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 4 × 10 K, making it the dominant fusion mechanism in smaller stars. A self-maintaining CNO chain requires a higher temperature of approximately 1.6 × 10 K, but thereafter it increases more rapidly in efficiency as the temperature rises, than does the proton–proton reaction. Above approximately 1.7 × 10 K, 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 1.57 × 10 K. As a main-sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle.