Clusters of Galaxies Can Form Larger Groups Called

Star Assignment3- Galaxies and the Universe Read and then Answer the following questions (pgs. 397-398 in Science Worktext) GALAXIES Most stars exist in galaxies. GALAXIES are huge collections of stars. The universe has hundreds of billions of galaxies. Each galaxy can contain hundreds of billions of stars. DARK MATTER Gravity holds stars together. Gravity also holds galaxies together. Astronomers study how galaxies rotate and how gravity affects their movement. They have found that most matter in galaxies is invisible. Matter that emits no light at any wavelength (distance between crests of a wave) is called DARK MATTER. Scientists now believe that more than 90 percent of the universe’s mass is dark matter. They do not know what this invisible dark matter contains. TYPES OF GALAXIES There are three major types of galaxies. Galaxies can be spiral, elliptical, or irregular. SPIRAL GALAXIES- The stars, gas and dust in a spiral galaxy exist in spiral arms that begin at a central disk. Some spiral arms are long and symmetrical and some are short and stubby. Spiral galaxies are thicker near the center, a region called the central bulge. A spherical halo of globular clusters and older redder stars surrounds the disk. ELLIPTICAL GALAXIES- Elliptical galaxies do not have an internal structure. Some are spheres, like basketballs and others are shaped like footballs. Elliptical galaxies have more red stars than spiral galaxies. Scientists believe that elliptical galaxies form as the gravity of two or more spiral galaxies come together. IRREGULAR GALAXIES- Galaxies that are oddly shaped are called irregular galaxies. Many form from the gravitational pull of neighboring galaxies. Irregular galaxies contain many young stars and have areas of intense star formation. GROUPS OF GALAXIES- Galaxies are not evenly distributed in the universe. Gravity holds them together in groups called clusters. Some clusters of galaxies are enormous. The VIRGO CLUSTER contains about 2,000 galaxies. Most clusters exist in even larger structures called superclusters. Between superclusters are VOIDS, which are empty space. Scientists hypothesize that the large-scale structure of the universe looks like a sponge. QUESTIONS: 1. What are galaxies? 2. What is dark matter? 3. What are the three main types of galaxies?