in the cores of lower-mass main-sequence stars such as the sun, the dominant energy production process is the proton–proton chain reaction. this creates a helium-4 nucleus through a sequence of chain reactions that begin with the fusion of two protons to form a deuterium nucleus (one proton plus one neutron) along with an ejected positron and neutrino. in each complete fusion cycle, the proton–proton chain reaction releases about 26.2 mev. the proton–proton chain reaction cycle is relatively insensitive to temperature; a 10% rise of temperature would increase energy production by this method by 46%, hence, this hydrogen fusion process can occur in up to a third of the star's radius and occupy half the star's mass. for stars above 35% of the sun's mass, the energy flux toward the surface is sufficiently low and energy transfer from the core region remains by radiative heat transfer, rather than by convective heat transfer. as a result, there is little mixing of fresh hydrogen into the core or fusion products outward.