3C58 supernova remnant. Coloured X-ray image of astronomical body 3C58, the remnant of a supernova explosion. The supernova occurred in the year 1181 and was recorded by astronomers in China and Japan. 3C58 consists of a pulsar (a fast- spinning, superdense body) and a nebula (gas/dust cloud). The pulsar was originally thought to be composed of neutrons, but recent observations suggest that it is cooler than expected for a neutron star and that it may be composed of denser exotic matter. 3C58 is about 10,000 light years away. From Earth it appears in the constellation of Cassiopeia. Photographed by NASA's orbiting Chandra X-Ray Observatory.