redshift tell us? A redshifted spectrum tell us that an object __________. ANSWER: s very far away s moving away from us mits 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. ANSWER: 17/46 12/4/2019 Ch 16 HW Reset   Help      All galaxies in the Coma cluster of galaxies have redshifted spectra. Irregular galaxies outside the Local Group are moving toward us. Galaxies 200 million light-years away move away from us twice as fast as galaxies 100 million light-years away. Galaxy speeds are faster in summer than in winter. The measured rate of expansion is the same in all directions. The Andromeda Galaxy, a member of our Local Group, is moving toward us. Spiral galaxies move away from us 10% faster than elliptical galaxies at the same distances. Correct 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. Now, use the following questions to help make sure you understand all the answers from Part A. Part B Consider the hypothetical observation "Irregular galaxies outside the Local Group are moving toward us." From Part A, 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? ANSWER: regular galaxies only piral galaxies only lliptical galaxies only ll galaxies ANSWER: ecause the idea that the universe is expanding should be observable only within our Local Group. ecause irregular galaxies are expected to show a different pattern of expansion than other galaxy types. ecause Hubble’s law predicts that all galaxies outside our Local Group should be moving away from us. ecause while irregular galaxies are expected to move with irregular speeds, they should still be moving away from us. 18/46 12/4/2019 Ch 16 HW Correct 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? ANSWER: No, because the Big Bang shot some galaxies out at higher speeds than others. No, because galaxy speeds are also affected by the gravity of other galaxies near them. es. ANSWER: ecause the Andromeda Galaxy is a spiral galaxy. ecause the galaxies of the Local Group are gravitationally bound together. ecause expansion is expected to cause some galaxies to move toward us. ecause the Local Group is located at the center of the expansion. Correct 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. 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. ANSWER: he same at all times of year, but varies in different directions aries with both the time of year and the direction in which we look he same in all directions, but varies with the time of year he same no matter when or in which direction we measure it 19/46 12/4/2019 Ch 16 HW Correct 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 Part A 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. Prelecture Narrated Figure: The Cosmic Distance Scale—Hubble's Law Learning Goal: To understand how we calibrate Hubble's law and use it to estimate galaxy distances. First, launch the video below. Then, close the video window and answer the questions at right. You can watch the video again at any point. Part A Hubble's law expresses a relationship between __________. Hint 1. What is a galaxy's recession velocity? What do we mean by a galaxy's recession velocity? ANSWER: he speed at which particles ejected from its central black hole are receding from us. he speed at which its outermost stars are orbiting its galactic center. he speed at which it is moving away from Earth due the expansion of the universe. ANSWER: he recession velocity of a galaxy and the speed at which it is moving away from us he distance of a star from the center of its galaxy and its orbital speed he distance of a galaxy and the speed at which it is moving away from us ubble's constant and the recession velocity of a galaxy Correct Hubble's law is a relationship between galaxy distances and the speeds at which they are moving away from us due to the expansion of the universe, also known as their recession velocities. Other factors also affect total galaxy speeds, such as the gravitational influence of neighboring galaxies, so Hubble's law works well only when these other factors are small compared to the speed due to expansion. This is why Hubble's law works well only for very distant galaxies: Their recession speeds are so great that any motions caused by neighboring galaxies are tiny in comparison. Part B We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first 20/46 12/4/2019 Ch 16 HW Hint 1. Definition of "calibration" in the context of Hubble's law The word "calibrate" means "to determine the precise measurement." In the context of Hubble's law, this means determining precisely how galaxy recession velocities vary with distance. Therefore, to calibrate Hubble's law, we must measure both the recession velocities and distances of many galaxies. The distance measurements must come from some technique that does not depend on Hubble's law itself, such as a standard candle technique using Cepheids or white dwarf supernovae. ANSWER: measuring the distances to many distant galaxies with parallax measuring the distances to many distant galaxies with a standard candle technique sing Hubble's law to estimate the distances of many galaxies etermining the age of the universe Correct We need distant standard candles, such as white dwarf supernovae, to measure the distances of galaxies so that we can plot them on a graph of recession velocity (which we can always measure from redshift) versus distance. It is from this graph that we learn Hubble’s law and find the value of Hubble's constant. Part C Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 (3 \times 10^4) kilometers per second. According to Hubble's law, approximately how far away is the galaxy? Hint 1. Finding the answer with the Interactive Figure You can use the Interactive Figure to find the answer to Part C. From the main screen, click on the blue label for "Hubble's Law," then click the "Use Hubble's Law" button. You can now use the slider to select the recession velocity and read off the distance for the galaxy. Note that 1000 million