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CHAPTER 29
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Date
The Sun
SECTION
1 Structure of the Sun
KEY IDEAS
As you read this section, keep these questions in mind:
• How do the sun’s radiative and convective zones
compare?
• What are the three layers of the sun’s atmosphere?
• How does the sun convert matter to energy in its core?
What Are the Sun’s Layers?
Scientists cannot see inside the sun. They use models
to figure out what the sun’s interior is like. They also
study the sun’s surface to learn more about the inside of
the sun. The sun has four main layers:
•
•
•
•
the core
READING TOOLBOX
Organize On a separate
piece of paper, create a
graphic organizer describing
the layers of the sun.
the radiative zone
the convective zone
the atmosphere
READING CHECK
1. Identify What are the four
main layers of the sun?
THE CORE
The core is the sun’s center. Like the rest of the sun,
the core is made up of ionized gas. Because the sun’s
mass is so large, the gas in the core is under a great deal
of pressure. In fact, the pressure is so great that the core
is as dense as iron.
THE RADIATIVE ZONE
The energy produced in the core moves through
two other zones until it reaches the sun’s atmosphere.
In the radiative zone, energy moves in the form of
electromagnetic waves, or radiation.
THE CONVECTIVE ZONE
The next layer is the convective zone. In the convective
zone, energy moves by convection. Convection is the
transfer of energy by moving matter. In the convective
zone, hot gases transfer energy to the sun’s surface. As the
gases approach the sun’s surface, they become cooler and
denser. The cooler, denser gases sink back to the bottom
of the convective zone, and the cycle begins again.
Critical Thinking
2. Infer Where is the
temperature probably
higher—at the bottom of
the convective zone or at
the top?
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The Sun
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Structure of the Sun continued
Layers of the
sun’s interior
Layers of the
sun’s atmosphere
Convective Zone
2,000,000 ºC
Corona
>1,000,000 ºC
Chromosphere
6,000 ºC
to 50,000 ºC
Radiative Zone
2,000,000 ºC to
7,000,000 ºC
Core
15,000,000 ºC
Photosphere
6,000 ºC
LOOKING CLOSER
3. Compare What is the
hottest layer of the sun?
Sunspot
3,800 ºC
Reactions in the sun’s core produce energy. This energy moves toward the outer
parts of the sun through radiation and convection.
What Are the Layers of the Sun’s
Atmosphere?
READING CHECK
4. Describe What are the
three layers of the sun’s
atmosphere?
Although the sun itself is made up of gases, scientists
consider the uppermost gases as the atmosphere. The
sun’s atmosphere surrounds the convective zone. The
sun’s atmosphere has three layers: the photosphere, the
chromosphere, and the corona.
THE PHOTOSPHERE
The photosphere is the layer closest to the convective
zone. It is made up of gases that have risen from the
convective zone. The photosphere gives off most of its
energy in the form of visible light. The visible light we
see from Earth comes from the photosphere. The other
layers of the sun’s atmosphere are transparent. Thus,
scientists sometimes refer to the photosphere as the sun’s
“surface.”
LOOKING CLOSER
The dark spots in the figure are called
sunspots. Sunspots are cooler regions
in the sun’s photosphere.
5. Define What are
sunspots?
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The Sun
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Structure of the Sun continued
THE CHROMOSPHERE
The chromosphere is the thin layer above the
photosphere. It is made up of gases that glow with
a reddish light. These gases move outward from the
photosphere.
THE CORONA
The corona is the outermost layer of the sun’s
atmosphere. The corona is not very dense, but its magnetic
field can stop most subatomic particles from escaping
into space. However, some particles do escape into space.
Some of these particles are electrons, and others are
electrically charged particles called ions.
The charged particles from the corona make up the
solar wind, which flows from the sun. We cannot usually
see the chromosphere or the corona from Earth. However,
during a solar eclipse, these layers become visible.
Normally, we cannot see the
sun’s corona. During a solar
eclipse, however, the moon
blocks the photosphere and
the corona becomes visible.
Talk About It
Use Word Roots Use a
dictionary or the Internet
to learn the meaning of
the prefix chromo-. With a
partner, talk about why the
reddish layer of the sun is
called the chromosphere.
LOOKING CLOSER
6. Explain Why is the
corona visible during a
total solar eclipse?
Where Does the Sun’s Energy Come From?
The sun produces energy through a process called
nuclear fusion. During nuclear fusion, the nuclei of
small atoms fuse, or combine, to form larger nuclei.
Nuclear fusion releases huge amounts of energy.
On Earth, atoms are made of a nucleus surrounded
by electrons. The nucleus and electrons stay together.
However, the high temperature and pressure in the sun’s
core cause the electrons to separate from the nucleus.
The nuclei in the sun tend to push away from each other.
