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.

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

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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.

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Variable Stars: • Stars that pulsate in brightness

because of the expansion and contraction of their outer layers.

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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.

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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.

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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.

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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.

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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.