2. Unit 12 Lesson 3 What Are Stars and Galaxies?
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3. 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.
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4. 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.
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5. 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.
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6. A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 6
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7. 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.
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8. 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.
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9. A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 9
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10. A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 10
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11. A Star Is Born Unit 12 Lesson 3 What Are Stars and Galaxies? 11
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12. 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.
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13. Going Galactic Unit 12 Lesson 3 What Are Stars and Galaxies? 13
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14. 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.
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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.
Pinwheel-shaped galaxies are called spiral galaxies.
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16. 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.
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17. 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.
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18. 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.
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19. More Types of Galaxies Unit 12 Lesson 3 What Are Stars and Galaxies?
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20. More Types of Galaxies Unit 12 Lesson 3 What Are Stars and Galaxies?
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21. 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.
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22. 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.
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23. Cosmic Crashes Unit 12 Lesson 3 What Are Stars and Galaxies? 23
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24. Unit 12 Lesson 3 What Are Stars and Galaxies?
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25. Unit 12 Lesson 3 What Are Stars and Galaxies?
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