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The interior of the sun cannot be observed directly. For that reason, all
it is based on information acquired from the energy it
radiates and from theoretical studies. The source of the sun's energy
was not discovered until the late 1930s.
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For: t.inks on nuclear fusion in
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Visitl www. SciLinks.org
Nuclear Fusion
Deep in its interior, the sun produces energy by
a process known as nuclear fusion. This nuclear reaction converts four
hydrogen nuclei into the nucleus of a helium atom. Tremendous
energy is released. W During nuclear fusion, energy is released
because solne matter is actually converted to energyr as shown in
Figure 18. How does this process work? Consider that four hydrogen
atoms have a combined atomic mass of 4.032 atomic mass units (4 X
1.008) whereas the atomic mass of helium is 4.003 atomic mass units,
or 0.029 less than the combined mass of the hydrogen. The tiny missing mass is emitted as energy according to Einstein's equation:
E:
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mc2
E equals energy, m equals mass) and c equals the speed of light.
Because the speed of light is very great (300,000 km/s), the amount of
energy released from even a small amount of mass is enormous.
The conversion ofjust one pinhead's worth of hydrogen to heliurn
generates more energy than burning thousands of tons of coal. Most
of this energy is in the form of high-energy photons that work their
way toward the solar surface. The photons are absorbed and reemitted
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many times until they reach a layer just below the photosphere. Here,
convection currents help transport this energy to
the solar surface, where it radiates through the
transparent chromosphere and corona.
Only a small percentage of the hydrogen ln
the nuclear reaction is actually converted to
energy. Nevertheless, the sun is consuming an
estimated 600 million tons of hydrogen each
second; about 4 million tons are converted to
energy. As liydrogen is consumed, the product of
this reaction-helium-forms the solar core,
which continually grows in size.
Figure 18 Nuclear Fusion
During nuclear fusion, four
hydrogen nuclei combine to form
one helium nucleus.some matter
is converted to energy.
Hca,Jinrg What hoppens during the
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A great deal is known about the universe beyond
our solar system. This knowledge hinges on the fact
that stars, and even gases in the "empty" space
between stars, radiate energy in all directions into
space. The key to understanding the universe is to
collect this radiation and unravel the secrets it
holds. Astronomers have devised many ways to do
just that. We will begin by examining some prop-
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erties;of stars, such as color, temperature, and mass.
Stan Color and Temrperature
Study the stars in Figure 2 and
note their color. ffi Color is a clue to a star's temperature. \r/ery hot
stars with surface temperatures above 30,000 K emit most of their
energy in the fcrrm of sh,rrt-wavelength light and therefbre appear blue.
Red stars are much cooler, anci most of their energy is emitted as
longer-wavelengtl-r recl light. Stars with temperatures between 5000 and
6000 K appear yeilolv,like the sun.
Figure 2 Stars of Orion This
time-lapse photograph shows
stars as streaks across the night
sky as Earth rotates. The streaks
ciearly show different star colors.
Binary Stars emd Ste8[mr Mas$
In the early
nineteenth century, astronomers discovered that many
stars orbit each other. These pairs of stars, pulled toward
binary stars" More than
50 percent of the stars in the universe may occllr in pairs
or multiples.
ffiBinary stars are used to determine the star
property most diffieult to calculate-its mass. The
mass of a body can be calculated if it is attached try gravity to a partner. 'Ihis is the case for any binary star
system. As shown in Figure 3, birrary stars orbit each
other around a cornmon point called the center of mass.
For stars of equal mass, the center of mass iies exactly
halfviay between them. If one star is more massive than
its pilrtner, their common center will be closer to the
more massive one, If the sizes of their orbits are known,
each other by gr:avity, are calied
the stars'masses can be cletermined.
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Whot is a binory star systemT
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Figure 3 Connmon Center of Mass
iq For stars of equal mass. the center of mass lies in
the middle. B A star twice as massive as its partner is
twice as close to the center of mass. lt therefore has
a smaller orbit than its less massive partner.
Eeyontl Our Solar
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Early in the twentieth century, Einar Hertzsprung and Henry Russell
independently developed a graph used to study stars. It is now called
a Hertzsprr.urg-Russell diagran-r (H-R ciiagram). ffiA HertzsprungRussell diagram shgws the relationship between the absolute
magnitude and temperature of stars. By studying H-R diagrams) we
learn a great deal about the sizes, colors, and temperatures of stars.
In the H-R diagram shown in Figure 5, notice that the stars are not
unifornrly distributed, About 90 percent are main-sequence stars that
fall along a band that runs from the upper-left corner to the lowerright corner of the diagram. As you can see, the hottest main-sequence
stars are the brightest, and the coolest main-sequence stars are the
din-rmest.
Figure.5 Hertzsprung-Russell
Diagram ln this idealized chart,
stars are plotted according to
temperature and absolute
magnitude.
7O4
Chapter 25
The brightness of the rnain-sequence stars is also related to their
irlass. The hottest blue stars are about 50 times more massive than the
sun, while the coolest red stars are only 1/10 as massive. Therefore, on
the H-R diagram, the main-sequence stars appear in decreasing order,
from hotter, rnore massive biue stars to cooler, less massive red stars.
Above and to the right of the main sequence in the H-R diagram lies
a group of very bright stars called red giants. The size of these giants can
be estimated by comparing them with stars of known size that have the
same surface temperature. Objects witir equal surface temperatlues
radiate the same amount of energy per unit area. Therefore, any difference in the brightness of two
stars having the same surface
temperature is due to their relative sizes. Some stars are so large
that they are called supergiants.
Betelgeuse, a bright red supergiant in the constellation Orion,
has a radius about 800 times
that of the sun.
Stars in the lower-central
part of the H-R diagrarn are
much fainter than mainsequence stars of the salne
temperature. Some probably
are no bigger than Earth. This
group is called white dwarfs,
although not all are white.