What happens when neutron stars collide?

When stars collapse, they can leave behind incredibly dense but relatively small and cold remnants called neutron stars. If two stars collapse in close proximity, the leftover binary neutron stars spiral in and eventually collide, and the interface where the two stars begin merging becomes incredibly hot. New simulations of these events show hot neutrinos — tiny, essentially massless particles that rarely interact with other matter — that are created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. During this time, the simulations show that the neutrinos can weakly interact with the matter of the stars, helping to drive the particles back toward equilibrium — and lending new insight into the physics of these powerful events.