What dictates the primary energy route between PP chain and CNO cycle?
The star's core temperature and density, determined by mass.
The specific nuclear pathway a main sequence star uses to convert hydrogen into helium is fundamentally dictated by the mass of that star, which in turn sets the required core temperature and density. Stars classified on the lower main sequence, specifically those less massive than about 1.5 times the mass of the Sun ($ ext{M}_{ ext{odot}}$), possess cores that are not hot enough to efficiently run the CNO cycle. Therefore, they rely primarily on the simpler, multi-step proton-proton (PP) chain reaction. Conversely, more massive stars exceed a critical temperature threshold in their cores, enabling them to successfully employ the CNO cycle. This process is much more effective at higher temperatures because it utilizes carbon, nitrogen, and oxygen as catalysts to facilitate the hydrogen-to-helium conversion much more rapidly than the PP chain.
