A mystery surrounding the first recorded supernova - seen by Chinese astronomers in AD185 - has been solved. The supernova RCW 86 lit up the sky for eight months, documented at the time as a "guest star". In more recent times, astronomers have wondered how it grew so large, so fast. Space telescope observations now suggest that before exploding, a wind of material from the star blew a cavity around it, into which the supernova could expand much more quickly. The supernova, about 8,000 light-years away, is huge - if the infrared light it emits could be seen by our eyes, it would appear to be as large in the sky as the full Moon.