Nearly 1000 years ago, in May 1006, a new star suddenly appeared, and in a few days became the brightest star ever recorded - in fact it was a supernova, and for about three months was visible even in daylight. It was visible to the naked eye at night for another three years after its dramatic appearance. In all, there have been seven historical supernovae over the last 2000 years that were close enough for everyone to see. You might remember the last one that exploded in 1987 if you were lucky enough to be in the Southern Hemisphere at the time. All were easily seen at night for months, and the brightest few were also visible during the day. However, only a tiny fraction of the supernova energy is radiated as visible light, and most of the energy goes into ejecting the gases in the outer layers of the star. The gas is blown out at up to 15,000 km/s (about 30 million m.p.h.!) and is heated to over one million degrees as it plunges into the gas in the environment, producing a huge amount of X-rays.

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