For about one-fifth of a second on the morning of Dec. 4, 1969, an otherwise obscure star in Camelopardalis, the constellation of the giraffe, burst into extraordinary brightness.

For one brief, shining moment the star poured forth as much thermonuclear energy as could be produced from 15 billion tons of atomic fuel. The star, normally about 80 times larger than the sun, flashed with a luminosity 6,000 times brighter than the sun's.

Then, as quickly as it began, the flash ended.

The event was captured on videotape by a National Aeronautics and Space Administration airplane making routine observations of the northern lights 16 years ago, but nobody noticed it until this year when the tapes were being reviewed. The flashing star is estimated to be 466 light years from Earth (a light year is the distance light travels in one year, roughly 6 trillion miles).

Reporting in the Astrophysical Journal, Thomas J. Wdowiak of the University of Alabama at Birmingham and K. Stuart Clifton of NASA's Marshall Space Flight Center in Huntsville, Ala., said they had no idea what caused the burst or the few others like it that have been recorded in recent decades.

One possibility is that a comet or other object plunged into the star, its mass being converted into energy almost instantly.