Proton-proton chain (or PP chain or P-P chain or PP-chain) is the name of a fusion sequence leading from hydrogen to helium. Temperatures necessary to trigger it are lower than that required to trigger the CNO cycle. It begins with 1H atoms colliding and fusing, then through subsequent reactions, eventually producing the typical helium isotope, 4He. The latter part of the cycle varies, i.e., there are different branches, paths to the helium, and the proportion of fusion on each branch depends upon the temperature triggering the reaction.
In small main sequence stars, such as red dwarves, proton-proton chain reactions in the stellar core power the star. In larger 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.