For stars heavier than the sun, theoretical models show that the CNO (carbon-nitrogen-oxygen) cycle of nuclear fusion is the dominant source of energy generation. The cycle results in the fusion of four hydrogen nuclei (¹H, protons) into a single helium nucleus (⁴He, alpha particle), which supplies energy to the star in accordance with Einstein's equation. Ordinary carbon,¹²C, serves as a catalyst in this set of reactions and is regenerated. Only relatively low energy neutrinos ()are produced in this cycle.

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