1. Unit 2 Lesson 3 What Are Stars and Galaxies?
Copyright © Houghton Mifflin Harcourt Publishing Company 1

2. Unit 2 Lesson 3 What Are Stars and Galaxies?
Florida Benchmark
SC.5.E.5.1 Recognize that a galaxy consists of gas, dust, and many stars, including any objects orbiting the stars. Identify our home galaxy as the Milky Way.
Copyright © Houghton Mifflin Harcourt Publishing Company 2

3. Unit 2 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

4. Unit 2 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 is the closest star to Earth.
The sun looks larger than other stars only because it is so close to Earth.
Copyright © Houghton Mifflin Harcourt Publishing Company 4

5. Unit 2 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. Eventually, they start releasing heat and light.
Stars are classified by color, temperature, brightness, and size.
Copyright © Houghton Mifflin Harcourt Publishing Company 5

6. Unit 2 Lesson 3 What Are Stars and Galaxies?
A Star Is Born
How does the brightness, size, and color of the sun compare to other stars in the image below?
Copyright © Houghton Mifflin Harcourt Publishing Company 6

7. Going Galactic Features of Galaxies
Unit 2 Lesson 3 What Are Stars and Galaxies?
Going Galactic
Features of Galaxies
The universe is everything that exists.
The universe is full of billions of galaxies.
A galaxy is a group of billions of stars, the objects that orbit the stars, gas, and dust.
Copyright © Houghton Mifflin Harcourt Publishing Company 7

8. Features of Galaxies Our home galaxy is known as the Milky Way.
Unit 2 Lesson 3 What Are Stars and Galaxies?
Features of Galaxies
Our home galaxy is known as the Milky Way.
Large distances separate galaxies.
Powerful telescopes help scientists to study galaxies far away.
Copyright © Houghton Mifflin Harcourt Publishing Company 8

9. Types of Galaxies Galaxies are classified by their shapes.
Unit 2 Lesson 3 What Are Stars and Galaxies?
Types of Galaxies
Galaxies are classified by their shapes.
Pinwheel-shaped galaxies are called spiral galaxies.
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 9

10. Unit 2 Lesson 3 What Are Stars and Galaxies?
More Types of Galaxies
Irregular galaxies 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 10

11. Unit 2 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 11

12. Unit 2 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 12

13. Unit 2 Lesson 3 What Are Stars and Galaxies?
Cosmic Crashes
A starburst, or rapid formation of many stars, can happen when galaxies collide.
Scientists think that many irregular galaxies were once spiral or elliptical galaxies that collided.
Copyright © Houghton Mifflin Harcourt Publishing Company 13