X-ray image of the supernova remnant Cassiopeia A (3C 461), taken with the Einstein Observatory X- ray satellite. Cassiopeia A is the remnant of a catastrophic stellar explosion, or supernova, which occured approximately 300 years ago. The X- ray emission comes from a spherically expanding cloud of gas expelled by the explosion. As the gas travels outwards, it collides with the surrounding interstellar medium, forming a shock front. This heats the expanding gas to temperatures of 10 to 50 million degrees, causing it to emit X-ray radiation.