The supernova that wasn't?

Some 1,800 years ago, Chinese astronomers witnessed the sudden appearance of a brilliant "guest star." Scientists now widely regard this celestial event, carefully noted in an official history at the time, as the oldest supernova ever recorded. Indeed, researchers have suggested several candidates for the glowing stellar remnant cast off during this apparent supernova, the explosive death of a massive star.

But a new analysis of the historical record suggests the ancient Chinese didn't see a supernova explosion after all. Instead, two radioastronomers now argue, the Dec. 7, 185, discovery was in fact a comet.

Yi-Nan Chin of the University of Bonn in Germany and Yi-Long Huang of National Tsing Hua University in Hsinchu, Taiwan, base their assertion on a reinterpretation of a key passage in the Houhanshu, the official history of the Later Han dynasty. Their translation, detailed in the Sept. 29 NATURE, differs significantly from that of several other scientists.

Researchers who previously had studied the passage, written in 185 by astronomers at the imperial observatory of Lo-Yang in central China, interpreted the wording to mean that the guest star remained fixed in the sky and stayed visible for about 19 months. This description is consistent with a supernova. In contrast, a comet moves, and its orbit would cause it to fade from view more rapidly.

But Chin and Huang argue that key words had been translated according to their meaning in modern Mandarin Chinese rather than the original definitions used by ancient astronomers. A more accurate translation, they maintain, reveals that the bright object moved across the sky and lasted for only 7 months.

They note that an additional passage in the text forecasts civil war in China. This kind of astrological prediction is based on a bright body moving across the sky at the same position as the guest star, Chin and Huang note. They add that the supernova remnant RCW86, a favorite candidate for the guest star, looks too old to have appeared in the sky only 1,800 years ago. "It seems clear that the guest star was a comet and not a supernova," the radioastronomers conclude.

Astronomer Bradley E. Schaefer of Yale University finds the new report intriguing but says it's too early to settle the controversy. He maintains that the ancient Chinese discovered the object low in the sky just before sunrise. A comet at the reported position would have had to have been very bright to be seen at all. And a bright comet is either unusually large or passing very near Earth. In the latter scenario, the comet couldn't remain visible for as long as 7 months, Schaefer says.