The most common form of helium, helium 4, contains two protons and two neutrons in its nucleus. Helium 3 differs from this in having two protons but only one neutron. Both helium 3 and helium 4 are produced in the Sun as a consequence of nuclear fusion processes. The Sun produces energy by fusing hydrogen to helium. This may be accomplished in a number of ways but in the Sun, a process known as the proton-proton chain is thought to be primarily responsible for energy generation.
Different numbers indicate different isotopes.
Helium 3 is produced by fusing hydrogen with deuterium but is destroyed by fusion with either helium 3 or helium 4. Because helium 3 reactions are very temperature sensitive, they occur only in the hottest parts of the solar core. Therefore, after a period of time, the helium 3 is depleted in the core. However, further away from the core helium 3 is produced but not burned, leading to an overabundance compared to initial values. A steep gradient in chemical composition develops and eventually an instability sets in.