 Helium-3

This stable helium isotope contains two protons and one neutron. In solar proton-proton burning it is an intermediate: two such nuclei can form helium-4 and return two protons, or one can capture helium-4 to enter the second chain branch. The balance between production and destruction determines the helium-3 equilibrium abundance and its helium-3 relaxation time.

Helium-3 (He-3) is a light, stable isotope of helium with two protons and one neutron in its nucleus, giving it an atomic mass of approximately three atomic mass units. It is a rare isotope compared to the more common helium-4 (He-4), which has two protons and two neutrons.