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Chapter 20: Galaxies and the Foundation of Modern Cosmology  Island of Stars o How are the lives of galaxies connected with the history of the universe  100 billion galaxies  Cosmology- the study of the structure of galaxies o What are the three major types of galaxies  Spiral Galaxy  Has a disk with spiral arms, a central bulge, and a halo  Barred Spiral Galaxy: has a bar across the middle o Lenticular Galaxy  Intermediate between Elliptical and spiral galaxy  Elliptical Galaxy  Older, smaller stars  No disk, random rotation of stars  Big ball of stars, very little dust or gas  No stars forming  Irregular Galaxies  Young galaxies  Bunch of stars gravitational bound together  Blue white o How are galaxies grouped together  They are grouped in between whether they are spiral or more disk like.  Spiral galaxies are often found in groups  Clusters of galaxies  Measuring Galactic Distances o How do we measure the distances to galaxies  First determine the size of the solar system using radar.  Use parallax to determine distances of nearby stars  Use a standard candle (Object that we know the luminosity without knowing the distance) to measure the distance of further star clusters.  White Dwarf Supernovae, and Cepheid variables can be used as a standard candle o How did Hubble prove that galaxies lie beyond the Milky way  Hubble uses the Cepheid variables to show that other galaxies are far away. o What is Hubble’s Law  Galaxies are moving away from us, and they are redshifted. Faster moving away, the further away they are  Find distance using redshifts  The age of the Universe o How does Hubble’s Law tell us the age of the universe  The expansion rate appears same everywhere  The universe had no center and no edge.  Cosmological Principle  The universe looks about the same no matter where you are in it.  Cosmological Horizon  Horizon in time not distance  We cannot see beyond the begin of the universe