light-years is the same as 1 billion light-years. Hint 2. Finding the answer with a calculation You can find the answer to Part C using the Interactive Figure. However, if you prefer to use an equation, it is easy to calculate with Hubble's law. The law states v = H_0 \times d where \texttip{v}{v} is the recession velocity, \texttip{d}{d} is the distance, and \texttip{H_{\rm 0}}{H_0} is Hubble's constant. Solving for \texttip{d}{d}, we find \large{d = \frac{v}{{H_0 }}} Therefore, for a recession velocity of 30,000 \rm km/s, the distance is \large{d = \frac{{30,000{\rm{~km/s}}}}{{22~{\rm{ }}\frac{{{\rm{km/s}}}}{{{\rm{million~ light \textrm{-} years}}}}}}} You can complete the calculation with a calculator. ANSWER: 5 million light-years .7 billion light-years .4 billion light-years 10 million light-years Correct You should now see the value of Hubble's law: Once it has been calibrated, which requires measurements of only a relatively small number of galaxies, it can be used to estimate the distance of any other distant galaxy. Part D 21/46 12/4/2019 Ch 16 HW Based on what you have learned, which of the following best describes the meaning of Hubble's constant (\texttip{H_{\rm 0}}{H_0} )? Hint 1. Reading the units of Hubble's constant The graph in the Interactive Figure shows that the best-fit value of Hubble's constant is H_0 = 22\; \rm km/s/million~light\textrm{-}years. We read this as "22 kilometers per second, per million light-years." Look closely at the axes on the graph and you should see where these units come from. Hint 2. What if Hubble's constant were larger? Suppose that Hubble's constant is actually larger than the current best-fit value of 22 km/s/million light-years. Compared to what we now calculate from Hubble's law, the larger value of Hubble's constant would tell us that a galaxy located 1 billion light-years away is __________. ANSWER: s more massive but still moving at the same speed moving away from us slower due to the expansion of the universe moving away from us faster due to the expansion of the universe ANSWER: describes the distances at which galaxies are located in an expanding universe. describes the expansion rate of the universe, with higher values meaning less rapid expansion. describes the expansion rate of the universe, with higher values meaning more rapid expansion. describes the total mass of our expanding universe. Correct The larger the value of Hubble's constant, the faster galaxies are being carried away from us with the expansion of the universe. For example, if a galaxy's velocity depended only on the expansion of the universe, then a galaxy that is 1 million light-years away would have a recession velocity of 22 kilometers per second, a galaxy 10 million light-years away would have a recession velocity of 220 kilometers per second, and a galaxy 100 million light-years away would have a recession velocity of 2200 kilometers per second. In other words, Hubble's constant essentially tells us the expansion rate of the universe, since it tells us how fast galaxies are moving at different distances away from us. A higher value of Hubble's constant would mean higher recession speeds—which would mean a more rapid expansion rate for the universe. The Cosmic Distance Scale—Cepheids Learning Goal: To understand the properties of Cepheids and how they are used to measure distances to nearby galaxies. First, launch the interactive figure below. Cepheids are a key link in the chain of techniques used to measure distances throughout the universe. This tutorial will guide you through exploring Cepheids in detail. Part A 22/46 12/4/2019 Ch 16 HW 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 __________. Hint 1. What are Cepheids? Cepheids are a type of _____. ANSWER: as cloud tar alaxy ANSWER: epheids with longer periods are more likely to vary in luminosity epheids with longer periods have higher luminosities epheids with longer periods have lower luminosities epheids become more luminous as more time passes Correct The graph shows how the average luminosity of a Cepheid variable star depends on its period. Each dot represents a different Cepheid. The data points suggest a clear trend: The longer the period of a Cepheid, the higher its luminosity. Part B A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun. Hint 1. Reading the graph The key to reading the graph is to focus on the solid red “best-fit” line, not on the individual dots. Find a period of 30 days on the horizontal axis, then look upward to find where this period meets the red line. Now, look left to find the luminosity that goes with the 30-day period. ANSWER: 0,000 0,000 00 000 Correct You can now see why the period-luminosity relation is so useful. It's relatively easy to measure a Cepheid's period, and once we've done that, we can use the period-luminosity relation to determine the Cepheid's average luminosity. Part C Next, click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Read the instructions that appear in the upper left hand corner and study the animation. What actually causes a Cepheid to vary in apparent brightness? . Hint 1. What factors determine the luminosity of a star? 23/46 12/4/2019 Ch 16 HW The two properties of a star that affect its luminosity are its __________. ANSWER: mass and distance mass and radius adius and surface temperature ANSWER: he Cepheid varies in mass, and its luminosity is greater when its mass is smaller. he Cepheid varies in radius, and its luminosity is greater when its radius is larger. he Cepheid varies in radius, and its luminosity is greater when its radius is smaller. he Cepheid varies in mass, and its luminosity is greater when its mass is larger. he Cepheid moves alternately toward and away from us, and its apparent brightness is greater when it is closer. Correct As the animation shows, a Cepheid's apparent brightness varies as the Cepheid varies in size. Because a Cepheid is at a fixed distance from Earth, these changes in apparent brightness