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Name Lesson 3 Summary Use with pp. 524–529 Lesson 3: How are stars grouped together? Vocabulary galaxy a huge system of stars, dust, and gas held together by gravity constellation an area of the sky and all the stars seen in that area A galaxy is a huge group of stars, dust, and gases. Gravity holds a galaxy together. There are billions of galaxies. Earth is part of the Milky Way galaxy. About three-fourths of the galaxies are spiral galaxies with wide centers. They also have thin arms that stretch out from the center like a pinwheel. Stars are in these arms. The stars move around the center of the galaxy. The Milky Way is a spiral galaxy. Elliptical galaxies are round or oval. They can look like footballs. The largest galaxies are elliptical. There are also elliptical galaxies that are smaller than the Milky Way. Some galaxies are called irregular because they are not spiral or elliptical. They do not have a definite shape. Irregular galaxies are probably young galaxies with their stars are still forming. Constellations Ursa Major is part of the Milky Way galaxy. Ursa Major is a constellation. A constellation is an area of the sky and all the stars seen in that area. A constellation is like a star’s address. Scientists use constellations to help them locate stars. For example, the Big Dipper is part of the constellation Ursa Major. There are 88 constellations. 128 Chapter 16, Lesson 3 Summary From Earth, the stars in a constellation may look close together. They may look this way because they are in the same direction from Earth. But the stars might actually be very far apart. People in different parts of the world see different constellations. Earth can be divided into two parts along the equator. The half to the north of the equator is called the Northern Hemisphere. The half to the south of the equator is called the Southern Hemisphere. The United States is in the Northern Hemisphere. Ursa Major can be seen in the Northern Hemisphere. But people in the Southern Hemisphere cannot see it. The constellation Centaurus can be seen only in the Southern Hemisphere. Stars on the Move You can see Ursa Major all year. But other constellations can only be seen at certain times of the year. Canis Major is a constellation we see only in the winter. The constellations change with the seasons. This is because Earth rotates, or spins. It takes Earth one year to move around the Sun. We see different parts of the sky as Earth moves. Nothing in the universe stands still. Stars hurtle though space at different speeds and in different directions. We cannot see this movement because the stars are too far away. Over time, the patterns of the stars will change. Some stars will move closer together or farther apart. Quick Study © Pearson Education, Inc. 5 Galaxies Name Lesson 3 Checkpoint Use with pp. 524–529 Lesson 3 Checkpoint 1. What type of galaxy is the Milky Way? 2. Which constellation is the Big Dipper in? 3. Which type of galaxy is almost round or football shaped? 4. How many constellations are there? © Pearson Education, Inc. 5 5. Compare and Contrast Ursa Major and the Milky Way. Quick Study Chapter 16, Lesson 3 Checkpoint 129