In theoretical models of the sun, the p—p chain of nuclear reactions illustrated here is the dominant source of energy production. Each reaction is labeled by a number in the upper left hand corner of the box in which it is contained. In reaction 1, two hydrogen nuclei (¹H, protons) are fused to produce a heavy hydrogen nucleus (²H, a deuteron). This is the usual way nuclear burning gets started in the sun. On rare occasions, the process is started by reaction 2. Deuterons produced in reactions 1 and 2 fuse with protons to produce a light element of helium (³He). At this point, the p—p chain breaks into three branches, whose relative frequencies are indicated in the figure. The net result of this chain is the fusion of four protons into a single ordinary helium nucleus (⁴He) with energy being released to the star in accordance with Einstein's equation. Particles called 'neutrinos' () are emitted in these fusion processes. Their energies are shown in the figure in units of millions of electron volts (MeV). Reactions 2 and 4 were not discussed by Hans Bethe.

The figure is adapted from J.N. Bahcall, Neutrinos from the Sun, Scientific American, Volume 221, Number 1, July 1969, pp. 28-37.