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 109 years to complete at the temperature of the Sun's core. Because of the slowness of this reaction the Sun is still shining; if it were faster, the Sun would have exhausted its hydrogen long ago.
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The sequence of
nuclear fusion reactions in a
plasma at around 107K in which a helium nucleus is built up by the successive collisions of hydrogen nuclei consisting of single protons. The proton-proton chain releases energy because the sum of the final masses is 0.7% less than the total original masses, the difference (the mass defect) being converted into energy. It is this exothermic reaction which is believed to power most
main sequence stars.
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