The outer layers of these stars are blown away at speeds of many thousands of kilometers an hour. The expelled matter may form nebulae called "supernova remnants". A well-known example of such a nebula is the Crab Nebula, left over from a supernova that was observed in China and North America in 1054. The core of the star or the white dwarf may either become a neutron star (generally a pulsar) or disintegrate completely in the explosion.