Supernova remnant Cassiopeia A. Coloured X-ray image of shells of gas expanding from the supernova remnant Cassiopeia A (Cas A). Colours represent the intensity of X-ray emission, from green (lowest) to magenta (highest). Cas A is the remains of a star which exploded around 300 years ago. The outrushing gas is colliding with the inter-stellar medium, heating it to around 50 million degrees Celsius and causing it to emit X- rays. This image was made with the Chandra X-ray Observatory in Earth orbit.