1. The Milky Way and Other Galaxies Chapter 20.2 Notes

2. Galaxies A galaxy is a collection of millions to billions of stars The Andromeda galaxy has millions to billions of stars bound together by gravity Young stars are often found near the nebular gas and dust where they were born; older stars may be found throughout the galaxy or in regions that contain no gas and dust The nearest galaxy to our own is millions of light-years from Earth Scientists have found as many as 100 billion galaxies

3. Gravity and Galaxies Without gravity, everything in space would be a spread out veil of gas Because of gravity, clouds of gas come together and collapse to form stars As the first stars in a galaxy begin to age, they throw off gas and dust to become supernovas; new stars then form All of the gas, dust, and stars form galaxies because of gravity The solar system revolves around the center of the galaxy because of gravity—it takes 226 million years to complete one orbit of our galaxy

4. Clusters of Galaxies Galaxies are not spread out evenly through space Galaxies are grouped together and bound by gravity into clusters The Milky Way and the Andromeda are two of the largest members of the Local Group—a cluster of more than 30 galaxies New members of the Local Group are being discovered by telescopes, such as the Hubble Space Telescope Clusters can form even larger groups called superclusters A typical supercluster contains thousands of galaxies that contain trillions of stars in individual clusters Superclusters are the largest known structures in the universe

5. Spiral Galaxies Spiral galaxies have spiral arms made of gas, dust, and stars The gas and dust is called interstellar matter—it contains materials that allow new stars to form The spiral arms appear blue, so scientists assume the arms are made of younger stars since hot stars emit more energy at blue wavelengths and burn out quickly We live in the Milky Way galaxy Like most spiral galaxies, the Milky Way has a “nucleus” made up of very dense and old stars The center appears red because it is comprised of older stars that use energy slowly Our solar system is located within a spiral arm

6. Elliptical Galaxies Elliptical galaxies have no spiral arms, little gas or dust, and a round shape Commonly appear red Assumed that they contain mostly older stars and have little interstellar matter Found in a wide range of sizes—giant elliptical galaxies contain trillions of stars, whereas dwarf elliptical galaxies contain a few million stars and are much smaller

7. Irregular Galaxies All other galaxies are irregular galaxies—they have no regular shape or defined structure Some contain little interstellar matter, while others have large amounts and contain mostly young blue stars

8. How Galaxies Change Scientists do not know what such a galaxy looks like now, but by studying closer galaxies that might be similar to ancient ones, they can slowly piece together the puzzle of how galaxies evolve Galaxies change over billions of years, so astronomers have never seen the whole process, but they have theories about how galaxies change over time Galaxies change because of collisions—as galaxies approach each other, the gravitational attraction changes their shapes

9. Quasars Quasars are the most distant and most radiant objects in space One explanation for the strong radiation is that quasars are enormous black holes that are surrounded by large disks of gas and dust Friction in the disks release energy at many wavelengths, especially radio waves Quasars may be the central parts of distant galaxies, seen as when they were very young