s-Process, r-Process & the Origin of Heavy Elements: Observation, Measurement & Evidence

The s-process operates mainly in evolved stars where moderate neutron sources expose seed nuclei over long timescales. The r-process requires extreme neutron-rich ejecta and produces nuclei far from stability that later decay toward stable heavy isotopes. The two pathways populate different portions of the chart of nuclides.

Element and isotope abundance patterns in stars show characteristic s- and r-process signatures. Observations of kilonovae associated with neutron-star mergers demonstrate production of heavy r-process material, while evolved giant stars show enrichment expected from s-process nucleosynthesis.

A neutron carries no electric charge, so it can enter a nucleus without overcoming the strong Coulomb repulsion faced by charged particles. In the s-process, capture and beta decay alternate along a path near stable isotopes. In the r-process, repeated rapid captures push nuclei to very neutron-rich states before a cascade of decays.

Stellar spectra show s- and r-process abundance signatures, presolar grains preserve isotopic patterns, and the kilonova following GW170817 provided direct evidence that neutron-star mergers synthesize heavy r-process material. Evolved stars display surface s-process enrichment.