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Proton-proton chain
The Proton-Proton Chain is a common set of fusion reactions that is the primary source of energy in stars such as the sun. The reactions transform mass-1 hydrogen isotopes into helium. The proton-proton chain was first proposed in 1926 by Arthur Eddington in his book Internal Constitution of the Stars.

The chain begins with two common reactions, followed by a set of possible branches of reactions. First, two mass-1 hydrogen isotopes combine to form a hydrogen-2 nucleus, while emitting a positive electron and a neutrino.

This is followed by the absorption of another hydrogen-1 nucleus, forming a helium-3 nucleus, and the emission of a gamma ray.

In the main branch, the final reaction involves the fusion of two Helium-3 nuclei formed by the previous two reactions. They form a helium-4 nucleus and eject two protons.