1 The Milky Way Galaxy We live on the quiet outskirts of a galaxy of approximately 100 Billion stars. This galaxy, the Milky Way, is roughly disk-shaped and is about 100,000 light years in diameter. Besides stars the Milky Way also contains enormous clouds of gas and dust The Milky Way as seen from some place on Earth far from city lights.

2 Parts of Our Galaxy Our Galaxy is a spiral galaxy. It is composed of: –a central bulge –a flat disk containing the spiral arms and –a halo of globular clusters there may also exist an extended halo of some unknown material commonly known as “dark matter”.

3 Gas and Dust in our galaxy account for about 15% of its mass. The spiral arms are composed mostly of clusters of bright young stars. The Sun orbits the center of the galaxy with a speed of about 220 kilometers per second. It takes the Sun 240 million years to go around the galaxy one time. The Milky Way as seen in infrared light. Dust and gas in our galaxy prevent us from seeing very far into the galaxy in visible light. In the infrared and radio we can see much farther. Using this information helps us to recognize that we live in a spiral galaxy. More Properties of the Milky Way

4 Learning About the Structure of the Milky Way William Herschel in 1784 recognized that the Sun was in a disk-shaped galaxy of stars. Because of dust and gas blocking the light of distant stars it appeared that the Sun was near the center of the galaxy. This same dust and gas makes it more difficult to observe other galaxies too especially along the disk of our galaxy and towards the center. Top. Herschel’s Map of the Milky Way Bottom. A picture of a dark dust cloud and a star cluster

5 Globular Clusters and the Center of the Galaxy In the 1920s an estimate was made for the size of the galaxy and the position of the Sun within it. It was found that certain dense clusters of stars known as globular clusters tended to be distributed in an elliptical region of space. The center would indicate the center of the galaxy. The Sun was found not to be at the center of this region. Later estimates placed this center at about 28,000 light years away. Globular Clusters contain some of the oldest stars in the galaxy.

6 There are several methods of determining the mass of the Milky Way. Astronomers comparing the estimated mass of all visible matter in the galaxy with the gravitational mass of the galaxy have found that most of the mass is not visible. This is where the theory that the galaxy must contain some kind of “dark matter” comes from. Using the same method used to measure the mass of planets and binary stars we can also use the modified form of Kepler’s 3rd Law to measure the mass of the Milky Way. Using the distance to the center of the galaxy and the orbital period of the Sun the mass of the galaxy is at least Solar masses. This method only measures the mass of the galaxy contained within the orbit of the Sun. Mass of the Milky Way

7 Spiral Arms - Density Waves The arms are traced out by massive bright, young stars. The arms do not “carry” stars with them but are regions of higher density that stars and gas pass through. An excess of mass in the arm exerts a slightly stronger gravitational pull. Clouds pulled into these arms collapse and form stars. Smaller stars live long enough to pass out of the arm as it sweeps by. Massive stars die before leaving the arm

8 Spiral Arms - SSF Another way that spiral arms form is due to supernova explosions of large stars. These explosions trigger the collapse of gas clouds causing new stars to form. The process continues as new large stars explode causing more clouds to collapse. As these stars orbit the galaxy the stars closer in orbit faster and form a spiral shape. This is called self- propagating star formation or SSF.

9 What is Lurking in the Center of Our Galaxy? Near the center of our galaxy there are roughly 10 million stars packed into one cubic light year. In the very center is an object smaller than the size of Jupiter’s orbit but with a mass several million times the mass of the Sun. Since so little light is visible from this object it is thought to be a very large black hole. This is called a supermassive black hole. Most other galaxies are also thought to have these huge black holes in their centers.