Stars and Galaxies - rcboe.org · shown in the figure below. Lower-mass stars do not have enough Visual Check mass to become supergiants. 2. Name what forms in only the most massive

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Life Cycle of a StarStars have life cycles that can be compared to the life cycles of

living things. They are “born,” and after millions or billions of years, they “die.” Stars die in different ways, depending on their masses. But all stars—from white dwarfs to supergiants—form in the same way.

Nebulae and Protostars

Stars form deep inside clouds of gas and dust. A cloud of gas and dust is a nebula (plural, nebulae). Star-forming nebulae are cold, dense, and dark. Gravity causes the densest parts to collapse, forming regions called protostars. Protostars continue to contract. As they contract, they pull in surrounding gas. Eventually, their cores are hot and dense enough for nuclear fusion to begin. As they contract, protostars produce enormous amounts of thermal energy.

Birth of a Star

Over many thousands of years, the energy produced by protostars heats the gas and dust around the protostars. Eventually, the gas and dust blow away, and the protostars become visible as stars. Some of this material might later become planets or other objects that orbit the star. During the star-formation process, nebulae glow brightly.

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before

column if you agree with the statement or a D if you disagree. After

you’ve read this lesson, reread the statements to see if you have

changed your mind.

Before Statement After

5. The more matter a star contains, the longer

it is able to shine.

6. Gravity plays an important role in the

formation of stars.

Key Concepts

• How do stars form?

• How does a star’s mass

affect its evolution?

• How is star matter

recycled in space?

Key Concept

Check

1. Summarize How do

stars form?

Ask Questions As you

read, write your questions

about stars on a sheet of

paper. Answer your

questions as you read the

lesson a second time.

Discuss any questions that

you can’t answer with your

teacher.

Stars and Galaxies

Evolution of Stars

LESSON 3

CHAPTER 20

Evolution of Stars Stars and Galaxies 637

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Main-Sequence Stars

Recall the main sequence of the Hertzsprung-Russell diagram. Stars spend most of their lives on the main sequence. A star becomes a main-sequence star as soon as it begins to fuse hydrogen into helium in the core. It remains on the main sequence for as long as it continues to fuse hydrogen into helium. Average-mass stars such as the Sun remain on the main sequence for billions of years. High-mass stars stay on the main sequence for only a few million years. Even though massive stars have more hydrogen than lower-mass stars, they process it at a much faster rate.

When a star’s hydrogen supply is nearly gone, the star moves off the main sequence. It begins the next stage of its life cycle, as shown in the figure below. Not all stars go through all phases shown in the figure below. Lower-mass stars do not have enough mass to become supergiants.Visual Check

2. Name what forms in

only the most massive stars.

Make a vertical five-tab

book to organize your notes

on the life cycle of a star.

A Massive Star’s Life Cycle

638 Stars and Galaxies Evolution of Stars

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End of a StarAll stars form in the same way. But stars die in different ways,

depending on their masses. Massive stars collapse and explode. Lower-mass stars die more slowly.

White Dwarfs

Average-mass stars, such as the Sun, do not have enough mass to fuse elements beyond helium. They do not get hot enough. After helium in their cores is gone, the stars cast off their gases, exposing their cores. The core becomes a white dwarf, a hot, dense, slowly cooling sphere of carbon.

The Sun as a Red Giant What will happen to Earth and the solar system when the Sun runs out of fuel? When the Sun runs out of hydrogen, in about 5 billion years, it will become a red giant. Once helium fusion begins, the Sun will contract. When the helium is gone, the Sun will expand again, probably absorbing Mercury, Venus, and Earth, and pushing Mars and Jupiter outward.

The Sun as a White Dwarf Eventually, the Sun will become a white dwarf, as shown in the figure below. Imagine the mass of the Sun squeezed a million times until it is the size of Earth. That’s the size of a white dwarf.

Scientists hypothesize that all stars with masses less than 8–10 times that of the Sun will eventually become white dwarfs. With a white dwarf at the center, the solar system will be a cold, dark place.

Reading Check

3. Point Out What

determines the way a star

will die?

Reading Check

4. Summarize What will

happen to Earth when the

Sun runs out of fuel?

The Sun as a White Dwarf

Visual Check

5. Locate Circle the

planet closest to the white

dwarf.



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Supernovae

Stars with more than 10 times the mass of the Sun do not become white dwarfs. Instead, they explode. A supernova (plural,

supernovae) is an enormous explosion that destroys a star.

In the most massive stars, a supernova occurs when iron forms in the star’s core. Iron is stable and does not fuse. After a star forms iron, it loses its internal energy source. Without its energy source, the core collapses quickly under the force of gravity. The collapse of the core releases so much energy that the star explodes. When it explodes, a star can become 1 billion times brighter and form elements even heavier than iron.

Neutron Stars

Have you ever eaten cotton candy? A bag of cotton candy is spun from just a few spoonfuls of sugar. Cotton candy is mostly air. Similarly, atoms are mostly empty space. During a supernova, the outer layers of the star are blown away and the core collapses under the heavy force of gravity. The space in atoms disappears as protons and electrons combine to form neutrons.

