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Chapter 19
Section 3 Galaxies
Objectives
• Identify three types of galaxies.
• Describe the contents and characteristics of
galaxies.
• Explain why looking at distant galaxies reveals
what young galaxies looked like..
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Chapter 19
Section 3 Galaxies
• A galaxy is a collection of stars, dust, and gas held
together by gravity.
Types of Galaxies
• Spiral Galaxies have a bulge at the center and
spiral arms.
• The Milky Way Astronomers think that our solar
system is in a spiral galaxy.
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SPIRAL GALAXY
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Chapter 19
Section 3 Galaxies
Types of Galaxies, continued
• Elliptical Galaxies About one-third of all galaxies
are simply massive blobs of stars. These are called
elliptical galaxies.
• Irregular Galaxies Galaxies that do not fit into any
other class are called irregular galaxies.
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ELLIPTICAL GALAXY
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IRREGULAR GALAXY
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Chapter 19
Section 3 Galaxies
Contents of Galaxies
• Gas Clouds A large clouds of gas and dust in
interstellar space is called a nebula.
• Star Clusters A globular cluster is a tight
group of stars that looks like a ball and contains
up to 1 million stars.
• An open cluster is a group of stars that are
close together relative to surrounding stars.
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Chapter 19
Section 3 Galaxies
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Chapter 19
Section 3 Galaxies
Origin of the Galaxies
• Because if takes light time to travel through space,
looking at distant galaxies reveals what early galaxies
looked like.
• Quasars A very luminous, starlike object that
generates energy at a high rate is called a quasar.
Some scientists think that quasars may be the core of
young galaxies that are in the process of forming.
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Chapter 19
Section 4 Formation of the Universe
Objectives
• Describe the big bang theory.
• Explain evidence used to support the big bang
theory.
• Describe the structure of the universe.
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Chapter 19
Section 4 Formation of the Universe
Objectives, continued
• Describe two ways scientists calculate the age of
the universe.
• Explain what will happen if the universe expands
forever.
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Chapter 19
Section 4 Formation of the Universe
• Cosmology is the study of the origin, properties,
processes, and evolution of the universe.
Universal Expansion
• Galaxy Movement To understand how the universe
formed, scientists study the movement of galaxies.
• A Raisin-Bread Model The universe, like the rising
raisin bread dough, is expanding. Think of the raisins
in the dough as galaxies. As the universe expands, the
galaxies move farther apart.
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Chapter 19
Section 4 Formation of the Universe
Big Bang Theory
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Chapter 19
Section 4 Formation of the Universe
The Big Bang Theory
• A Tremendous Explosion The theory that the
universe began with a tremendous explosion is called
the big bang theory.
• Cosmic Background Radiation In 1964, two
scientists using a huge antenna accidentally found
radiation coming from all directions in space. One
explanation for this radiation is that it is cosmic
background radiation left over from the big bang.
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Chapter 19
Section 4 Formation of the Universe
The Structure
of the Universe
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Chapter 19
Section 4 Formation of the Universe
How Old Is the Universe?
• Age of the Universe Scientist use to methods to
study the age of the universe.
• By measuring the distance between Earth and
various galaxies, scientists can predict the rate of
expansion and calculate the age of the universe.
• Because the universe must at least be as old as the
oldest stars it contains, the ages of the stars provide a
clue to the age of the universe.
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Chapter 19
Section 4 Formation of the Universe
A Forever Expanding Universe
• The expansion of the universe depends on the
amount of matter it contains. A large enough quantity
of matter would cause gravity to stop the expansion.
The universe could start collapsing.
• Scientist now think that there may not be enough
matter in the universe, so the universe would continue
to expand forever and become cold and dark as all the
stars die.
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End of Chapter 19 Show
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Chapter 19
Standardized Test Preparation
Passage 2 If you live away from bright outdoor lights,
you may be able to see a faint, narrow band of light and
dark patches across the sky. This band is called the
Milky Way. Our galaxy, the Milky Way, consists of stars,
gases, and dust. Between the stars of the Milky Way
are clouds of gas and dust called interstellar matter.
These clouds provide materials that form new stars.
Continued on the next slide
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Chapter 19
Standardized Test Preparation
Passage 2, continued Every star that you can see in
the night sky is a part of the Milky Way, because our
solar system is inside the Milky Way. Because we are
inside the galaxy, we cannot see the entire galaxy.But
scientists can use astronomical data to create a picture
of the Milky Way.
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Chapter 19
Standardized Test Preparation
1. In the passage, what does the term interstellar
matter mean?
A stars in the Milky Way
B the Milky Way
C a narrow band of light and dark patches across
the sky
D the clouds of gas and dust between the stars in
the Milky Way
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 19
Standardized Test Preparation
1. In the passage, what does the term interstellar
matter mean?
A stars in the Milky Way
B the Milky Way
C a narrow band of light and dark patches across
the sky
D the clouds of gas and dust between the stars in
the Milky Way
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Chapter 19
Standardized Test Preparation
2. Based on the information in the passage, what
can the reader conclude?
F The Milky Way can be seen in the night sky near
a large city.
G The entire Milky Way can be seen all at once.
H Every star that is seen in the night sky is a part
of the Milky Way.
I Scientists have no idea what the entire Milky Way
looks like.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 19
Standardized Test Preparation
2. Based on the information in the passage, what
can the reader conclude?
F The Milky Way can be seen in the night sky near
a large city.
G The entire Milky Way can be seen all at once.
H Every star that is seen in the night sky is a part
of the Milky Way.
I Scientists have no idea what the entire Milky Way
looks like.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.