The s Process and Beyond

Neutron captures produce the vast majority of abundances of elements heavier than iron in the Universe. Beyond the classical slow (s) and rapid ® processes, there is observational evidence for neutron-capture processes that operate at neutron densities in between, at different distances from the valley of $\beta$ stability. Here, we review the main properties of the s process within the general context of neutron-capture processes and the nuclear physics input required to investigate it. We describe massive stars and asymptotic giant branch stars as the s-process astrophysical sites and discuss the related physical uncertainties. We also present current observational evidence for the s process and beyond, which ranges from stellar spectroscopic observations to laboratory analysis of meteorites.