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The Question
(Submitted September 12, 1997)
How do stars create their energy?
The Answer
Stars create their energy through the process of nuclear fusion.
Fusion is the process in which light atoms combine to form heavier
atoms, giving off excess energy in the process. There are a number of
different ways in fusion might take place in stars, depending
on the temperature in the core of the star. Each of them is a
multistep process in which other elements may be used or momentarily
created. The simplest is the proton-proton chain, which occurs in
stars having core temperatures less than 15 x 10^6 K. The proton-
proton chain uses two steps to convert Hydrogen first into 3He, and
then combines two 3He nuclei into He, and giving back 2 H. In the
process, neutrinos and gamma-rays are emitted.
Other nuclear processes which occur in stars with higher core
temperatures are the Carbon cycle and the Carbon-Nitrogen cycle.
These cycles use Carbon or Carbon and Nitrogen to mediate the
conversion of H into He. At even higher temperatures ( ~ 10^8 K),
Helium fuses into Carbon via the "triple alpha" process.
Energy is given off because, e.g. the sum of the mass of 4 H nuclei
is more than the mass of a He nucleus. The "excess" mass is converted
into energy as part of the process.
Textbooks in astronomy or modern physics will have further details
about these processes.
Jim Lochner
for Ask a High-Energy Astronomer
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