Note: The text on this page has not been updated since it was written in 1996.
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.
Energy Generation in the Sun
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.
H is hydrogen, D is deuterium (heavy hydrogen), He is helium, Li is lithium, Be is beryllium, and B is boron. 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. β+ represents a positron, ν a neutrino and γ a gamma ray. The Homestake experiment detects only the highest energy neutrinos produced by the Sun, the neutrinos produced by the beryllium/boron reactions.
Helium 3 Instability
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. The result is mixing in the core which means fresh fuel is brought into the energy-producing regions. It is unclear whether this mixing occurs at discrete intervals or whether continuous slow mixing is maintained.
When fresh fuel is brought into the core, the total energy generation rate increases at first. However, because the Sun is thermally stable, this increase is offset of an expansion of the core, thus reducing the central temperature and pressure. To maintain hydrostatic equilibrium, the solar envelope also expands and cools, reducing both solar luminosity and effective temperature. Because of lower core temperatures, the energy generation rate also decreases. This has important consequences for neutrino fluxes and may provide an explanation for the solar neutrino problem.
Fusion still occurs, albeit more slowly, in the solar core, and the helium 3 is gradually destroyed. This reduces the temperature sensitivity in the core and mixing eventually ceases. At this point the Sun goes back to its normal main sequence evolution. However, because of the earlier mixing, the Sun is less chemically inhomogeneous and thus appears “younger” than it actually is. Thus, the core values, surface luminosity, and effective temperature are still somewhat lower than before the mixing occurred. As the Sun ages, the chemical composition gradients are re-established. Eventually the criteria for instability are again satisfied and the process of mixing begins again.
Alternatively, it has been suggested that instead of discrete mixing episodes occurring, continuous slow mixing is set up. In this case, when the composition gradient of helium 3 becomes steep enough, an instability occurs but the mixing is very slow. Thus, the gradient is maintained, not destroyed, and the mixing is continuous.
Ice ages occur periodically on Earth and it has been suggested that reductions in the solar luminosity or some other solar variation may be partially responsible. If this is so, then a helium 3 instability which causes mixing in the solar core at discrete intervals could be the cause of periodic ice ages on Earth. The time between theoretical core-mixing episodes in the Sun caused by the helium 3 instability and the intervals between major ice ages on the Earth are roughly similar.