Chapter 15 Quiz: Jessica Horn

1. What are the three major types of galaxies? Spiral galaxies have prominent disks and spiral arms, elliptical galaxies are rounder and redder than spiral galaxies and contain less cool gas and dust, and irregular galaxies are neither disklike nor rounded in appearance
2. How are galaxies grouped together? Spiral galaxies tend to collect in groups that contain up to several dozen galaxies. Elliptical galaxies are more common in clusters of galaxies, which contain hundreds of thousands of galaxies, all bound together by gravity
3. How do we measure the distances to galaxies? Our measurements of galaxy distances depend on a chain of methods. The chain begins with radar ranging in our own solar system and parallax measurements of distances to nearby stars, then relies on standard candles to measure greater distance. 
4. What is Hubble's law? Hubble's law tells us that more distant galaxies are moving away faster. It allows us to determine a galaxy's distance from the speed at which it is moving away from us, which we can measure from its Doppler shift. (v = H0 X d)
5. How do distance measurements tell us the age of the universe? Combining distance measurements with velocity measurements tells us Hubble's constant, and the inverse of Hubble's constant tells us how long it would have taken the universe to reach its present size if the expansion rate had never changed. Based on Hubble's constant and estimates of how it has changed with time, we now estimate the age of the universe at about 14 billion years, which restricts our view of the universe to objects with lookback times smaller than that age.
6. How do we observe the life histories of galaxies? Today's telescopes enable us to observe galaxies of many different ages because they are powerful enough to detect light from objects with lookback times almost as large as the age of the universe. 
7. How did galaxies form? The most successful models of galaxy formation assume that galaxies formed as gravity pulled together regions of the universe that were ever so slightly denser than their surroundings. Gas collected in protogalactic clouds, and stars began to form as the gas cooled. 
8. How do galaxies differ? Differences between present day galaxies probably arise both from conditions in their protogalactic clouds and from collisions with other galaxies. Slowly rotating or high density protogalactic clouds may form elliptical rather than spiral galaxies. ellipticals may also form through the merger of spiral galaxies.
9. What are quasars? Some galaxies have unusually bright centers knows as active galactic nuclei. A quasar is a particularly bright active galactic nucleus. Quasars are generally found at great distances from us, telling us that they were more common early in the history of the universe.
10. What is the power source for quasars and other active galactic nuclei? Supermassive black holes are thought to be the power sources for active galactic nuclei. As matter falls into a supermassive black hole through an accretion disk, its gravitational potential energy is transformed with enormous efficiency into thermal energy and then into light.