the type of hydrogen fusion process that dominates in a star is determined by the temperature dependency differences between the two reactions. the proton–proton chain reaction starts at temperatures about , making it the dominant fusion mechanism in smaller stars. a self-maintaining cno chain requires a higher temperature of approximately , but thereafter it increases more rapidly in efficiency as the temperature rises, than does the proton-proton reaction. above approximately , the cno cycle becomes the dominant source of energy. this temperature is achieved in the cores of main sequence stars with at least 1.3 times the mass of the sun. the sun itself has a core temperature of about . as a main sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its cno cycle.