However, the high temperature and pressure force the
nuclei close enough to fuse together.
The most common form of nuclear fusion in the sun is
the fusion of hydrogen into helium. This kind of nuclear
fusion has three main steps.
READING CHECK
7. Describe What causes
electrons to separate from
nuclei inside the sun?
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The Sun
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Structure of the Sun continued
HOW NUCLEAR FUSION HAPPENS
READING CHECK
8. Explain What happens
when two protons fuse?
In the first step of nuclear fusion, two hydrogen nuclei
collide and fuse to form a larger nucleus. Each hydrogen
nucleus contains only one proton, which has a positive
charge. When the two nuclei fuse, one of the protons
emits a particle called a positron. When the proton emits
a positron, the proton changes into a neutron. Therefore,
at the end of step 1, the nucleus has one proton and one
neutron.
During the second step of nuclear fusion, another
proton fuses with the new nucleus. The nucleus now
contains two protons and one neutron. It is a nucleus of
the element helium.
During the final step of nuclear fusion, two nuclei
from step 2 fuse together. As this fusion happens, two
protons are released. The remaining two protons and two
neutrons are fused together. The protons and neutrons
form a new nucleus of a different form of the element
helium.
Steps in Nuclear Fusion
LOOKING CLOSER
9. Identify All together, how
many protons have to fuse
together to form one 4He
nucleus? Explain your
answer.
Proton, hydrogen
nucleus, 1H
Positron
Hydrogen
nucleus, 2H
Helium
nucleus, 3He
Energy
released
Energy
released
Helium
nucleus, 4He
Energy
released
Neutron
Energy
released
Energy
released
Step 1
Step 2
Step 3
THE SUN’S ENERGY
One of the final products of this type of nuclear fusion
is a helium nucleus. The helium nucleus has about 0.7%
less mass than the hydrogen nuclei that formed it. The
lost mass has been converted into energy during the
fusion process. This energy causes the sun to shine and
to have a high temperature.
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The Sun
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Structure of the Sun continued
How Can Matter Change into Energy?
In 1905, Albert Einstein suggested that a small amount
of matter can become a large amount of energy. This
suggestion was part of Einstein’s theory of relativity.
His theory of relativity includes the equation E = mc2.
Scientists can use this equation to calculate how much
energy a certain amount of matter can become. In this
equation,
• E represents energy
• m represents mass
• c represents the speed of light (about 300,000 km/s)
You can see that c2 is a very large number. Therefore,
even a tiny amount of mass can become a very large
amount of energy.
Scientists have used Einstein’s equation to explain
how the sun produces so much energy. Each second, the
sun uses nuclear fusion to change about 4 million tons of
mass into energy.
READING CHECK
10. Restate In words, state
the relationship between
matter and energy that
Einstein’s equation describes.
What Is the Sun Made Of?
Scientists use a spectrograph to break up a star’s light
into a spectrum of colors. They can use this spectrum to
figure out what elements the star is made of. Dark lines
in the spectrum form when elements in the star’s outer
layers absorb certain wavelengths of light. Each element
produces a unique pattern because of the wavelengths it
absorbs. Astronomers use this information to infer which
elements are part of a star.
In this way, scientists have discovered that about 75%
of the sun’s mass is hydrogen. About 24% of the sun’s mass
is helium. However, the sun’s spectrum shows that the sun
contains small amounts of almost all chemical elements.
LOOKING CLOSER
Hydrogen
11. Explain Why are
there more lines in the
sun’s spectrum than in the
hydrogen spectrum?
Sun's
spectrum
The first spectrum shows which wavelengths of light are absorbed by hydrogen. The
second spectrum shows which wavelengths of light are absorbed by the sun.
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Section 1 Review
SECTION VOCABULARY
chromosphere the thin layer of the sun that is
just above the photosphere and that glows a
reddish color during eclipses
convective zone the region of the sun’s interior
that is between the radiative zone and the
photosphere and in which energy is carried
upward by convection
corona the outermost layer of the sun’s
atmosphere
nuclear fusion the process by which nuclei of
small atoms combine to form a new, more
massive nucleus; the process releases energy
photosphere the visible surface of the sun
radiative zone the zone of the sun’s interior that
is between the core and the convective zone
and in which energy moves by radiation
1. Explain Why does nuclear fusion happen only in the sun’s core?
2. Infer Will the amount of hydrogen in the sun increase or decrease over the next
million years? Explain your answer.
3. Compare What is the main difference between the radiative zone and the
convective zone?
4. Describe Complete the table below to describe the layers of the sun’s atmosphere.
Layer
Average Temperature
Other Characteristics
6,000 °C
Chromosphere
• glows with a reddish color
• above the photosphere
• generally visible only during solar eclipses
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The Sun