Neutron star merger

A neutron star merger is the stellar collision of neutron stars. When two neutron stars fall into mutual orbit, they gradually spiral inward due to the loss of energy emitted as gravitational radiation.^[1] When they finally meet, their merger leads to the formation of either a more massive neutron star, or—if the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit—a black hole. The merger can create a magnetic field that is trillions of times stronger than that of Earth in a matter of one or two milliseconds.^[2] These events are believed to create short gamma-ray bursts.^[3]

The merger of neutron stars momentarily creates an environment of such extreme neutron flux that the r-process can occur. This reaction accounts for the nucleosynthesis of around half of the isotopes in elements heavier than iron.^[4]

The mergers also produce kilonovae,^[5] which are transient sources of isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process.^[6] Kilonovae had been discussed as a possible r-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after multi-messenger event GW170817 was observed in 2017.