Supernova remnant Cassiopeia A, Hubble Space Telescope image. Cassiopeia A is an expanding ring of glowing gas and dust formed by the massive explosion of a star (a supernova) 340 years ago. Supernova remnants are a vital source of chemical elements heavier than oxygen, forming the basis for new stars and planets. The remnants are still expanding at speeds of many millions of kilometres per hour. The colours here correspond to oxygen (green), hydrogen and nitrogen (blue), and sulphur (red and purple). Cassiopeia A is 10,000 light years from Earth in the constellation of Cassiopeia. This composite was made from 18 separate images obtained in December 2004 by Hubble's Advanced Camera for Surveys sensor.