Stellar nucleosynthesis encompasses nuclear production during stellar evolution and explosions, including hydrogen burning, core helium burning, advanced burning and neutron-capture processes. Stellar mass, composition, mixing and mass loss govern which nuclei form and which eventually escape. Slow neutron-capture process and rapid neutron-capture process build many nuclei beyond the iron group.
Two alpha captures and an intervening beta-plus decay turn each nitrogen-14 nucleus into neon-22, incorporating eight baryons from helium and converting one proton to a neutron. With no loss or dilution and nuclear mass defects neglected, a nitrogen-14 mass fraction therefore yields neon-22 mass fraction , not . In otherwise symmetric carbon-oxygen matter this gives the displayed electron fraction in stellar matter. A final neon mass fraction and a pre-helium-burning metallicity must not silently be treated as identical.
Hot explosive material can use photodisintegration on pre-existing heavy seed nuclei through neutron, proton and alpha emission. This gamma process supplies some proton-rich isotopes that the principal neutron-capture paths bypass. It is a particular mechanism, rather than a synonym for every proposed proton-rich nucleosynthetic process.
At sufficiently large neutron density, successive neutron captures outrun relevant beta decays and produce very neutron-rich nuclei. Decay toward stability afterward yields many heavy isotopes, including some actinides. Neutron-rich ejecta from neutron star mergers provide an established site; particular magnetically driven collapse outflows are additional candidates. This does not imply that every ordinary core-collapse supernova produces a complete heavy-element pattern.
