The r-process (rapid neutron-capture process) is a nucleosynthesis process consisting of repeated neutron capture under conditions where beta decay is suppressed (or a nucleus generally captures another neutron before beta decay occurs), resulting in synthesis of isotopes with an increase in mass number more than one but remaining at the same atomic number. When conditions revert to allowing beta decay, such decays happen until the nucleus reaches a stable configuration. The r-process explains the abundances of germanium, xenon, and platinum and many elements heavier than iron and nickel.
The process is presumed to happen in core collapse supernovae due to models but observational evidence has only been indirect via the models. The first observational evidence of spectra showing the characteristic radioactivity (beta decay from heavy elements) is from GW detection GW170817, a neutron star merger. These are termed kilonovae, which had been presumed to be another place where the process occurs.
Neutron capture requires free neutrons, which have a limited lifetime (a mean lifetime of about 880 seconds) and in events that produce such free neutrons, the ratio of neutrons to all nucleons at each point in space and time is of interest, typically cited as the electron fraction (Ye), the fraction of nucleons that are protons (because wherever there are protons, there must be a similar number of electrons in the general region, and vice versa, or else the region has a strong electric charge).
The spectral-line signatures of r-process-created nuclei can be used to identify and age old stars. Very old (high redshift?) galaxies have been observed with r-process-produced elements but not s-process-produced elements, suggesting that at an early time, the r-process was working but the s-process was not. Individual stars with similar characteristics are presumed to be of similar age.