1. Galaxies A galaxy is a collection of millions to billions of stars, dust, and gas bound together by gravity. There are over 1 billion galaxies in the universe and the largest galaxy has more than 1 trillion stars. There are three main types of galaxies: 1.Elliptical Galaxies 2. Spiral Galaxies 3.Irregular Galaxies

2. Elliptical Galaxy looks like a round or flattened ball. has no spiral arms contain billions of stars but very little gas and dust. older stars and have little interstellar matter often have a reddish color, because older stars are red

3. Spiral Galaxy has a bulge in the middle and arms that spiral outward like a pinwheel. Our solar system is located in the milky way galaxy – it is a spiral galaxy

4. Irregular Galaxies usually smaller. lack regular shapes and do not have a well-defined structure. may be oddly shaped because the gravitational influence of nearby galaxies distorts their spiral arms

5. Quasars Quasars may be infant galaxies. –In 1960, a faint object was matched with a strong radio signal. This object was called a quasar. –Each quasar has a huge central black hole and a large disk of gas and dust around it. quasar: quasi-stellar radio sources; very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe

6. Beyond Galaxies Gravity holds galaxies together in clusters. –Galaxies are not spread evenly throughout space. –The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies. –Clusters of galaxies can form even larger groups, called superclusters. cluster: a group of stars or galaxies bound by gravity

7. How Galaxies Change Over Time Galaxies change over time. Galaxies change as they use up their stores of gas and dust. Galaxies also change as a result of collisions. As galaxies approach each other, mutual gravitational attraction changes their shape. Collisions of gas and dust may cause new stars to begin forming.