Cassiopeia A, also called Cas A, is a supernova remnant located in the constellation Cassiopeia. The supernova that originated this remnant was within the Milky Way, at a distance of approximately eleven thousand light years. Today, Cassiopeia A is the brightest astronomical radio source outside the Solar System.
According to current data, the expanding cloud of the remaining material of this supernova is ten light years from one end to the other. In spite of its luminosity in radio it is barely visible, and it can only be observed in long exposure photographs.
The light of the stellar explosion is estimated to have reached Earth for the first time about 300 years ago, although there are no historical data on the observation of the parent star. It is believed that this was because interstellar dust absorbed visible radiation before it reached our planet.
Among the possible explanations, there is the idea that the original star was unusually massive, and that it could have expelled many of its outer layers. These layers could have covered up the star and reabsorbed much of its light released when the inside of the star collapsed.
The expanding layers of Cassiopeia A reach a temperature of almost 30 million degrees Celsius, and travels at more than 16 million kilometers per hour. In 1999, the Chandra X-ray Observatory, an artificial satellite launched by NASA, found a hot point source near the center of the nebula, which could be a neutron star or a black hole.