In the cores of stars like the Sun, energy is generated in a fusion reaction called the proton-proton chain. In this process, two protons come together so fast that they overcome their mutual electric repulsion and fuse. Almost immediately, one of the protons turns into a neutron via inverse beta decay, releasing a neutrino and a positron (antimatter counterpart of an electon). The result is an isotope of hydrogen called deuterium. From elsewhere, another proton flys in and fuses giving off a gamma ray and forming an isotope of heliom called helium-3. Another helium 3 from the environment flys in and fuses to the other, forming a nucleus with 2 protons and 2 neutrons, and giving off 2 protons to start the reaction again. The resulting helium-4 is the byproduct of the reaction. Created by Jeff Bryant (jeffb@wolfram.com)