reflect changes in luminosity. This fact, along with the fact that all Cepheids have similar surface temperature, explains the period-luminosity relation: A larger Cepheid will be more luminous and will take longer to pulsate in size than a smaller Cepheid. Part D From here, click the Next button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. What is the approximate luminosity of the Cepheid whose light curve is shown in the graph? Hint 1. What is the Cepheid's period? Approximately what is the period of the Cepheid whose light curve is shown? ANSWER: 0 days 00 days 0 days Hint 2. Using the graph to find the Cepheid's luminosity Follow this procedure to find the Cepheid's luminosity: Step 1: Place your cursor over the light curve, and it will show you the position in days along the horizontal axis. Measure the number of days from one peak to the next; this is the period. Step 2: Now place your cursor over the graph of the period-luminosity relation. As you move it, you should see two labeled yellow lines, as shown in the figure below. For the point where the lines intersect, the vertical line tells you the period in days and the horizontal line tells you the luminosity in solar units. Be sure that you place the cursor on the blue curve and that you set the period to the period you found in Step 1. You can then read the luminosity that goes with this period. Note that the cursor is very sensitive, but as long as you are approximately correct, you should get the right answer. 24/46 12/4/2019 Ch 16 HW ANSWER: 000 \texttip{L_{\rm Sun}}{L_Sun} 0,000 \texttip{L_{\rm Sun}}{L_Sun} 000 \texttip{L_{\rm Sun}}{L_Sun} 000 \texttip{L_{\rm Sun}}{L_Sun} Correct By mousing over the Cepheid light curve, you should have found that the period is about 20 days. Then, by mousing over the graph of the periodluminosity relation, you'll find that a Cepheid with a period of 20 days has a luminosity of about 8000 \texttip{L_{\rm Sun}}{L_Sun}. You've essentially just done what an astronomer does: You've taken the measured period of a Cepheid and used to find the Cepheid's luminosity. Part E Notice that the window with the light curve also has a box that tells you the average apparent brightness of the Cepheid, which shows a value of 1.3 \times 10^4. (The units are shown in the box, but you only need to focus on the numerical value.) Based on this apparent brightness and the luminosity you found in Part D, what is the approximate distance to this Cepheid? Hint 1. Procedure for finding the distance with the Interactive Figure Follow this procedure to find the Cepheid's distance: Step 1: Be sure you are on the correct screen of the Interactive Figure; you should see a light curve at the top left and a graph of the period-luminosity relation at the top right. If you are not on the correct screen, go back to the main screen, click the blue Cepheid label, then click the Cepheids as Standard Candles button, and then click the Next button at the lower left. Step 2: From Part D, you already know that the luminosity of the Cepheid is about 9600 \texttip{L_{\rm Sun}}{L_Sun}, which in scientific notation is 9 \times 10^3 \texttip{L_{\rm Sun}}{L_Sun}. Set the luminosity slider so the readout is as close as possible to this value. It doesn't have to be exactly 9. Step 3: Now move the Distance slider until the "Predicted brightness" matches the "Apparent brightness" listed for this Cepheid; you have now found the Cepheid's distance. You should see that as the distance increases, the "Predicted brightness" gets smaller (because moving an object farther away makes it appear less bright). ANSWER: 00,000 light-years million light-years 7 million light-years 5,000 light-years 25/46 12/4/2019 Ch 16 HW Correct To find this answer, set the luminosity slider to the value you found in Part D. Then adjust the distance slider until the "Predicted brightness" matches the Cepheid's apparent brightness of 1.3\times 10^4 (in the units shown). Notice that you've now found the Cepheid's distance by starting only from its period, which is generally easy to measure. Part F Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements? In all cases, assume that the Cepheid's apparent brightness has been carefully measured through observations. ANSWER: Step 1: Measure the period of the Cepheid's brightness variations. Step 2: Use the period to determine the amount that the Cepheid changes in radius as it pulsates. Step 3: Use the period-luminosity relation to determine the Cepheid's distance. Step 1: Measure the luminosity of the Cepheid. Step 2: Use the period-luminosity relation to determine the Cepheid's period. Step 3: Find the Cepheid's distance from its period and luminosity. Step 1: Measure the changes in the Cepheid's radius as it pulsates. Step 2: Calculate the Cepheid's luminosity from its radius and surface temperature. Step 3: Calculate the Cepheid's distance from its luminosity and apparent brightness. Step 1: Measure the period of the Cepheid's brightness variations. Step 2: Use the period-luminosity relation to determine the Cepheid's luminosity. Step 3: Calculate the Cepheid's distance from its luminosity and apparent brightness. Correct Using this simple process, we can measure the distance to any galaxy in which we can identify an individual Cepheid variable star. We can now identify Cepheids (and the galaxies in which they reside) up to about 100 million light-years away from Earth. The Cosmic Distance Scale—From the Solar System to the Universe Learning Goal: To understand how we measure cosmic distances through a chain of techniques that begins with measurements in our own solar system. First, launch the interactive figurebelow. The Interactive Figure shows how astronomers use a chain of techniques, starting with radar, to measure distances throughout the universe. Before answering the following questions, click on each of the labeled blue strips in the Interactive Figure to learn how the techniques relate to one another. Part