Hydrogen nuclear fusion in stars, illustration. The fusion process shown here (known as the proton-proton chain reaction) is the one that occurs in Sun-like stars that are fusing hydrogen and helium. At top, two protons (hydrogen nuclei, red spheres) fuse to form a nucleus of deuterium (an isotope of hydrogen, also known as hydrogen-2), as well as a neutrino (blue) and a positron (anti-electron, white). The deuterium nucleus consists of a proton (red) and a neutron (grey). The second fusion reaction (middle) is a proton and a deuterium nucleus fusing to produce a helium-3 nucleus and gamma radiation (blue wave symbol). The third fusion reaction (bottom) is two helium-3 nuclei fusing to form a helium-4 nucleus and two protons. Most of the fusion reactions in such stars are of hydrogen, with helium fusion typically occurring once the hydrogen in the star's core has been used up.