Questions for Chapter 21

1. What is this?
2. What do we mean by galaxy evolution?
3. Briefly describe the starting assumptions for galaxy formation, and how models suggest that these starting conditions can lead to the formation of a spiral galaxy.
4. Describe two ways in which conditions in a protogalactic cloud might lead to the birth of an elliptical rather than a spiral galaxy.
5. Account for the large variety of sizes (2) and types (3) of galaxies by describing their formation and evolution.
6. What happens when two galaxies collide? How might elliptical galaxies form from collisions of spiral galaxies? What evidence supports the idea that galaxy collisions sometimes occur?
7. Briefly explain why we expect collisions between galaxies to be relatively common, while collisions between stars are extremely rare. Why should galaxy collisions have been more common in the past than they are today?
8. What is a starburst galaxy? How do observations of starbursts help us understand why the stars in elliptical galaxies are so old?
9. Briefly explain why starburst galaxies often appear ordinary when they are observed in visible light but extraordinary when they are observed in infrared light.
10. What is a galactic wind? What causes it? How is it similar to a superbubble in the Milky Way, and how is it different?
11. Briefly describe the discovery of quasars. What evidence convinced astronomers that the high redshifts of quasars really do imply great distances? Why can we learn more about quasars by studying nearby active galaxies and radio galaxies?
12. List at least three definitely observed properties of quasars and indicate briefly how one theory fits them.
13. Summarize the supermassive black hole model for the energy output of quasars and other active galactic nuclei. What evidence suggests that such black holes really exist?
14. What is this? The red shows radio waves.
15. What is a radio galaxy? Describe jets and radio lobes. Why do we think that the ultimate energy sources of radio galaxies lie in quasar like galactic nuclei?
16. Suggest one energy generating source for radio galaxies and /or active galaxies and explain how it results in the observed phenomena.
17. The Milky Way has a black hole at its center. Why isn't it an AGN?
18. In an active galaxy where does the x-ray, ultraviolet, visible, infrared and radio radiation come from? 
19. This is a spectrum from a galaxy. Can you tell what might be going on?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all these have definitive answers, so your explanation is more important than your chosen answer.

20. Galaxies that are more than 10 billion years old are too far away for us to see even with our most powerful telescopes.
21. Elliptical galaxies are more likely to form in denser regions of space.
22. If the Andromeda Galaxy someday collides and merges with the Milky Way, the resulting galaxy may be elliptical
23. NGC 9645 is a starburst galaxy that has been forming stars at the same furious pace for some 10 billion years.
24. The energy from supernova explosions can drive a large proportion of the interstellar gas out of a small galaxy.
25. Astronomers proved that quasar 3C 473 contains a supermassive black hole when they discovered that its center is completely dark,
26. The black hole at the center of our own galaxy may once have powered an active galactic nucleus,
27. Radio galaxies emit only radio waves and no visible light.
28. Analyses of quasar light can tell us about intergalactic clouds that might otherwise remain invisible.