- The remains left over by a supernova explosion. These burnt out stars sound dull but are in fact among the most exciting objects in the sky. A supernova sheds huge amounts of its mass in the course of exploding, leaving behind a core which collapses to a neutron star, which may can be visible as a pulsar, if it is of less than about three solar masses. Beyond that the core is subject to complete gravitational collapse and becomes a black hole. The best-known such remnant is the Crab nebula in Taurus, the remains of a supernova observed in 1054AD.