A record-breaking star with flare.

The binary star V711 Tau, about 100 light-years from Earth, belongs to a class of binaries that have relatively high magnetic fields. Noting that the sun and other magnetically active stars produce flares with the sudden release of stored magnetic energy, researchers reasoned that sooner or later, V711 Tau might erupt. But after 15 years of study -- the last eight with a robotic telescope on Arizona's Mount Hopkins -- astronomers found no evidence that this star or any of the other 110 binaries in the same class that the observed had ever undergone such sudden brightening.

V711 Tau's status changed irrevocably on Dec. 14, 1989. Astronomers at the Beijing (China) Astronomical Observatory noted a rapid brightening of the star over just a few hours. After years of negative results, Gregory W. Henry of nennessee State University in Nashville figured the Chinese had erred. But checking with the Mount Hopkins telescope, he found that 12 hours after the Beijing observation, the star had undergone another, more intense jump in luminosity. That flare may represent not only the first observed in this stellar class, but also the most intense observed on any star, Henry says.

At the peak of the flare-up, which lasted about six hours, the total amount of energy released by the star climbed to 5 X [10.sup.34] ergs/second -- a rate about 10 times as large as the sun's total output and 100 million times as intense as a typical white-light solar flare, report Henry and Douglas S. Hall of Vanderbilt University in Nashville.

They also found that the star's overall luminosity remained slightly elevated for three months after the record-breaking outburst. This, coupled with the star's orientation relative to Earth, indicates that the flare may have temporarily erased a large, light-obscuring spot -- perhaps similar to a sunspot -- believed to reside at one of the star's poles, Henry says. The luminosity slowly returned to normal levels, perhaps as the spot began to build back up, he adds.

The flare's origin remains unclear. Because its intensity far exceeded that of a solar flare, it may well have arisen from processes that differ from those causing outbursts on the sun. Even so, V711 Tau's magnetic activity might possibly undergo fluctuations resembling the 11-year sunspot cycle, Henry says.