This is Cassiopeia A. Astronomers call it Cas A for short. It’s the youngest supernova remnant in the Milky Way and brightest extrasolar source of radio waves in our sky. This image – taken in X-rays by NASA’s CHANDRA X-ray Observatory – shows the debris field left behind after a massive star exploded. The explosion should have appeared in Earth’s sky over 300 years ago, but there are no historical records of any sightings of the supernova. One theory is that interstellar dust absorbed the visible radiation before it reached Earth.