INTERLUDE 21-2 Nearby Supernovae
Only six Galactic supernovae have been recorded in the past 1000 years. The accompanying figure shows their positions in the Milky Way. They are labeled by the year in which they first appeared. The supernova Cassiopeia A (Cas A) apparently went unnoticed optically, although modern radio studies suggest that the first light from the explosion should have reached Earth midway through the seventeenth century. The combined radio—optical—X-ray image is dramatic proof that this supernova remnant, although invisible optically, makes quite an impact at other wavelengths. (Blue is radio, red is optical, and green is X-ray.)

Most astronomers assume that many more stars than these six have blown up in our galaxy. Why haven't we seen them? Possibly because they were too distant to be detected by the naked eye, or most likely because dark dust clouds in the Galactic plane kept them from our view. Each of the Milky Way supernovae mapped in the first figure is in our "neighborhood"—that is, in our quadrant of the Galaxy—and each is at least 100 pc above or

below the Galactic plane. Studies of the rate at which supernovae occur suggest that we can expect one within 100 pc of our Sun only every 500,000 years. Thus, a truly "close" supernova would be a rare event indeed. Humanity may be destined to see all supernovae from a distance.

Despite their rarity, nearby supernovae may conceivably play an important role in determining the development of life on Earth. A supernova at a distance of a few parsecs would bombard our planet with high-energy radiation for a period of several months, possibly causing substantial long-term atmospheric changes, particularly in the ozone layer. Some scientists have even gone so far as to suggest that such an event might have been responsible for episodes of mass extinction that are known to have occurred hundreds of millions of years ago and in which, according to the fossil record, over 95 percent of all life on our planet vanished in a very short period of time. (Recall, though, that this is not necessarily the only "astronomical" explanation for mass extinctions on our planet—see Interlude 14-1.)