Supernova remnant. Hubble Space Telescope (HST) image of Cassiopeia A. This is the remains of a star that exploded around 300 years ago. The out- rushing gas is colliding with the stellar medium, which causes a shock wave. This heats the gas to around 50 million degrees Celsius and causes it to emit X-rays. The bright dot at far right may be the neutron star or black hole left after the supernova explosion. The colours seen represent various chemical elements: blue is oxygen and red is sulphur.