S-process

The slow neutron-capture process builds many elements heavier than iron inside stars. Neutron captures usually occur slowly enough for unstable nuclei to undergo beta decay before capturing another neutron.

Neutron capture: A nuclear reaction in which an atomic nucleus absorbs a neutron. Each capture adds a neutron to a nucleus and advances the s-process.
Stellar nucleosynthesis: The production of new atomic nuclei through reactions inside stars. The s-process is one pathway by which stars synthesize elements heavier than iron.
Solar-system abundances of the elements: The measured relative amounts of chemical elements in the Solar System. Their isotope patterns reveal how much material came from slow neutron capture.
R-process: A nucleosynthesis process in which nuclei capture neutrons rapidly compared with their radioactive decay. Its rapid captures produce neutron-rich nuclei beyond the s-process path.
Beta decay: Radioactive decay in which a neutron converts into a proton, or a proton into a neutron, with emission of other particles. Between captures, beta decay raises a nucleus’s proton number and moves it toward a stable isotope.
Asymptotic giant branch star: A late evolutionary stage of a low- or intermediate-mass star, marked by alternating hydrogen- and helium-burning shells. Many s-process elements form in these stars and are later dispersed into space.
Presolar grain: A microscopic mineral grain that formed around a star before the Solar System existed. Some grains preserve isotope signatures of s-process nucleosynthesis in their parent stars.
P-process: A set of processes that produces rare, proton-rich isotopes not made efficiently by neutron capture. It creates proton-rich nuclei that neither the s-process nor the r-process readily supplies.
Chart of nuclides: A map of atomic nuclei arranged by their numbers of protons and neutrons. The s-process follows a path near stable nuclei across this map.
S-process in massive stars: Slow neutron capture during helium and carbon burning in massive stars. This weak component mainly produces lighter s-process nuclei before the stars explode.

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