Supernova remnant. Optical image of NGC 6995, part of the Cygnus Loop, a large supernova remanant. A supernova remnant is a glowing gas and dust cloud formed from the explosion of a large star. The Cygnus Loop star exploded some 20-30,000 years ago and the explosion flung the outer layers of the star against the interstellar medium. The rate of expansion is now around 100 kilometres per second, still enough to cause the gas and dust to heat up and glow. The expanding dust cloud contains the material needed to form planets around new stars. The Cygnus Loop is about 2600 light years distant in the constellation Cygnus.