 Students will be able to determine the size and shape of our galaxy.  Students will be able to distinguish the different kinds of variable stars.  Students will be able to identify different kinds of stars in a galaxy and their location.

 Galaxy: Any of the very large groups of stars and associated matter found throughout the universe.

 Main Idea, Page 862  Stars with varying light output allowed astronomers to map the Milky Way which has a halo, spiral arms, and a massive blackhole at its center.

 Page 862  Milky Way Galaxy: Disk shape collection of stars where we are located.

 Page 862  Variable Stars: 1920s astronomers try to map the locations of globular clusters of stars. To do so, they estimated the distances to the clusters by identifying variable stars in them.

 Page 862  Variable Stars: Stars that pulsate in brightness because of the expansion and contraction of their outer layers.

 Page 863  Different Types of Variable Stars: 1. RR Lyrae variables: Pulse from 1.5 hours to ever 1 day. 2. Cepheid variables: Pulse from 1 day to 100 days

 Page 863  Different Types of Variable Stars: Longer period of pulsation = greater luminosity. Luminosity is then compared to star’s magnitude (brightness.)  Allows astronomers to calculate how far away a star must be to appear so bright.

 Page 863  The Galactic Center: A region of high star density. RR Lyrae variables used to determine distance to globular clusters of stars orbiting the center of the Milky Way. Clusters’ distribution was centered on a point 28,000 light years away.

 Page 863  The Shape of the Milky Way: Determined by mapping with radio waves.  Radio waves penetrate gas and dust without being disturbed. Map showed that galactic center is surrounded by a nuclear bulge. Around the bulge is a disk shaped halo.

 Page 864  Spiral Arms: Along with the disk shape and bulge, the Milky Way has 4 Major Spiral Arms and numerous minor arms. Arms were discovered using hydrogen emissions spectra.

 Page 864  Spiral Arms: Some galaxies are shaped like a bar and not a disk. The shape of a galaxy is determined by the density of gas in its halo. Bulge: Made of older red stars. Bar: Made of young stars and discs that form from hydrogen gas.