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.
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.