Neutron stars merging. Supercomputer simulation of a pair of neutron stars merging. Colours represent strength of magnetic field from blue (weakest), red, orange and yellow (strongest). Neutron stars are the remnants of stars that have run out of fuel and exploded as supernova, eventually collapsing into a superdense core. A typical neutron star has a mass of between 1.3 and 2.1 solar masses but measures only 10km in diameter. Neutron stars can exist as paired, or binary, stars. This simulation shows that the merger of two neutron stars causes a violent burst of magnetic radiation lasting milliseconds. It is suggested that this is the source of short gamma ray bursts.