| definition |
A series of thermonuclear reactions, taking place mainly in low-mass stars, such as
the Sun, which transforms four hydrogen nuclei (protons) into one helium (Helium-4)
nucleus and thereby generates energy in the stellar core. First, two protons (Helium-1)
combine to form a deuterium nucleus (Helium-2) with the emission of a positron and
a neutrino. The deuterium nucleus then rapidly captures another proton to form a
Helium-3 nucleus, while emitting a gamma ray. There are three alternatives for the
next step. In the proton-proton chain, occurring in 86% of the cases, two Helium-3
nuclei fuse to a final Helium-4 nucleus while two protons are released. The mass of
the resulting Helium-4 nucleus is less than the total mass of the four original protons.
The difference, ~ 0.7% of the total mass of the protons, is converted into energy
and radiated by the Sun. In this process, the Sun loses some 4 million tons of its
mass each second.
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