Unit 12 Lesson 3 What Are Stars and Galaxies? Copyright © Houghton Mifflin Harcourt Publishing Company 2

Unit 12 Lesson 3 What Are Stars and Galaxies? Twinkling Stars Astronomy is the study of objects in space and their characteristics. Astronomers are scientists who study astronomy. Astronomers use tools such as telescopes to study objects in space. Copyright © Houghton Mifflin Harcourt Publishing Company 3

Unit 12 Lesson 3 What Are Stars and Galaxies? Twinkling Stars Stars are huge balls of hot, glowing gases that produce their own heat and light. The sun looks larger than other stars only because it is so close to Earth. Copyright © Houghton Mifflin Harcourt Publishing Company 4

A Star Is Born Stars form when Unit 12 Lesson 3 What Are Stars and Galaxies? A Star Is Born Stars form when gravity causes gas and dust particles in space to pull together. The particles are squeezed together under great pressure. Eventually, they release heat and light. Copyright © Houghton Mifflin Harcourt Publishing Company 5

A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 6 Copyright © Houghton Mifflin Harcourt Publishing Company 6

Unit 12 Lesson 3 What Are Stars and Galaxies? A Star Is Born Stars are classified by color, temperature, brightness, and size. The color of a star can tell us about its temperature. For example, blue stars are the hottest, at about 15,000 °C. Copyright © Houghton Mifflin Harcourt Publishing Company 7

Unit 12 Lesson 3 What Are Stars and Galaxies? A Star Is Born Stars are classified by color, temperature, brightness, and size. The size of stars vary also. White dwarf stars, for instance, can be as small as a planet. The largest stars are usually the brightest. A star’s brightness is related to the amount of visible light it gives off. Copyright © Houghton Mifflin Harcourt Publishing Company 8

A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 9 Copyright © Houghton Mifflin Harcourt Publishing Company 9

A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 10 Copyright © Houghton Mifflin Harcourt Publishing Company 10

A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 11 Copyright © Houghton Mifflin Harcourt Publishing Company 11

Going Galactic The universe is everything that exists. Unit 12 Lesson 3 What Are Stars and Galaxies? Going Galactic The universe is everything that exists. There are billions of galaxies in the universe, and our home galaxy is known as the Milky Way. Copyright © Houghton Mifflin Harcourt Publishing Company 12

Going Galactic Unit 12 Lesson 3 What Are Stars and Galaxies? 13 Copyright © Houghton Mifflin Harcourt Publishing Company 13

Unit 12 Lesson 3 What Are Stars and Galaxies? Features of Galaxies A galaxy is a group of billions of stars, as well as the objects that orbit the stars, gas, and dust. Galaxies are held together by gravity. Large distances separate galaxies. Powerful telescopes help scientists to study galaxies far away. Copyright © Houghton Mifflin Harcourt Publishing Company 14

Unit 12 Lesson 3 What Are Stars and Galaxies? Types of Galaxies Astronomer Edwin Hubble was the first to study galaxies in the 1920s. He classified them by shape. Pinwheel-shaped galaxies are called spiral galaxies. Copyright © Houghton Mifflin Harcourt Publishing Company 15

Unit 12 Lesson 3 What Are Stars and Galaxies? Types of Galaxies Astronomer Edwin Hubble was the first to study galaxies in the 1920s. He classified them by shape. Barred spiral galaxies, such as the Milky Way, are spiral galaxies with a center shaped like a long bar. Copyright © Houghton Mifflin Harcourt Publishing Company 16

Unit 12 Lesson 3 What Are Stars and Galaxies? More Types of Galaxies Irregular galaxies are dimmer and do not have a particular shape. The stars are randomly scattered. Irregular galaxies have lots of gas and dust to form new stars. About 20 percent of all galaxies are irregular. Copyright © Houghton Mifflin Harcourt Publishing Company 17

Unit 12 Lesson 3 What Are Stars and Galaxies? More Types of Galaxies Elliptical galaxies are brightest at their center. They can be shaped like a perfect sphere or a flattened globe. Elliptical galaxies have old stars and too little gas and dust to form new stars. About 60 percent of all galaxies are elliptical. Copyright © Houghton Mifflin Harcourt Publishing Company 18

More Types of Galaxies Unit 12 Lesson 3 What Are Stars and Galaxies? Copyright © Houghton Mifflin Harcourt Publishing Company 19

More Types of Galaxies Unit 12 Lesson 3 What Are Stars and Galaxies? Copyright © Houghton Mifflin Harcourt Publishing Company 20

Unit 12 Lesson 3 What Are Stars and Galaxies? Cosmic Crashes Sometimes galaxies collide, or crash together, in space. Gravity pulls galaxies toward each other. Galaxies are always moving. When galaxies collide, large amounts of dust and gas get pressed together. Copyright © Houghton Mifflin Harcourt Publishing Company 21

Unit 12 Lesson 3 What Are Stars and Galaxies? Cosmic Crashes A starburst, or rapid formation of many stars, can happen when galaxies collide. This collision can cause a smaller galaxy to become part of a larger galaxy, or it can form a large, irregular galaxy. Scientists think that many irregular galaxies were once spiral or elliptical galaxies that collided. Copyright © Houghton Mifflin Harcourt Publishing Company 22

Cosmic Crashes Unit 12 Lesson 3 What Are Stars and Galaxies? 23 Copyright © Houghton Mifflin Harcourt Publishing Company 23

Unit 12 Lesson 3 What Are Stars and Galaxies? Copyright © Houghton Mifflin Harcourt Publishing Company 24

Unit 12 Lesson 3 What Are Stars and Galaxies? Copyright © Houghton Mifflin Harcourt Publishing Company 25