Dear PF Forum, I'm just wondering about P+P reaction in star. Because of its pressure, in the core of the star, hydrogens fuse to become deuterium. Its the complete reaction P + P -> D D + P -> He3 He3 + He3 -> He3 + P + P and if the star is big enough, it can undergo CNO cycle, But what if the star is smaller. Can it only able to process P + P -> D and it can't continue to D + P -> He, etc...? What if the star is somewhat bigger. Can it continue to Carbon and then stop. It can't produce nitrogen, or the once initiated CNO cycle automatically completed?