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Astronomers Puzzled by X-Ray Burst from Normally Stable Star Embargoed for release 6 p.m. EDT

September 15, 1994

WASHINGTON (AP) _ One of the brightest stars in the sky suddenly emitted a large burst of X- rays, startling astronomers who are now scrambling to explain the first flare ever seen on this normally sedate type of star.

Two German astronomers report Friday in the journal Science that they spotted the burst on Zeta Orionis, the left-most star in the belt of the famed Orion constellation.

″Hot stars″ are huge, young stars that are enormously bright and constant, beloved by astronomy buffs for their dependable lights in the night sky.

Not only does their luminescence not fade, these stars also give off an unvarying stream of X-rays. Until now.

The finding has astronomers puzzled, but hopeful that the unique flare will shed light on the mysterious stellar winds that produce shock waves of hot gases throughout galaxies.

″These observations produce the most direct evidence so far for the scenario of shock-heated gas in the winds of hot stars,″ wrote Thomas Berghofer and Jurgen Schmitt of Munich’s Max Planck Institute for Extraterrestrial Physics.

Berghofer and Schmitt were analyzing long-awaited data from the German satellite ROSAT earlier this year when they spotted the burst. During two days in September 1992, Orionis’ emission of high-energy X-rays suddenly rose 30 percent.

When the satellite passed by again five months later, the high-energy X- rays had dropped some but the low-energy X-rays were up 20 percent. Not until September 1993 did all the emissions return to normal.

″I’ve never heard of an event like that before,″ said astronomer Nolan Walborn of the Space Telescope Science Institute in Baltimore.

″It’s a new phenomenon,″ said Jean Swank, an astrophysicist at NASA’s Goddard Space Flight Center. Determining the cause ″will be pretty significant as far as understanding these stars.″

Even though hot stars are the most watched in the sky, astronomers still don’t know a lot about them - even what causes their normal X-ray emissions.

The best theory is that the star’s intense light drives gases off its surface, generating stellar winds. Those winds blow in regular gusts that create shock waves to heat the gases, resulting in steady radiation. Bits of those hot gases, meanwhile, are blown into space and eventually form new stars.

Berghofer and Schmitt argue that Orionis’ flare was caused by an abnormally large shock wave. They used the ROSAT data to develop a model in which Orionis emits a very dense cloud of gases that is hit by a fast-moving windstream. The shock wave propels gases outward at about 1,000 kilometers a second, generating high-energy X-rays. As the shock wave slows, it loses energy and emits low-energy X-rays before dying altogether.

U.S. astronomers haven’t had a chance to study the ROSAT data yet. The explanation might be simpler: Orionis could be part of a binary system, where a special hot star sends X-rays to an orbiting star hidden in its bright light, Walborn suggested.

But the Orionis burst also might provide important information on ″how the wind works from individual stars,″ said astronomer Richard White, who helped develop the theory of stellar wind shocks.

″That understanding of how stellar winds work feeds into our understanding the whole picture of the life cycle of stars.″

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