zanjire-ye proton-proton (#)
Fr.: chaîne proton-proton
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 (4He) nucleus and thereby generates energy in the stellar core. First, two protons (1H) combine to form a → deuterium nucleus (2H) with the emission of a → positron (e+) and a → neutrino (ν): 1H + 1H → 2H + e+ + ν. The deuterium nucleus then rapidly captures another proton to form a helium-3 nucleus (3He), while emitting a → gamma ray (γ): 2H + 1H → 3He + γ. There are three alternatives for the next step. In the PP I chain, occurring in 86% of the cases, two 3He nuclei fuse to a final 4He nucleus while two protons are released: 3He + 3He → 4He + 1H + 1H. The mass of the resulting 4He nucleus is less than the total mass of the four original protons used to produce 4He (→ mass defect). 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. See also → CNO cycle.