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Stars and Galaxies
Evolution of Stars
Key Concepts
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
• How do stars form?
• How does a star’s mass
affect its evolution?
• How is star matter recycled
in space?
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.
3TUDY#OACH
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Life Cycle of a Star
Stars 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.
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.
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.
Key Concept Check
1. Summarize How do
stars form?
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.
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Stars and Galaxies
217
Main-Sequence Stars
Make a vertical five-tab book
to organize your notes on
the life cycle of a star.
Protostar
Main
Sequence
Red Giant
Re
Supergdian
t
Superno
va
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. It remains on the main
sequence for as long as it continues to fuse hydrogen into
helium. Lower-mass stars such as the Sun stay 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
leaves 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, such
as the Sun, do not have enough mass to become supergiants.
Visual Check
2. Name what forms in
only the most massive stars.
A Massive Star’s Life Cycle
Massive star
When the star’s hydrogen supply is gone, gravity causes the
core to contract and heat up. Thermal energy in the star's center
causes the star’s outer layers to expand and cool. The star
becomes a red giant. Eventually, the interior becomes hot
enough to resume nuclear fusion. The outer layers contract,
and the star begins to fuse helium nuclei and form carbon.
Helium
fuses and
forms
carbon.
Hydrogen fuses
and forms helium.
Red giant
Hydrogen
Helium
Carbon
Neon
Oxygen
Silicon
Iron
Red supergiant
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Stars and Galaxies
Hydrogen
fuses and
forms
helium.
Larger red giant
When helium in the core runs low, the
core again collapses under gravity and
the outer layers expand. The star
becomes a red giant for a second time,
but this time it is even larger. It again
contracts when it begins to fuse carbon
and form other elements.
The process repeats again and again. The star
becomes a red supergiant as different elements
are formed during fusion. Iron nuclei form in
the most massive stars.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Helium
fuses and
forms
carbon.
Carbon fuses
and forms
other elements.
End of a Star
All 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.
Reading Check
3. Point Out What
White Dwarfs
Lower-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.
determines the way a star
will die?
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
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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
4. Summarize What will
happen to Earth when the
Sun runs out of fuel?
The Sun as a White Dwarf
White dwarf
Mars
Asteroid belt
Visual Check
5. Locate Circle the planet
Jupiter
Reading Essentials
closest to the white dwarf.
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219
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.
Reading Check
6. Explain Why does a
massive star lose its internal
energy source when iron
forms in its core?
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
REVIEW VOCABULARY
neutron
a neutral particle in the
nucleus of an atom
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 collapse is so violent that it eliminates the
normal spaces inside atoms, and a neutron star forms.
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
7. Explain How does a
star’s mass determine if it will
become a white dwarf, a
neutron star, or a black hole?
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 Matter
At 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.
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Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Key Concept Check
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.
Planetary Nebulae
You read that lower-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.
Reading Check
8. Relate How are a white
dwarf and a planetary
nebula related?
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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
Key Concept Check
9. Describe How do stars
recycle matter?
Reading Essentials
Stars and Galaxies
221
Mini Glossary
black hole: an object whose gravity is so great that no light
can escape
neutron star: a dense core of neutrons left from a supernova
supernova: an enormous explosion that destroys a star
nebula: a cloud of gas and dust
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
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
nebulae
supernova
What do you think
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 disagree.
Did you change your mind?
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Stars and Galaxies
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