1. Introduction to Galaxies 5/23/2013

2. BR: Milky Way Scale The Milky Way has a diameter of approximately 8.25 x 10 9 AU (8.25 billion AU). 206,265 AU = 3.26 ly What is the diameter of the Milky Way in light- years?

3. Modeling the Milky Way The orbit of Pluto has a diameter of 80 AU. What is this diameter in ly? If we think of the extent of our solar system being Pluto's orbit, what percentage of the diameter of the Milky Way is the size of our solar system? (Take your answer about Pluto and divide by your bell ringer answer.)

4. Milky Way Galaxy spiral The Milky Way is made up of approximately 100 billion stars including our Sun. The center or nucleus of the Milky Way galaxy is in the center of a disk. The nucleus is surrounded by a nuclear bulge. The galaxy is shaped like a disk with a halo around it.

5. Galactic Center Globular clusters on the outside of the Milky Way, which are located far from our Solar System, are centered on a specific point and appear to orbit the center of the Milky Way. The center of the Milky Way is a region of very high star density, most of which is obscured by interstellar gas and dust. Motion of stars that orbit close to the galactic center indicate that this area has about 2.6 million times the mass of the Sun but is smaller than our solar system. Something this massive could only be a black hole.

6. Spiral Arms Many galaxies including the Milky Way are shaped like disks with a central bulge and spiral arms. It is been more difficult for astronomers to determine the shape of the Milky Way than other galaxies because astronomers have no way to get outside of the galaxy and look down on the disk. As a result, they have to use measurements of hydrogen emission to detect the four major spiral arms and numerous minor arms of the Milky Way.

7. Galaxy Formation

8. Other Galaxies Astronomers have been aware of the existence of galaxies outside the Milky Way for centuries, though it took until Edwin Hubble's discovery of Cepheid variable stars in 1924 and his measurements of their distance to determine that they were too far away to be located in our galaxy and therefore must be in another galaxy (in this case the Andromeda Galaxy).

9. Galaxy Classification – Spiral Galaxies Normal spiral galaxies Barred spiral galaxies (elongated central region)

10. Galaxy Classification Elliptical galaxies Irregular galaxies