The r -process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r -process therefore must occur in locations where there exists a high density of free neutrons. Early studies theorized that 10 free neutrons per cm would be required, for temperatures of about 1 GK, in order to match the waiting points, at which no more neutrons can be captured, with the mass numbers of the abundance peaks for r -process nuclei. This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova, as part of supernova nucleosynthesis, or decompression of neutron star matter thrown off by a binary neutron star merger in a kilonova. The relative contribution of each of these sources to the astrophysical abundance of r -process elements is a matter of ongoing research as of 2018.