Cygnus Loop supernova remnant, optical image. This is a series of shells of gas cast off by a supernova, the explosive death of a massive star. The loop has an apparent diameter four times the width of the full Moon. The shells of gas impact the interstellar medium at great speeds, compressing it into a plasma and causing the gases to emit light. Light emitted by different ionised gases is colour coded: sulphur is red, hydrogen is green and oxygen is blue. It is thought that the supernova responsible for this remnant occurred between 5-8000 years ago. Its distance is uncertain, but is thought to be between 1400-2600 light years. It lies in the constellation Cygnus.