Supernova remnant Cassiopeia A. Coloured X-ray image showing silicon ion concentration in the supernova remnant Cassiopeia A (Cas A). Colours represent the intensity of X-ray emission, from green (lowest) through magenta to yellow (highest). Cas A is the remains of a star which exploded as a supernova around 300 years ago. The shape of the silicon ion map suggests that the explosion was asymmetric. Silicon is produced in giant stars (more than 3 solar masses) close to the end of their lives. This image was made with the Chandra X-ray Observatory in Earth orbit.