The sun! In stars like the sun the principal reaction is called the proton-proton cycle that was first described by the German-American physicist Hans Bethe (1906-2005) in 1939. In the first step two protons fuse into a deuterium (²H) nucleus. The deuterium has a proton and a neutron, so the second proton changed into a neutron, releasing a positron and a neutrino. The positron annihilates with an electron, producing two gamma rays. In the second stage the deuterium fuses with another proton to produce helium-3 (³He) plus a gamma ray. In the sun the third stage is primarily a fusion between two ³He nuclei producing ⁴He plus two protons. So the net reaction is an input of four protons and an output of two ⁴He plus six gammas and two neutrinos, as well as a lot of energy due to the loss of 0.7% of the mass of the protons. In order to accomplish these reactions the protons must be moving with a large amount of kinetic energy, the equivalent of a temperature of about 10 million kelvins.