In the year 185 AD, Chinese astronomers noticed something odd in the sky. They labeled it a "guest star" and it stayed for eight months. In the 1960s, scientists pinpointed the event as a supernova, a star's explosion, but they remained unsure why it had been as big as it was. Now, NASA scientists say that the explosion, a Type Ia event, took place in a low-density "cavity" which allowed it to expand much faster than is normal. The supernova, RCW 86, is about 8,000 light years away. If we were able to see infrared light, it would appear in the sky larger than a full moon. The image above is a composite of data from four NASA space telescopes.