Editing Proton–proton chain (section)

==History of the theory==
The theory that proton–proton reactions are the basic principle by which the Sun and other stars burn was advocated by [[Arthur Eddington]] in the 1920s. At the time, the temperature of the Sun was considered to be too low to overcome the [[Coulomb barrier]]. After the development of [[quantum mechanics]], it was discovered that [[quantum tunnelling|tunneling]] of the [[wavefunction]]s of the protons through the repulsive barrier allows for fusion at a lower temperature than the [[classical physics|classical]] prediction.

In 1939, [[Hans Bethe]] attempted to calculate the rates of various reactions in stars. Starting with two protons combining to give a [[deuterium]] nucleus and a [[positron]] he found what we now call Branch II of the proton–proton chain. But he did not consider the reaction of two {{chem|3|He}} nuclei (Branch I) which we now know to be important.<ref>{{cite journal |last1=Hans Bethe |title=Energy Production in Stars |journal=[[Physical Review]] |date=Mar 1, 1939 |volume=55 |issue=5 |pages=434–456 |doi=10.1103/PhysRev.55.434 |bibcode=1939PhRv...55..434B |doi-access=free }}</ref> This was part of the body of work in [[stellar nucleosynthesis]] for which Bethe won the [[Nobel Prize in Physics]] in 1967.