s-process · r-process Nucleosynthesis · Stellar Element Factories
Chart of Nuclides (Z=26–92)
Stable
β⁻ unstable
s-path
r-path
Current Nucleus
Fe-56
Iron-56 — Z=26, N=30
Stable
Most tightly bound nucleus per nucleon. Natural endpoint of stellar fusion.
About half of all elements heavier than iron are forged by the s-process (slow neutron capture) in AGB stars; the other half by the r-process. The s-process adds neutrons one at a time — slower than beta decay — threading along the valley of stability. The r-process requires extreme neutron fluxes (neutron stars, neutron star mergers — confirmed by GW170817) and captures dozens of neutrons before decay, creating very neutron-rich nuclei far from stability. Both paths terminate at lead-208, bismuth-209, and beyond at the actinides. Gold, platinum, and uranium are r-process products: born in neutron star collisions.