The Crab Nebula (previous page) is a supernova in our galaxy, the remnants of a particularly violent stellar explosion that occurred in the summer of 1054 A.D. Western Europe was too primitive to notice such things then, although it is surprising that Arabian astronomers missed it. But the Japanese and the royal astronomers of the Sung Dynasty in China took note of it. Three such violent supernovae have been observed in our own galaxy in recorded history, in 1054, 1572, and 1604.

But a search for supernovae in other galaxies conducted at the Palomar Observatory for many years suggests that a typical galaxy might produce a supernova every thirty years. The Crab Nebula is a hotbed of turmoil, emitting radio noise and X-rays as well as light, and having a pulsar at its core. The theoretical astronomer has no trouble explaining why stars become supernovae; rather the problem is to explain why such catastrophes are not seen more often.