Chapter 15 Jane Lucas

Jane Lucas
Astronomy 100-Week 16 – Chapter 15
Island of Stars
Distances of Galaxies
Galaxy Evolution
Quasars and Other Active Galactic Nuclei
Island of Stars – There are three major types of galaxies: 1) Spiral galaxies which have prominent disks and spiral arms. 2) Elliptical galaxies which are rounder and redder than spiral galaxies and contain less cool gas and dust. 3) Irregular galaxies which are neither dislike nor rounded in appearance.  Spiral galaxies tend to collect in groups of galaxies, which contain up to several dozen galaxies.  Elliptical galaxies are more common in clusters of galaxies, which contain hundreds to thousands of galaxies, all bound together by gravity.  
Distance of 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.  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.  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 at about 14 billion years, which restricts our view of the universe to lookback times smaller than that age.  
Galaxy Evolution – 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.  We can therefore assemble “family albums” of galaxies at different distances and lookback times.  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.  Differences between present-day galaxies probably can 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 collision and merger of two spiral galaxies.  
Quasars and Other Active Galactic Nuclei – 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 very great distances, telling us that they were much more common early in the history of the universe.  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 into thermal energy and then into light with enormous efficiency.  Observations of orbiting stars and gas clouds in the nuclei of galaxies suggest that all galaxies may harbor supermassive black holes at their centers.  
In conclusion we learned in Chapter 15 that the universe in filled with galaxies that come in a variety of shapes and sizes.  We learned what types of galaxies there are, and how they are grouped together.    We learned about radar ranging in order to measure the distance to galaxies.  We covered Hubble’s law that tells us that more-distant galaxies are moving faster away from us.  We observed the life histories of galaxies, how they formed, and how they differ. Finally, we learned about quasars, and their power source.