A 26/46 12/4/2019 Ch 16 HW 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? Hint 1. Definition of an AU One astronomical unit, or 1 AU, is defined as the average distance between Earth and the Sun. Hint 2. Why did Kepler know planetary distances in AU but not in kilometers or miles? Kepler was able to discover his third law of planetary motion, which relates planetary distances in AU to planetary periods in years, because the relative distances in AU were already known. (They were first worked out by Copernicus.) But Kepler did not know the absolute distance represented by an AU. Why not? ANSWER: here was no known way to measure the angles at which we viewed the planets at the time. here was no known way to measure the Earth-Sun distance at the time. he correct order of the planets was not yet known. ANSWER: he distances of all the planets in our solar system from Earth he Earth-Sun distance he distance from Earth to Venus he distance to the nearest star besides the Sun Correct As the Interactive Figure shows, parallax measurements depend on knowing the precise value of the Earth-Sun distance, or 1 AU. Therefore, the primary goal of using radar is to establish this value. We cannot bounce radar directly off the Sun, because the Sun absorbs radio waves, but we can use radar to measure the distance to Venus and then use Kepler's laws and a little geometry to calculate the value of an AU. Part B Which cosmic distance measurement techniques are considered standard candle techniques? Select all that apply. Hint 1. What is a standard candle? In the context of distance measurement, a standard candle is __________. ANSWER: n object whose luminosity can be measured, even if we do not know its distance n object whose luminosity can be calculated from its apparent brightness and distance n object whose apparent brightness can be measured ANSWER: 27/46 12/4/2019 Ch 16 HW ubble's law adar epheids hite dwarf supernovae (distant standards) ain-sequence fitting arallax Correct A standard candle technique requires knowing the luminosities of the objects whose distances we are trying to find. This is the case for mainsequence fitting, which relies on knowing the luminosities of main-sequence stars; it is the case for Cepheids, because we know their luminosities from the period-luminosity relation; and it is the case for white dwarf supernovae, which are all thought to have about the same luminosity. In contrast, radar and parallax are direct measurement techniques based on applying geometry, and Hubble's law is an estimation technique based on a rule about how galaxy distances depend on recession velocities. Part C 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? Hint 1. How do we know the luminosities of white dwarf supernovae? We know the luminosities of white dwarf supernovae because __________. ANSWER: we have used Hubble's law to measure the distances to the galaxies in which they occur ome have been observed in nearby galaxies for which we can measure distances with parallax ome have been observed in nearby galaxies for which we can measure distances with Cepheids ANSWER: We would still be able to measure the distances of distant galaxies using white dwarf supernovae, but we would not be able to use Hubble's law. We would still be able to measure the distances of distant galaxies, but not of nearby galaxies. We would not be able to measure the distances of distant galaxies. We would still be able to measure the distances of distant galaxies using Hubble's law, but we would not be able to use white dwarf supernovae. Correct Cepheids provide a key link in the cosmic distance chain. Cepheids are used to measure the distances of nearby galaxies in which white dwarf supernovae have been detected, which allows us to determine the luminosities of these supernovae. White dwarf supernovae, in turn, allow us to calibrate Hubble's law. Therefore, without Cepheids, we would not be able to use either white dwarf supernovae or Hubble's law, and we would be unable to measure the distances of distant galaxies. Part D Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases? Hint 1. How often do white dwarf supernovae occur? In a typical galaxy, a white dwarf supernova occurs approximately __________. ANSWER: 28/46 12/4/2019 Ch 16 HW few times each month nce a year nce every few hundred years ANSWER: We could use either method for any galaxy, but Hubble's law is simpler. We can detect white dwarf supernovae only in relatively nearby galaxies. ubble's law gives a more reliable estimate of distance than white dwarf supernovae. We have not observed white dwarf supernovae in most galaxies. Correct Supernovae of any type are relatively rare events, so we have not observed supernovae in the vast majority of galaxies. We are therefore left with Hubble's law as our best means of estimating the distances to the majority of distant galaxies.  Part E 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?  ANSWER: arallax would then be off by 20%, main-sequence fitting by 30%, Cepheids by 40%, and so on. arallax would also be off by 10%, but the other methods would be just as accurate. he error in radar would affect our measurements in the solar system, but it would have no effect on measurements with the other techniques. hey would all be off by the same 10%. Correct Each step in the chain can be only as reliable as the prior steps. If the Earth-Sun distance were off by 10%, then all the other methods would also have to be off by the same 10% (or more, if there are other errors at later stages of the chain). Part F Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?  Hint 1. Finding 10 million light-years on the Interactive Figure Notice that the main screen of the Interactive Figure shows distances in light-years, labeled with powers of 10 (along the top of the black cone). Recall that 10 million is 10^7, so 10 million light-years is the next arc to the right of the one labeled 10^6 light-years.  