These wisps of interstellar gas in the constellation Cassiopeia may look like gentle clouds, but they are actually debris from an astron- omical event of unimaginable violence: a cloud of gas hurled outward by a star in its death throes, or a supernova. A close look at this image reveals knots or thickenings of matter. These provide clues to the birth of Cas A. Astronomers have demonstrated that some of the knots are moving apart at speeds up to 6,000 kilometers per second, although they appear to slow as they encoun- ter resistance from interstellar gas and dust. Reasoning backward from the present speed of the knots, astronomers have determined that in 1658 all the nebula’s material would have appeared to be in the