What main characteristic of a star's central temperature dictates whether it primarily uses the proton-proton chain or the CNO cycle to burn hydrogen into helium?

The main characteristic of a star's central temperature that dictates whether it primarily uses the proton-proton chain or the CNO cycle to burn hydrogen into helium is the *absolute value of that temperature*, which directly determines the kinetic energy available to overcome the specific Coulomb barriers associated with each fusion pathway. The proton-proton (PP) chain is a nuclear fusion process where four protons (hydrogen nuclei) combine to form one helium nucleus, releasing energy. This process is dominant in stars with lower central temperatures, typically below about 15 million Kelvin. The fundamental step in the PP chain involves two protons fusing. Since protons each carry a single positive charge, the electrostatic repuls....