Hydrogen burning

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.