proton-proton chain

Proton-proton chain (or PP chain or P-P chain or PP-chain) is a type of a fusion sequence leading from hydrogen to helium (hydrogen burning). Temperatures necessary to trigger it are lower than that required to trigger the other such fusion sequence, the CNO cycle. A PP chain's beginning is ¹H nuclei (i.e., protons) colliding and fusing. Following that, there are multiple branches of the chain, each consisting of a sequence of additional reactions that lead to the typical helium isotope, ⁴He. The proportion of fusion on each branch of the chain depends upon the temperature of the material. In small main sequence stars, such as red dwarfs, proton-proton chain reactions in the stellar core power the star. In large main-sequence stars, the hotter core triggers the CNO cycle, but the proton-proton chain is also present and extends outward beyond the region hot enough for the CNO cycle.