 This difference stems from temperature dependency differences between the two reactions; pp-chain reaction starts at temperatures around  (4 megakelvin), making it the dominant energy source in smaller stars.A self-maintaining CNO chain starts at approximately , but its energy output rises much more rapidly with increasing temperatures so that it becomes the dominant source of energy at approximately .The Sun has a core temperature of around , and only of nuclei produced in the Sun are
born in the CNO cycle.