A neutron star is a dense core of neutrons that remains after a supernova. Neutron stars are only about 20 km wide. Their cores are so dense that a teaspoonful would weigh more than 1 billion tons.

Black Holes

For the most massive stars, atomic forces holding neutrons together are not strong enough to overcome so much mass in such a small volume. Gravity is too strong, and the matter crushes into a black hole. A black hole is an object whose gravity is so great that no light can escape.

A black hole does not suck matter in like a vacuum cleaner. But a black hole’s gravity is very strong because all of its mass is concentrated in a single point. Because astronomers cannot see a black hole, they only can infer its existence. For example, if they detect a star circling around something but they cannot see what that something is, they suspect it is a black hole.

Recycling MatterAt the end of a star’s life cycle, much of its gas escapes into

space. This gas is recycled. It becomes the building blocks of future generations of stars and planets.

Reading Check

6. Explain Why does a

massive star lose its

internal energy source

when iron forms in its core?

Key Concept

Check

7. Explain How does a

star’s mass determine if it

will become a white dwarf,

a neutron star, or a black

hole?

REVIEW VOCABULARYneutrona neutral particle in the nucleus of an atom



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Planetary Nebulae

You read that average-mass stars, such as the Sun, become white dwarfs. When a star becomes a white dwarf, it casts off hydrogen and helium gases in its outer layers. The expanding, cast-off matter of a white dwarf is a planetary nebula. Most of the star’s carbon remains locked in the white dwarf. But the gases in the planetary nebula can be used to form new stars.

Planetary nebulae have nothing to do with planets. They are called “planetary” because early astronomers thought they were regions where planets were forming.

Supernova Remnants

During a supernova, a massive star comes apart. This sends a shock wave into space. The expanding cloud of dust and gas is called a supernova remnant. Like a snowplow pushing snow, a supernova remnant pushes on the gas and dust it encounters.

In a supernova, a star releases the elements that formed inside it during nuclear fusion. Almost all of the elements in the universe other than hydrogen and helium were created by nuclear reactions inside the cores of massive stars and released in supernovae. This includes the oxygen in air, the silicon in rocks, and the carbon in you.

Gravity causes recycled gases and other matter to clump together in nebulae and form new stars and planets. As you will read in the next lesson, gravity also causes stars to clump together into even larger structures called galaxies.

Reading Check

8. Relate How are a

white dwarf and a planetary

nebula related?

Key Concept

Check

9. Describe How do

stars recycle matter?



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Mini Glossary

black hole: an object whose gravity is so great that no light can escape

nebula: a cloud of gas and dust

neutron st ar: a dense core of neutrons left from a supernova

supernova: an enormous explosion that destroys a star

white dwarf: a hot, dense, slowly cooling sphere of carbon

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes how a supernova and a neutron star are related.

2. Complete the life cycle of a massive star by writing the following in the correct sequence in the circles of the diagram: larger red giant, protostar, red giant, red supergiant, supernova remnants.

massive star

main-sequence

star

supernova

nebulae

Reread the statements at the beginning of the

lesson. Fill in the After column with an A if you

agree with the statement or a D if you dis-

agree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.



Stars and Galaxies 643

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Skim Lesson 3 in your book. Read the headings and look at the photos and illustrations. Identify three things you want to learn more about as you read the lesson. Record your ideasin your Science Journal.

Lesson 3 Evolution of Stars

Life Cycle of a Star

I found this on page .


Sequence the change of a nebula to a visible star.

A nebula begins as a , , and

cloud of and .

causes the parts

to forming .

A protostar contracts until

.

The around the protostar ,

and the protostar becomes .

Order the changes in the life cycle of a massive star.

Stage Elements Formed

Massive star

and red giant

• hydrogen →

• helium →

Larger red

giant

• hydrogen →

• helium →

• carbon →

Red

supergiant

hydrogen → helium → →

→ → →

Explain why the Sun will not become a supergiant. Identify what it will become.

End of a Star


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644 Stars and Galaxies

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Lesson 3 | Evolution of Stars (continued)


Sequence what will happen to the solar system when the Sun runs out of fuel.

will run out of

and become .

The Sun

will begin to fuse ,

and it will .

will run out of and

, absorbing

.

will push farther out

and will finally become a .

Order the formation of a supernova.

Iron

forms in

the star’s

core.

The star

explodes.

Characterize neutron stars.

Neutron

Star

about

wide

teaspoonful

would weigh

normal space

inside atoms

dense core of

left from




Stars and Galaxies 645

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Lesson 3 | Evolution of Stars (continued)

Recycling Matter


Compare and contrast a black hole with the star from whichit formed.

Original Star Black Hole

Size

Mass

Gravity

Appearance

Diagram the cycle of a planetary nebula.

gases form

average-mass

star becomes a

white dwarf

casts off

forms

Identify three examples of elements found on Earth that are released in supernova remnants.

1. in

2. in

3. in


Summarize how the force of gravity factors into at least three processes in the formation of stars.



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Stars and Galaxies - rcboe.org · shown in the figure below. Lower-mass stars do not have enough Visual Check mass to become supergiants. 2. Name what forms in only the most massive