The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller. Overcoming electrostatic repulsion between two hydrogen nuclei requires a large amount of energy, and this reaction takes an average of more than [citation needed] years to complete at the temperature of the Sun's core. Because of the slow nature of this reaction the Sun is still shining; if it were faster, the Sun would have exhausted its hydrogen long ago.