152 PART 2^ |^ THE STARS
Convective
zoneRadiative
zoneCore energy
generationPhoton follows a
random path as
it drifts outward.Creating one helium nucleus makes only a small amount of
energy, hardly enough to raise a housefl y one-thousandth of an
inch. Because one reaction produces such a small amount of
energy, it is obvious that many reactions are necessary to supply
the energy needs of a star. Th e sun, for example, completes 10^38
reactions per second, transforming 5 million tons of matter into
energy every second. It might sound as if the sun is losing mass
at a furious rate, but in its entire 10-billion-year lifetime, the sun
will convert less than 0.07 percent of its mass into energy.
It is a Common Misconception that nuclear fusion in
the sun is tremendously powerful. After all, the fusion of a mil-
ligram of hydrogen (roughly the mass of a match head) produces
as much energy as burning 30 gallons of gasoline. However, at
any one time, only a tiny fraction of the hydrogen atoms are fus-
ing into helium, and the nuclear reactions in the sun are spread
through a large volume in its core. Any single gram of matter
produces only a little energy. A person of normal mass eating a
normal diet produces about 4000 times more heat per gram than
the matter in the core of the sun. Gram for gram, you are a much
better heat producer than the sun. Th e sun produces a lot of
energy because it contains a lot of grams of matter in its core.
Fusion reactions can occur only when the nuclei of two
atoms get very close to each other. Because atomic nuclei carry
positive charges, they repel each other with an electrostatic force
called the Coulomb force. Physicists commonly refer to this
repulsion between nuclei as the Coulomb barrier. To overcome
this barrier and get close together, atomic nuclei must collide
violently. Suffi ciently violent collisions are rare unless the gas is
very hot, in which case the nuclei move at high enough speeds.
(Remember, an object’s temperature is related to the speed with
which its particles move.)
Nevertheless, the fusion of two protons is a highly unlikely
process. If you could follow a single proton in the sun’s core, you
would see it encountering and bouncing off other protons mil-
lions of times a second, but you would have to follow it around
for roughly a billion years before it happened to penetrate the
Coulomb barrier and combine with another proton.
Because of the dependence of nuclear reactions on particle
collisions, the reactions in the sun take place only near the center,
where the gas is hot and dense. A high temperature ensures that
collisions between nuclei are violent, and a high density ensures
that there are enough collisions, and thus enough reactions, to
make energy at the sun’s rate.
Energy Transport in the Sun
Now you are ready to follow the energy from the core of the sun
to the surface. You will learn in a later chapter that astronomers
have computed models indicating that the temperature at the
center of the sun is about 15 million K. Compared with that, the
sun’s surface is very cool, only about 5800 K, so energy must fl ow
from the high temperature core outward to the cooler surface
where it is radiated into space.
Because the core is so hot, the photons bouncing around
there are gamma rays. Each time a gamma ray encounters an
electron, it is defl ected or scattered in a random direction, and,
as it bounces around, it slowly drifts outward toward the surface.
Th at carries energy outward in the form of radiation, so astrono-
mers refer to the inner parts of the sun as a radiative zone.
To examine this process, imagine picking a single gamma ray
and following it to the surface. As your gamma ray is scattered
over and over by the hot gas, it drifts outward into cooler layers,
where the cooler gas tends to emit photons of longer wavelength.
Your gamma ray will eventually be absorbed by the gas and
reemitted as two X-rays. Now you must follow those two X-rays
as they bounce around, and soon you will see them drifting out-
ward into even cooler gas, where they will become a number of
longer-wavelength photons.
Th e packet of energy you have been following, now repre-
sented by many photons, eventually reaches the outer layers of
the sun, where the gas is so cool that it is not very transparent to
radiation. Th ere, energy backs up like water behind a dam, and
the gas begins to churn in convection. Hot blobs of gas rise, and
cool blobs sink. In this region, known as a convective zone, your
packet of energy is carried outward not as photons but as circu-
lating gas (■ Figure 8-10). Rising hot gas carries energy outward,
of course, but sinking cool gas is a necessary part of the cycle that
results in the net transport of energy outward.■ Figure 8-10
A cross-section of the sun. Near the center, nuclear fusion reactions gener-
ate high temperatures. Energy fl ows outward through the radiative zone as
photons are randomly defl ected over and over by electrons. In the cooler,
more opaque outer layers, the energy is carried by rising convection currents
of hot gas (red arrows) and sinking currents of cooler gas (blue arrows).