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THE SOURCE OF
ENERGY IN STARS
IS NUCLEAR FUSION
ENERGY GENERATION
IN CONTEXT
KEY ASTRONOMER
Hans Bethe (1906–2005)BEFORE
1919 Francis Aston discovers
that four hydrogen nuclei
(protons) have more mass
than a helium nucleus.1929 Welsh astronomer Robert
Atkinson and Dutch physicist
Fritz Houtermans calculate
how the fusion of light nuclei
within stars could release
energy in accordance with
mass−energy equivalence.AFTER
1946 Ralph Alpher and
George Gamow describe
how nuclei of helium and
some other nuclei could have
been synthesized during
the Big Bang.1951 Ernst Öpik describes
the triple-alpha process,
which converts the nuclei
of helium-4 into carbon-12
in the cores of red giant stars.Low- to medium-mass
stars are fueled by the proton−proton
chain, turning hydrogen into helium.High-mass stars
are fueled by the CNO
cycle, which turns hydrogen
into helium in the presence
of carbon and nitrogen
as catalysts.The fusion of hydrogen
nuclei to form helium
turns mass into energy.U
ntil a brilliant young
German-born physicist
named Hans Bethe
figured it out in 1938, no one
knew for sure why the sun and
other stars emitted so much
light, heat, and other radiation,
or where their energy came from.
A step toward the correct
answer had been made in 1905
with Albert Einstein’s special
theory of relativity, which proposed
that mass and energy have an
equivalence. The significanceThe source of energy
in stars is nuclear fusion.of this discovery was that a small
loss of mass could be accompanied
by a very large release of energy.
In 1919, British chemist Francis
Aston found that an atom of helium
(the second lightest element) had a
mass slightly less than that of four
atoms of hydrogen (the lightest
element). Soon afterward, British
astrophysicist Arthur Eddington
and French physicist Jean Baptiste
Perrin independently proposed that
stars might obtain their energy by
combining four hydrogen nuclei to