ANSWER: epheids arallax main-sequence fitting hite dwarf supernovae 29/46 12/4/2019 Ch 16 HW Correct Cepheid distance measurements are the most commonly used technique for measuring the distances of galaxies from a few million to a few tens of million light-years away, which is why Cepheids are such a critical link in the distance chain. Visual Skills Check 16.1 Answer questions in parts A-C based on the . Answer questions in parts D-G based on the . Part A Which distance measurement technique is most suited to measuring objects at a distance of 10 million light-years? ANSWER: ubble's Law Main-Sequence Fitting arallax adar istant Standards epheids 30/46 12/4/2019 Ch 16 HW Correct Part B Which distance measurement technique is most suited to measuring objects at a distance of 10 light-years? ANSWER: epheids ubble's Law istant Standards Main-Sequence Fitting arallax adar Correct Part C Which standard-candle technique is best for measuring the distances of very distant galaxies? ANSWER: Main-Sequence Fitting arallax istant Standards epheids Correct Part D The image in Figure 16.36 combines observations made with visible light and radio telescopes. Which color in the image represents the radio emission? ANSWER: hite ack urple nkish-white All attempts used; correct answer displayed Part E What is the approximate distance from the far edge of one lobe to the far edge of the other? ANSWER: 31/46 12/4/2019 Ch 16 HW pproximately 1,000,000 light-years pproximately 2,100,000 light-years pproximately 500,000 light-years pproximately 250,000 light-years Correct Part F What is the approximate size of the visible galaxy producing the jets? ANSWER: bout 800,000 light-years in diameter bout 500,000 light-years in diameter bout 100,000 light-years in diameter bout 200,000 light-years in diameter Correct Prelecture Overview: Galaxy Evolution First, launch the videobelow. Then, close the video window and answer the questions that follow. You can watch the video again at any point. Part A Consider the first image shown in the video, which is the Hubble Extreme Deep Field. Which of the following statements about this image are true? Select all the true statements. ANSWER: areful study of the image shows that the youngest galaxies were mostly irregular in shape. We see the more distant galaxies as they were when they were quiteyoung. he galaxies in this image are part of a large galaxy cluster, bound together by gravity. he image includes galaxies that are elliptical, spiral, and irregular. areful study of the image shows that all present-day galaxies are spirals. Answer Requested As discussed in the video, we see more distant galaxies as they were when they were younger, which is what allows us to use this image to contructthe family ablum of galaxies also shown in the video. The image shows galaxies of all types, but most young galaxies are irregular. The present-dayuniverse  has galaxies of all three types: elliptical, spiral, and irregular. 32/46 12/4/2019 Ch 16 HW Part B We see a galaxy with a lookback time of 12 billion years when its age could not have been morethan ___________. ANSWER: 00 million years billion years billion years 0 billion years 2 billion years Correct As explainedin the embedded pause and predict question in the video, you find the answer by subtracting the galaxies lookback time of 12 billion years from the universe's approximate age of 14 billion years. Part C - Which of the following are likely to play a role in determining whether a galaxy is spiral or elliptical? Select all that apply. ANSWER: ollisions or other interactions that the galaxy has had with other galaxies in the past he rotation rate of the protogalactic cloud from which the galaxy was born he densityof the protogalactic cloud from which the galaxy was born he age of the universe at the time the galaxy first formed Correct As explained in the video and your textbook, the density and rotation rate of a protogalactic clouds are birth conditions that likely influence a galaxy's type, but types can also change through later interactions. Part D Which of the following statements about galactic centersare true? Select all the true statements. ANSWER: vidence suggests that many or most galaxies have a supermassive black hole in their centers. upermassive black holes are interesting but unlikely to have any effect on a galaxy outside its bulge. he power source for active galactic nuclei is thought to be the accretion disk around a central black hole. he mass of a galaxy's central black hole is closely linked to the mass of a galaxy's bulge. heblack hole in the center of the Milky Way Galaxy is one of the most massive known. All attempts used; correct answer displayed As discussed in the video,evidence suggests that these central black holes may play an important role in galaxy evolution. Sorting Task: Spiral and Elliptical Galaxies 33/46 12/4/2019 Ch 16 HW Part A 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? ANSWER: piral galaxies are more massive than elliptical galaxies. he oldest stars in spiral galaxies are younger than the oldest stars in elliptical galaxies. lliptical galaxies lack a disk. ANSWER: Reset       Help  contain abundant clouds of cool gas and dust contain many bright, hot stars contain primarily old, low-mass stars have significant, ongoing star formation are more reddish in color have a flattened disk of stars are rare in central regions of galaxy clusters Correct 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. Sorting Task: Galaxy Formation—Spiral or Elliptical? Learning Goal: To understand how conditions in a protogalactic cloud or later interactions with other galaxies can determine whether a galaxy becomes spiral or elliptical. Introduction. Most galaxies in the universe are either spiral or elliptical, but why do galaxies come in these two categories? In this tutorial, we'll explore possible answers to this question. Part A Astronomers suspect that a galaxy’s type can be affected both by the conditions in the protogalactic cloud from which it forms ("initial conditions") and by later interactions with other galaxies. Each item below describes either a condition in a protogalactic cloud or a later interaction. Match these items to the galaxy type you would expect to form as a result. 