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Neutron star merger
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<p>A neutron star merger is the <a href="/facts/Stellar_collision/cLI3rvKm">stellar collision</a> of <a href="/facts/Neutron_star/4enfoxBA">neutron stars</a>. When two neutron stars fall into mutual orbit, they gradually <a href="/facts/Inspiral/c9rj3aK6">spiral inward</a> due to the loss of energy emitted as <a href="/facts/Gravitational_radiation/4XWN3NtG">gravitational radiation</a>. 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 <a href="/facts/Tolman%25E2%2580%2593Oppenheimer%25E2%2580%2593Volkoff_limit/msQN8GkI">Tolman–Oppenheimer–Volkoff limit</a>—a <a href="/facts/Black_hole/1r2lmkcF">black hole</a>. 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. The immediate event creates a <a href="/facts/Gamma-ray_burst/oRRU6Fhn">short gamma-ray burst</a> visible over hundreds of millions, or even billions of <a href="/facts/Light_year/IGc5NdQb">light years</a>. </p><p>The merger of neutron stars momentarily creates an environment of such extreme neutron flux that the <a href="/facts/R-process/wEDL01yc"><i>r</i>-process</a> can occur. This reaction accounts for the <a href="/facts/Nucleosynthesis/F9MIeADF">nucleosynthesis</a> of around half of the isotopes in elements heavier than iron. </p><p>The mergers also produce <a href="/facts/Kilonovae/okpV1akI">kilonovae</a>, which are transient sources of <a href="/facts/Isotropic/mc3QEY2x">isotropic</a> longer wave electromagnetic radiation due to the <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a> of heavy <i>r</i>-process nuclei that are produced and ejected during the merger process. Kilonovae had been discussed as a possible <i>r</i>-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after <a href="/facts/Multi-messenger_astronomy/7pvCx4PL">multi-messenger</a> event <a href="/facts/GW170817/q78LSs5w">GW170817</a> was observed in 2017. </p>