Isotopic Ratios: The Key to Elemental Abundances and Nuclear Reactions in the Sun

Differences between the abundances of isotopes of elements in the planetary system, in the solar wind, and in solar flares suggest that a) the most abundant nuclear species in the sun is Fe (the ash of violent thermonuclear reactions at equilibrium) rather than H (the best fuel for thermonuclear reactions), and b) nuclear reactions in the outer regions of the sun produce N by the N(H, βν)N reaction. Diffusion in the sun enriches light-weight elements like H and He at the solar surface, where thermonuclear reactions occur, but the most abundant elements in the bulk sun are Fe, Ni, O, Si, S, Mg, and Ca.