The Veil nebula, also known as the Cygnus Loop, is an enormous region of diffuse gas emission, covering several degrees on the sky. Although this image is over a degree across (more than 40 light years), using the full wide-field capability of the Schmidt telescope, it still shows only the western segment (NGC6960) of the entire object (over 100 light years in width). The nebula is the remnant of a supernova explosion which occurred more than 20000 years ago. It consists mostly of interstellar matter swept up by the material flung off by the exploding star, and it still shines because of excitation due to the collision between this expanding shock wave and the interstellar medium. The Veil nebula also emits X-rays, although they are weaker than those from younger supernova remnants such as Cassiopeia A, since the shock loses energy as it plows through its surroundings. Supernova explosions are perhaps the most spectacular events in our Galaxy, occurring when a star throws off its outer layers at speeds of ten to twenty thousand kilometers per second, leaving behind sometimes nothing, sometimes a shriveled remnant neutron star, or sometimes even a totally collapsed black hole.