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. The theory that proton–proton reactions were the basic principle by which the Sun and other stars burn was advocated by Arthur Stanley Eddington in the 1920s. At the time, the temperature of the Sun was considered too low to overcome the Coulomb barrier. After the development of quantum mechanics, it was discovered that tunneling of the wavefunctions of the protons through the repulsive barrier allows for fusion at a lower temperature than the classical prediction.
Even so, it was unclear how proton-proton fusion might proceed, because the most obvious product, helium-2, is unstable and immediately dissociates back into a pair of protons. In 1939, Hans Bethe proposed that one of the protons could beta decay into a neutron via the weak interaction during the brief moment of fusion, making deuterium the initial product in the chain. This idea was part of the body of work in stellar nucleosynthesis for which Bethe won the Nobel Prize.
In the Sun, deuterium-producing events are rare enough that a complete conversion of its hydrogen would take more than 1010
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 faster, the Sun would have exhausted its hydrogen long ago.
The pp chain reaction
The first step involves the fusion of two hydrogen nuclei 1
H (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron.
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