34/46 12/4/2019 Ch 16 HW Drag the appropriate items to their respective bins. Hint 1. What is the most fundamental difference between elliptical and spiral galaxies? Which statement below best describes the most fundamental difference between spiral and elliptical galaxies? ANSWER: piral galaxies have more stars than elliptical galaxies. piral galaxies have a disk of gas and elliptical galaxies do not. lliptical galaxies form from protogalactic clouds and spiral galaxies do not. Hint 2. Is there a difference in rotation between elliptical and spiral galaxies? Which of the following structures exhibits clear, organized rotation patterns? ANSWER: he halo of a spiral galaxy n elliptical galaxy he disk of a spiral galaxy ll of the above ANSWER: Reset     Help   stars form rapidly as the protogalactic cloud shrinks protogalactic cloud has high density a galaxy collision strips away gas protogalactic cloud has high angular momentum most protogalactic gas settles into a disk protogalactic cloud rotates very slowly Correct Notice that processes that allow gas to settle into a disk lead to spiral galaxies, while those that do not allow gas to settle into a disk (or that strip gas away) lead to elliptical galaxies. The remaining questions should help make sure you understand the answer to Part A. Part B 35/46 12/4/2019 Ch 16 HW Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy? Hint 1. What is the most fundamental difference between elliptical and spiral galaxies? Which statement below best describes the most fundamental difference between spiral and elliptical galaxies? ANSWER: piral galaxies have more stars than elliptical galaxies. piral galaxies have a disk of gas and elliptical galaxies do not. lliptical galaxies form from protogalactic clouds and spiral galaxies do not. ANSWER: he loss of gas will cause the galaxy to start rotating faster. he collision will cause stars that were on circular orbits to change to elliptical orbits. galaxy cannot have a disk if it does not have gas. he collision will disrupt the orbits of stars in the galaxy’s halo. Correct The most fundamental difference between the galaxy types is that spiral galaxies have a rotating disk of gas in which ongoing star formation can occur, while elliptical galaxies do not. Therefore, once gas is stripped out of a galaxy, it can no longer have a gaseous disk. (It could in principle still have its stars arranged in a disk, but the collision will generally disrupt that as well.) Part C 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? Hint 1. Effects of early star formation You should recognize that rapid star formation early in the history of a protogalactic cloud tends to use up most of the available gas in the cloud. Think about this, along with the fact that the most fundamental difference between spirals and ellipticals is that spirals have a gaseous disk and ellipticals do not. ANSWER: tars that form early in a galaxy’s history tend to have orbits with more random inclinations. lliptical galaxies are characterized by higher rates of ongoing star formation than spiral galaxies. apid star formation means that there may not be enough gas left to make a disk. Correct Rapid star formation in a protogalactic cloud can mean there is not enough gas left to form a disk, and without a disk a galaxy cannot be a spiral. Part D High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy? Hint 1. Disks throughout the universe The disks of spiral galaxies arise for essentially the same physical reasons as many other disklike structures in the universe, including the disks in which planets form around stars and the accretion disks that can form around white dwarfs, neutron stars, or black holes in binary systems. Think 36/46 12/4/2019 Ch 16 HW ANSWER: tars in spiral galaxies orbit the galactic center faster than stars in elliptical galaxies. piral arms rotate like giant pinwheels, and therefore they need to rotate very fast. aster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud. ndividual stars in spiral galaxies rotate faster than those in elliptical galaxies. Correct Note that this is essentially the same reason why the rotation of a cloud that forms a star tends to lead to the formation of a spinning disk around the star in which planets can form. Vocabulary in Context: Galaxy Evolution Part A 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. ANSWER: 37/46 12/4/2019 Ch 16 HW 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. Correct Visual Activity: Exploring the Active Galactic Nucleus in M87 Click on the image below to launch the video: Exploring the Active Galactic Nucleus in M87. Once you have watched the entire video, answer the graded follow-up questions on the right. You can watch the video again at any point. Part A Why do astronomers hypothesize that a massive black hole lies at the center of M87? Hint 1. What is the significance of the jet of particles in the video? Notice the jet of particles shooting out (at very high speed) from the active nucleus of M87. The existence of this jet tells us that the active nucleus __________. ANSWER: must contain a black hole s releasing an enormous amount of energy s disintegrating 38/46 12/4/2019 Ch 16 HW ANSWER: istorical records show that a supermassive star at the center of M87 exploded as a supernova, leaving behind a black hole. ime-lapse images from space telescopes show stars falling to the center of M87 and then disappearing from view. mages of M87 made with powerful telescopes show a well-defined black region devoid of any stars. very small region at the center of M87 releases an enormous amount of energy. Correct The video shows that no matter how much we zoom in, we still