Supernova remnant Cassiopeia A. Coloured X-ray image showing iron 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 distribution of iron suggests that the star's atmosphere was overturned before the explosion. Iron and nickel are the heaviest elements produced in giant stars (more than 3 solar masses) before they finally explode. This image was made with the Chandra X-ray Observatory in Earth orbit.