Supernova remnant Cassiopeia A, X-ray image. Cassiopeia A (Cas A) is a remnant of a supernova star that exploded around 320 years ago, the youngest in the Milky Way galaxy. The shells of gas are constantly expanding, and within them electrons are accelerated to extremely high speeds (blue wispy arcs). The red and green areas are material from the star that has been heated to millions of degrees, causing it to emit radiation. Cas A is about 10 light years in diameter and is found around 10,000 light years from Earth in the constellation Cassiopeia. Image taken by NASA's Chandra X-ray observatory in 2004.