An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ proton; → chain.