cannot resolve the bright source at the galactic center. The brightness of the source (and the fact that it is shooting out a jet of particles) tells us that it is releasing an enormous amount of energy. The fact that we cannot resolve its structure tells us that it is very small in size. Calculations show that the only way that such a small region can release so much energy is if its central power source is the infall of matter into a supermassive black hole. Part B The third image in the video (with the most detailed view of the galactic center) is labeled "gas disk." Which of the following best describes what we are seeing in this photo? Hint 1. How large (in the photos) is the central black hole in M87? If you could actually see the event horizon of the suspected black hole in the video sequence, how large would it appear? ANSWER: would cover most of the galaxy shown in the first image. is about the size of the square in the middle photo that contains the "active nucleus." would be a tiny point in the third (most detailed) photo. ANSWER: he bright central region is the bulge of the galaxy, and around it we see spiral arms. he bright central region is a place where many young stars are being born, and the surrounding material is the gas from which these stars are made. he black background in this photo is the supermassive black hole, and we see bright spots on top of it where nearby gas is emitting light. he black hole is located deep within the bright central region, and around this region we see gas that is orbiting the central black hole. Correct The bright center marks where most of the energy is coming from, and from Part A you already know that the energy source is thought to be a supermassive black hole. The immense gravity of the black hole causes surrounding gas (and stars) to orbit around it. Part C Assuming that the bright core of M87 is powered by a supermassive black hole, which of the following best describes the source of energy that makes the core appear so bright? Hint 1. Where is the singularity of a black hole located? True of False? The singularity of a black hole is located within the event horizon of the black hole. ANSWER: 39/46 12/4/2019 Ch 16 HW rue alse Hint 2. What role do stars play in generating energy near the black hole? Suppose that there were no stars at all within a few hundred light-years of the supermassive black hole in M87, but there was plenty of gas. In that case __________. ANSWER: here would be no light of any kind coming from the center of the galaxy he center of the galaxy would still be a source of radio waves, but it would not emit any visible light he center of the galaxy would still be quite bright ANSWER: he black hole emits intense light as its huge mass is squeezed to infinite density at the black hole’s singularity. Gravitational potential energy is converted to thermal energy as matter from the surrounding gas disk spirals into the central black hole. he immense gravitational force exerted by the central black hole in M87 triggers nearby stars to explode as supernovae. Gas and dust clouds form stars at a rapid rate due to forces from the central black hole in M87. Correct The black hole itself does not emit any light. Rather, the bright light comes from hot gas surrounding the black hole. This gas becomes hot in the process of falling toward the black hole because a large fraction (up to 40%) of its gravitational potential energy is converted into thermal energy (through friction in the disk) as it falls inward. Most of this energy is ultimately radiated away as light, explaining why the central region is so bright. Process of Science: Understanding a Correlation in Galaxy Data Learning Goal: To examine real data for galaxies and show how astronomers use an observed correlation to infer results, make discoveries, and decide what the next step should be in a scientific investigation. Introduction. The graph shows actual data for galaxies that are the brightest in their clusters. The "infrared color ratio," plotted on the vertical axis, tells us whether the galaxy emits more strongly at longer or shorter infrared wavelengths. 40/46 12/4/2019 Ch 16 HW Part A Which statement describes the trend among the galaxies shown on this graph? Hint 1. What is a statistical trend? Consider a sample of people of different weights and heights. Which statement is true? ANSWER: Height has no effect on weight, because the average weight is the same for people of all heights. aller people tend to weigh more than shorter people, although there are exceptions for people who are overweight or unusually thin. We cannot claim any trend linking height and weight, because two people of the same height can have very different weights. ANSWER: here is no trend because there are some exceptions. he galaxies fall into two distinct groups: nearby galaxies with a small infrared color ratio and distant galaxies with a large infrared color ratio. More distant galaxies tend to have a smaller infrared color ratio, but there are some exceptions. More distant galaxies tend to have a larger infrared color ratio, but there are some exceptions. Correct In other words, the data points tend to slope upward from left to right. Part B You discover a new cluster of galaxies, and the brightest galaxy in this new cluster has an infrared color ratio of 0.65. Based on the data in the graph, what is the approximate distance to this new cluster? 