Seeking stellar flares in a deceptive sky.

Seeking stellar flares in a deceptive sky

Stargazers through the years have noted many odd flashes of light in the night sky. These have been traced to airplanes, camera glitches, reflections off satellites, imagination, asteroids and even fireflies. Astronomer Bradley E. Schaefer of the NASE Goddard Space Flight Center in Greenbelt Md., thinks some of these flashes may originate from more distant, stellar sources.

Schaefer suggests that some flashes could be outbursts from ordinary-looking but temperamental stars. In the Feb. 15 ASTROPHYSICAL JOURNAL, he lists 24 stars that appear to have given off solar-flare-like flashes, several of them 10 times more powerful than the star's normal brightness. Some of these flare-ups were photographed, while others were seen through telescopes or with the naked eye.

Schaefer's list excludes "exotic" stars, whose behavior is poorly understod or more unstable than that of other stars. He also disqualifies objects known as "flare stars," which are so dim to start with that they need only a little energy to flash many times their original brightness. He lists those stars whose flashes he thinks least likely to be flukes. One flash was noted independently by three people in different countries. Others simply started up too gradually or lasted too long to be explained away as instrument glitches or satellite glints. The 24 listed stars are all considered "normal," like our sun, except they may flare up vigorously perhaps every 100 or 1,000 years.

Our sun has been relatively serene--at least as long as people have recorded its behavior. It does have flares, but they are tiny compared to a flash that can be seen from another star. Solar flares are caused by unstable patches in the sun's magnetic field, and a similar change in magnetic field could explain the more distant flashes: If part of the field were annihilated, its potential energy could explode out as light. But a magnetic mechanism might not provide enough energy to account for the biggest flashes Schaefer lists. Another explanation, he suggests, is a comet crashing into a star's unknown white-dwarf companion -- but again, it's difficult to account for the amount of energy released. "You have to believe in very big supercomets and there's no evidence -- but they are possible," he says.

But many other things can trigger a flash in the sky and fool even professional astronomers. Canadian researchers recorded some spectacular flashes in the direction of the constellation Perseus several years ago. Later, they discovered they had recorded glints of light reflected from a Soviet satellite (SN: 6/20/87, p.397). In another instance, French astronomers reported some dramatic, potassium-laced stellar flares, only to learn later that the flares came from a night watchman who was lighting matches near the observing equipment.

Despite the hazards of scientific embarrassment, Schaefer says more astronomers should be watching what the sky does on a short time scale. However, confirming observations of brief events can be difficult. "What can science do to reproduce things that are so rare?" Schaefer asks. "We just have to wait."

One thing that might speed up the search is a project to systematically watch for flashes. Up on Kitt Peak in Arizona, a sensitive imaging device called the Explosive Transient Camera, or ETC, is taking quick consecutive pictures of stars to spot sudden changes. It was set up to catch mysterious explosions producing gamma-ray bursts (see story below), but the ETC can catch other sudden flashes, says co-designer and builder Roland Vanderspek of the Massachusetts Institute of Technology. The camera works in conjunction with the Rapidly Moving Telescope, which can swing over to a suspect star within a second and zoom in on it before it stops flashing.

Schaefer implores astronomers to design more experiments to detect flashes from stars. He also urges people not to be afraid to report any flash they observe. "There are too many cases of bright flashes on normal stars to be dismissed," he says. "If we watch for these things we might learn something new from them."