"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.
In general, proton–proton fusion can occur only if the temperature (i.e. kinetic energy) of the protons is high enough to overcome their mutual electrostatic or Coulomb repulsion.
In the Sun, deuterium-producing events are so rare (diprotons, the much more common result of nuclear reactions within the star, immediately decay back into two protons) that a complete conversion of the star's hydrogen would take more than 1010 (ten billion) years at the prevailing conditions of its core. The fact that the Sun is still shining is due to the slow nature of this reaction; if it went more quickly, the Sun would have exhausted its hydrogen long ago." [Proton–proton chain reaction. Wikipedia]
The nuclear reaction diagram example "Proton-proton chain reaction" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Physics solution from the Science and Education area of ConceptDraw Solution Park.