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.