41/46 12/4/2019 Ch 16 HW Hint 1. What does each tick mark represent along the vertical axis? Each of the small tick marks along the vertical axis represents an increase in the infrared color ratio of _____ . ANSWER: .2 .1 .05 ANSWER: billion light-years billion light-years .5 billion light-years nknown, since the exceptions to the general trend make it impossible to estimate the cluster's distance. Correct You can estimate the cluster's distance by drawing a "best-fit" straight line through the data points. An infrared color ratio of 0.65 would fall on this line at a point corresponding to a distance of about 1 billion light-years. Part C A follow-up study showed that the galaxies that are exceptions to the general trend on the original graph have unusually brighter centers; that is, these galaxies contain active galactic nuclei. Based on this follow-up study, could you improve your confidence in your distance estimate for the new cluster discussed in Part B, and, if so, how? Hint 1. How was the second study conducted? Part C tells us that the follow-up study concluded that the exceptions to the trend in the original study are galaxies with unusually bright centers. What tests would scientists have done to reach this conclusion? ANSWER: Repeat the first study, but with a different set of galaxies that are each the brightest in their clusters. tudy some of the galaxies from the first study to see how the exceptions to the general trend compare to those that follow the trend. tudy only the galaxies in the first study that are exceptions to the general trend to learn what types of galaxies they are. ANSWER: es, by studying other galaxies in the new cluster to see if any of them have unusually bright centers. o. The follow-up study will not improve your confidence in your distance estimate, because you cannot know whether your new galaxy follows he general trend. es, by photographing the new cluster with better angular resolution to see whether its brightest galaxy has an unusually bright center. es, by studying many more galaxies that are the brightest in their clusters so that you can estimate the fraction of these galaxies that have both right centers and infrared color ratios of 0.65. 42/46 12/4/2019 Ch 16 HW Correct If the brightest galaxy does not have an unusually bright center, then you can be more confident that it follows the general trend. If it does have an unusually bright center, then you can use this fact to improve your estimate, because the exceptions tend to have higher-than-expected infrared color ratios for particular distances. Part D Consider both the original study (with data shown on the graph) and the follow-up study showing that the exceptions to the general trend are galaxies with unusually bright centers (as described in Part C). Which of the following conclusions are supported by this pair of studies? Select all that apply. ANSWER: he unusually bright nuclei in the exceptions are powered by energy released as gas falls into supermassive black holes. alaxies with unusually bright nuclei are more common at greater distances. alaxies with unusually bright nuclei were more common when the universe was young than they are today. n average, more distant galaxies have larger infrared color ratios. Correct We can conclude that, on average, more distant galaxies have larger infrared color ratios, because that is the general trend shown on the graph. We can conclude that galaxies with unusually bright nuclei are more common at greater distances, because the graph shows that there are more exceptions at large distances. We can conclude that galaxies with unusually bright nuclei were more common when the universe was young, because large distances mean looking farther back in time, which means to a time when the universe was younger. Problem 16.34 Choose the best answer. Part A Which of these galaxies would you most likely find at the center of a large cluster of galaxies? ANSWER: large spiral galaxy large elliptical galaxy small irregular galaxy Correct Problem 16.35 Choose the best answer. Part A We determine the distance of a Cepheid by ANSWER: 43/46 12/4/2019 Ch 16 HW measuring its parallax. nowing that all Cepheids have about the same luminosity and then applying the inverse square law for light. etermining its luminosity from the period-luminosity relation and then applying the inverse square law for light. Correct Problem 16.36 Choose the best answer. Part A Which kind of object is the best standard candle for measuring distances to extremely distant galaxies? ANSWER: white dwarf Cepheid variable star white dwarf supernova Correct Problem 16.37 Choose the best answer. Part A Why do virtually all the galaxies in the universe appear to be moving away from our own? ANSWER: ecause we are located near where the Big Bang happened. ecause we are located near the center of the universe. ecause expansion causes all galaxies to move away from nearly all others. Correct Problem 16.33 Choose the best answer. Part A Which of these galaxies is most likely to be oldest? ANSWER: 44/46 12/4/2019 Ch 16 HW galaxy in the Local Group galaxy observed at a distance of 5 billion light-years galaxy observed at a distance of 10 billion light-years Correct Problem 16.38 Choose the best answer. Part A When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching Earth, we are seeing that galaxy as it was when the universe was ANSWER: 0 billion years old. billion years old. billion years old. Correct Problem 16.39 Choose the best answer. Part A Which of these items is a key assumption in our most successful models for galaxy formation? ANSWER: he distribution of matter was perfectly uniform early in time. ome regions of the universe were slightly denser than others. Galaxies formed around supermassive black holes. Correct Problem 16.42 Choose the best answer. Part A Observations indicate that galaxies with more massive central black holes tend to also have ANSWER: 45/46 12/4/2019 Ch 16 HW more elliptical shape. greater overall luminosity. greater mass of stars in their central bulges. Correct Problem 16.40 Choose the best answer. Part A The luminosity of a quasar is generated in a region the size of ANSWER: he Milky Way. star cluster. he solar system. Correct Problem 16.41 Choose the best answer. Part A The primary source of a quasar's energy is ANSWER: hemical energy. uclear energy. ravitational potential energy. Correct Score Summary: Your score on this assignment is 95.1%. You received 38.04 out of a possible total of 40 points. 46/46