Supernova remnant Cassiopeia A, infrared Spitzer telescope image. Cassiopeia A (Cas A) is a remnant of a supernova star that exploded around 320 years ago, the youngest supernova in the Milky Way. The blue area is material that was heated by the explosion at the time and the red, yellow and green areas are material that was ejected by the explosion but heated by a second shockwave. The blue dots are neighbouring stars. Cas A is about 10 light years in diameter and is around 10,000 light years from Earth in the constellation Cassiopeia.