ATOMS, STARS, AND GALAXIES 183
form a helium nucleus with some
loss of mass, which was then
converted to energy. Eddington
thought this might allow the sun
to shine for tens of billions of years.
In 1929, Robert Atkinson and Fritz
Houtermans calculated how the
fusion of light nuclei, rather than
atoms, could create energy in stars,
but the reactions were unknown.
The proton−proton chain
In 1938, Bethe attended a physics
conference in Washington, D.C., to
discuss how energy is generated
in stars. During the conference,
he realized that, given the
abundance of hydrogen in stars,
the most likely first step in energy
generation was one in which two
hydrogen nuclei—which are single
protons—join to form the nucleus
of a deuterium (heavy hydrogen)
atom. Bethe knew that this reaction
generated energy. He then figured
out how two further reaction steps
could produce a nucleus of helium-4
(the most common form of helium).
He saw the entire sequence of
reactions, called the proton−proton
chain, as the main source of energy
production in stars up to about the
size of the sun.The CNO cycle
Whereas the core temperature
of a star rises slowly as star size
increases, the amount of energy it
produces rises much more rapidly.
The proton−proton chain could not
explain this, so Bethe investigated
reactions involving heavier atomic
nuclei. After hydrogen and helium,
the next heaviest element present
in appreciable amounts in higher
mass stars is carbon, so Bethe
looked at possible reactions of
carbon nuclei with protons. He
found a cycle of reactions, called
the CNO (Carbon–Nitrogen–
Oxygen) cycle, during which
hydrogen nuclei fuse to form
helium in the presence of heavier
elements, which seemed to work.
Bethe’s findings were quickly
accepted by other physicists. ■See also: The theory of relativity 146–53 ■ Nuclear fusion within stars 166–67 ■
The primeval atom 196–97
The proton−proton chain
joins protons to eventually
form helium-4 atoms, releasing
energy as gamma rays.
Proton NeutronGamma rayPositron NeutrinoHans Bethe
Hans Bethe was born in 1906
in Strasbourg, then part of
the German Empire. From a
very early age, he displayed
a high ability in mathematics.
By 1928, he had completed a
doctorate in physics. With the
rise of the Nazi regime, Bethe
emigrated first to Britain
and then to the US. His work
during World War II included
three years at the Los Alamos
Scientific Laboratory, which
was engaged in assembling
the first atomic (fission) bomb.
After the war, Bethe played
an important role in the
development of the hydrogen
(fusion) bomb. He later
campaigned against nuclear
testing and the arms race.
In addition to his work in
astrophysics and nuclear
physics, Bethe made major
contributions to other fields
of physics, including quantum
electrodynamics (QED).
He continued to work in all
these fields until his death
in 2005, at age 98.Key works1936–37 Nuclear Physics
(with Robert Bacher and
Stanley Livingston)
1939 Energy Production
in StarsDeuterium
nucleusHelium-4
nucleusHelium-3
nucleusFusion