|The Curve of
The key to energy production in stars lies in what nuclear physicists call the
curve of binding energy, which is illustrated in the following figure.
Curve of binding energy.
This plot shows the amount of binding energy per nucleon (A nucleon is either a
neutron or a proton.
The nucleon number is the sum of the number of
protons in a nucleus; thus, it is equal to the atomic mass number)
as a function of the atomic mass number A. The energy units
are MeV, which stands for "million electron-volts", a standard unit of energy in
This curve indicates how stable atomic nuclei are; the higher the curve the more
stable the nucleus. Notice the characteristic shape, with a peak near A=60. These
nuclei (which are near iron in the periodic table and are called
the iron peak nuclei) are the most stable in the
Universe. The shape of this curve suggests two possibilites for converting
significant amounts of mass into energy.
Note that this is the "upside down" version of the similar graph in text.
There the vertical scale increases downward, here it increases upward.
From the curve of binding energy, the heaviest nuclei are less stable than the
nuclei near A=60. This suggests that energy can be released if heavy nuclei split
apart into smaller nuclei having masses nearer A=60. This process is called
fission. It is the process that powers atomic bombs and nuclear power
The curve of binding energy suggests a second way in which energy could be released
in nuclear reactions. The lightest elements (like hydrogen and helium) have nuclei
that are less stable than heavier elements up to A~60. Thus, sticking two light
nuclei together to form a heavier nucleus can release energy. This process is
called fusion, and is the process that powers hydrogen (thermonuclear)
bombs and (perhaps eventually)
fusion energy reactors.
In both fission and fusion reactions the total masses after the reaction are
less than those before. The
"missing mass" appears as energy, with the amount
given by the famous Einstein equation.
Stellar Energy Production
Both fission and fusion reactions have the potential to convert a small amount of
mass into a large amount of energy and could conceivably account for the energy
sources of stars. However, stars are made from light elements (mostly hydrogen and
helium). Thus, fission cannot be initiated in stars as a source of energy, but
fusion is quite possible if the right conditions prevail. As we shall see, these
conditions can be found in the cores of stars, and thermonuclear fusion is the
primary source of stellar energy.