Unit 12 Lesson 3 - · More Types of Galaxies 3. Irregular galaxies are dimmer and do not have...

Unit 12 Lesson 3 What Are Stars and Galaxies?

Copyright © Houghton Mifflin Harcourt Publishing Company

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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.

12.3 images/astronomy.PNG

12.3 videos/WMAA What is Astronomy.mp4


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.

12.3 images/stars.PNG

A Star Is Born

• Stars form when 1. gravity causes gas and

dust particles in space to pull together.

2. The particles are squeezed together under great pressure.

3. Eventually, they release heat and light.



12.3 videos/How stars are formed and born.mp4

A Star Is Born



A Star Is Born

• Stars are classified by color, temperature, brightness, and size.

1. The color of a star can tell us about its temperature. For example, blue stars are the hottest, at about 15,000 °C.



A Star Is Born

• Stars are classified by color, temperature, brightness, and size.

2. The size of stars vary also. White dwarf stars, for instance, can be as small as a planet.

3. The largest stars are usually the brightest. A star’s brightness is related to the amount of visible light it gives off.



A Star Is Born



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.


12.3 images/universe.PNG

12.3 videos/The Detailed Universe This will Blow Your Mind..mp4

Going Galactic



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.



12.3 images/galaxy.PNG

Types of Galaxies

• Astronomer Edwin Hubble was the first to study galaxies in the 1920s. He classified them by shape.

1. Pinwheel-shaped galaxies

are called spiral galaxies.



Types of Galaxies

• Astronomer Edwin Hubble was the first to study galaxies in the 1920s. He classified them by shape.

2. Barred spiral galaxies, such as

the Milky Way, are spiral galaxies with a center shaped like a long bar.



More Types of Galaxies

3. 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



4. 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.



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.



12.3 videos/Cosmic Collisions Different Stages in Collisions Between Galaxies.mp4

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.



Cosmic Crashes



Unit 12 Lesson 3 - · More Types of Galaxies 3. Irregular galaxies are dimmer and do not have...

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