The Solar System

150 PART 2^ |^ THE STARS

Nuclear Fusion

in the Sun

Like soap bubbles, stars are structures balanced between oppos-
ing forces that individually would destroy them. Th e sun is a ball
of hot gas held together by its own gravity. If it were not for the
sun’s gravity, the hot, high-pressure gas in the sun’s interior would
explode outward. Likewise, if the sun were not so hot, its gravity
would compress it into a small dense body. In this section, you
will discover how the sun generates its heat.
Th e sun is powered by nuclear reactions that occur near its
center.* Th e energy keeps the interior hot and keeps the gas
totally ionized. Th at is, the electrons are not attached to atomic
nuclei, so the gas is an atomic soup of rapidly moving particles
colliding with each other at high velocities. Nuclear reactions
inside stars involve atomic nuclei, not whole atoms.
How exactly can the nucleus of an atom yield energy? Th e
answer lies in the force that holds the nuclei together.

Nuclear Binding Energy

Th e sun generates its energy by breaking and reconnecting the
bonds between the particles inside atomic nuclei. Th is is quite
diff erent from the way you would generate energy by burning
wood in a fi replace. Th e process of burning wood extracts energy
by breaking and rearranging chemical bonds among atoms in the
wood. Chemical bonds are formed by the electrons in atoms, and
you saw in Chapter 7 that the electrons are bound to the atoms
by the electromagnetic force. So the chemical energy released
when these bonds are broken and rearranged originates in the
electromagnetic force.
Th ere are only four forces in nature: the force of gravity, the
electromagnetic force, the weak force, and the strong force. Th e
weak force is involved in the radioactive decay of certain kinds of
nuclear particles, and the strong force binds together atomic
nuclei. Th e weak force and the strong force are both much stron-
ger than either gravity or the electromagnetic force, but they are
short-range forces that are eff ective only within the nuclei of
atoms. Nuclear reactions break and reform the bonds that hold
atomic nuclei together, so nuclear energy comes from the strong
force. (In a later chapter you will learn that physicists have dis-
covered the electromagnetic force and the weak force are actually
diff erent aspects of one force, and they have a goal of eventually
fi nding a way to “unify” all four forces, including gravity, within
one general mathematical description).
Th ere are two ways to generate energy from atomic nuclei.
Nuclear power plants on Earth generate energy through nuclear
fi ssion reactions that split uranium nuclei into less massive

8-2

■ Figure 8-8

The red line in this graph shows the binding energy per particle, the energy

that holds particles inside an atomic nucleus. The horizontal axis shows the

atomic mass number of each element, the number of protons and neutrons

in the nucleus. Both fi ssion and fusion nuclear reactions move downward in

the diagram (arrows), meaning the nucleus produced by a reaction is more

tightly bound than the nuclei that went into the reaction. Iron has the

most tightly bound nucleus, so no nuclear reactions can begin with iron and

release energy.

Binding energy per nuclear particle (10

–13

J)

Iron

Uranium

Carbon

Oxygen

Nitrogen

Helium

Lithium

Fusion

Fission

0 40 80 120

Mass number

160 200 240

5

10

0

15

Less

tightly

bound

More

tightly

bound

Hydrogen

*Astronomers sometimes use unhelpful words when they talk about nuclear
reactions inside stars. Th ey may use words like burn or ignite. What goes on
inside stars is not related to simple burning but is comprised of nuclear reactions.

fragments. A uranium nucleus contains a total of 235 protons

and neutrons, and when it decays, it splits into a range of possi-

ble fragments, each containing roughly half as many particles.

Because the fragments that are produced are more tightly bound

than the uranium nuclei, binding energy is released during ura-

nium fi ssion.

Stars don’t use nuclear fi ssion. Th ey make energy in nuclear

fusion reactions that combine light nuclei into heavier nuclei.

Th e most common reaction inside stars, the one that occurs in

the sun, fuses hydrogen nuclei (single protons) into helium

nuclei, which contain two protons and two neutrons. Because

the nuclei produced are more tightly bound than the original

nuclei, energy is released.

■ Figure 8-8 shows how tightly diff erent atomic nuclei are

bound. Th e lower in the diagram, the more tightly the particles

in a nucleus are held. Notice that both fusion and fi ssion reac-

tions move downward in the diagram toward more tightly bound

nuclei. Th ey both produce energy by releasing binding energy of

atomic nuclei.