In A.D. 1054, a brilliant new star blazed into view into the constellation of Taurus, the Bull. The Chinese astronomers who observed it called it a "guest star," and at its peak brightness they could see it even during the day. Over the following months it gradually faded and disappeared. When we train our telescopes on the location of the former guest star, we see an angry-looking cloud of gas called the Crab Nebula. Several other "guest stars" appear in the historical record, usually separated by intervals of a few hundred years. They are therefore quite uncommon. Two appeared in A.D. 1572 and A.D. 1604, but after then astronomers would have to wait more than 380 years before the next one.
Studies of the Crab Nebula show that it is expanding, as if the enormous cloud of gas had been flung outward from a central point. Modern stellar evolution theory has provided a reason for this: the "guest stars" were the final acts in the lives of a massive star. These stars end their lives as supernovae, massive explosions that blast the stars' outer layers into space. For a short time they can rival the brightness of a small galaxy; later, like a dying coal in a fire, they fade away.