Supernova remnant Cassiopeia A. Chandra X-ray Observatory image of shells of gas expanding from the supernova remnant Cassiopeia A (Cas A). This is the remains of a star which exploded around 300 years ago. The outrushing 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 study of the spectra of these X-rays will allow the composition of the old star to be determined. The bright dot at the centre of the gaseous shells may be the neutron star or black hole left after the supernova explosion.