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TAMU ASTR 101 - Lecture24a_2009C

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Copyright © 2009 Pearson Education, Inc. Chapter 15 Galaxies and the Foundation of Modern CosmologyCopyright © 2009 Pearson Education, Inc. What is Hubble’s law?Copyright © 2009 Pearson Education, Inc. What have we learned? • What is Hubble’s law? — The faster a galaxy is moving away from us, the greater its distance: velocity = H0 × distanceCopyright © 2009 Pearson Education, Inc. What have we learned? • How do distance measurements tell us the age of the universe? — Measuring a galaxy’s distance and speed allows us to figure out how long the galaxy took to reach its current distance. — Measuring Hubble’s constant tells us that amount of time: about 14 billion years.Copyright © 2009 Pearson Education, Inc. 15.3 Galaxy Evolution Our goals for learning: • How do we observe the life histories of galaxies? • How did galaxies form? • Why do galaxies differ?Copyright © 2009 Pearson Education, Inc. How did galaxies form?Copyright © 2009 Pearson Education, Inc. Our best models for galaxy formation assume that: • Matter originally filled all of space almost uniformly. • Gravity of denser regions pulled in surrounding matter.Copyright © 2009 Pearson Education, Inc. Denser regions contracted, forming protogalactic clouds. H and He gases in these clouds formed the first stars.Copyright © 2009 Pearson Education, Inc. Supernova explosions from the first stars kept much of the gas from forming stars. Leftover gas settled into a spinning disk. Conservation of angular momentumCopyright © 2009 Pearson Education, Inc. But why do some galaxies end up looking so different? M87 M101Copyright © 2009 Pearson Education, Inc. Why do galaxies differ?Copyright © 2009 Pearson Education, Inc. Why don’t all galaxies have similar disks?Copyright © 2009 Pearson Education, Inc. Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk. Conditions in Protogalactic Cloud?Copyright © 2009 Pearson Education, Inc. Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk. Conditions in Protogalactic Cloud?Copyright © 2009 Pearson Education, Inc. Distant Red Ellipticals Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe.Copyright © 2009 Pearson Education, Inc. We must also consider the effects of collisions.Copyright © 2009 Pearson Education, Inc. Collisions were much more likely early in time, because galaxies were closer together.Copyright © 2009 Pearson Education, Inc. Many of the galaxies we see at great distances (and early times) indeed look violently disturbed.Copyright © 2009 Pearson Education, Inc. The collisions we observe nearby trigger bursts of star formation.Copyright © 2009 Pearson Education, Inc. Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.Copyright © 2009 Pearson Education, Inc. Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical. Galaxy Collision AnimationCopyright © 2009 Pearson Education, Inc. Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together.Copyright © 2009 Pearson Education, Inc. Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies.Copyright © 2009 Pearson Education, Inc. Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years.Copyright © 2009 Pearson Education, Inc. The intensity of supernova explosions in starburst galaxies can drive galactic winds.Copyright © 2009 Pearson Education, Inc. The intensity of supernova explosions in starburst galaxies can drive galactic winds. X-ray imageCopyright © 2009 Pearson Education, Inc. What have we learned? • How do we observe the life histories of galaxies? — Deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages. • How did galaxies form? — Our best models for galaxy formation assume that gravity made galaxies out of regions of the early universe that were slightly denser than their surroundings.Copyright © 2009 Pearson Education, Inc. What have we learned? • Why do galaxies differ? — Some of the differences between galaxies may arise from the conditions in their protogalactic clouds. — Collisions can also play a major role because they can transform two spiral galaxies into an elliptical galaxy.Copyright © 2009 Pearson Education, Inc. 15.4 Quasars and Other Active Galactic Nuclei Our goals for learning: • What are quasars? • What is the power source for quasars and other active galactic nuclei? • Do supermassive black holes really exist?Copyright © 2009 Pearson Education, Inc. What are quasars?Copyright © 2009 Pearson Education, Inc. If the center of a galaxy is unusually bright, we call it an active galactic nucleus. Quasars are the most luminous examples. Active Nucleus in M87Copyright © 2009 Pearson Education, Inc. The highly redshifted spectra of quasars indicate large distances. From brightness and distance, we find that luminosities of some quasars are >1012LSun! Variability shows that all this energy comes from a region smaller than the solar system.Copyright © 2009 Pearson Education, Inc. Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant. B. They were more common early in time. C. Galaxy collisions might turn them on. D. Nearby galaxies might hold dead quasars.Copyright © 2009 Pearson Education, Inc. Thought Question All of the above! What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant. B. They were more common early in time. C. Galaxy collisions might turn them on. D. Nearby galaxies might hold dead quasars.Copyright © 2009 Pearson Education, Inc. Galaxies around quasars sometimes appear disturbed by collisions.Copyright © 2009 Pearson Education, Inc. Characteristics of Active Galaxies • Luminosity can be enormous (>1012LSun). • Luminosity can rapidly vary (comes from a space smaller than solar system). • They emit energy over a wide range of


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TAMU ASTR 101 - Lecture24a_2009C

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