Cassiopeia A (Cas A for short) is the debris field left behind after a massive star exploded.This explosion would have appeared in Earth's sky over 300 years ago.Scientists have combined data from Chandra, NASA's Spitzer Space Telescope, and ground-based facilities to construct a unique 3D model of the 300-year old remains of a stellar explosion that blew a massive star apart, sending the stellar debris rushing into space at millions of miles per hour.