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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 an Astrophysicist

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