Helium-3 (${}^3\mathrm{He}$)

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.