If correct, the discoverers say, the result marks the first example of stellar surface features that depend solely on atmospheric effects and not on magnetic fields—or in other words, weather. "No one has been able to find definite signatures of weather in stars before," says astrophysicist Saul Adelman of The Citadel in Charleston, S.C. "In most stars, what is seen is the result of magnetic activity."
Sunspots—the most familiar stellar markings—occur when strong magnetic fields roil up from the sun's interior and block the outward flow of energy. Similarly, researchers believe that magnetic fields create patterns on some large, bright, young stars by holding in place elements that have become concentrated in certain parts of the atmosphere.
In 2002, however, Adelman and colleagues reported finding a few large, bright stars that lacked magnetic fields but were chemically blotchy anyway. In particular, they noticed a fluctuating amount of mercury in the atmosphere of Alpha Andromedae, a relatively young Milky Way star that is more than twice as wide as the sun and 30 times brighter, Adelman says.
To detail the changes, they scanned for mercury at two-year intervals from 1998 to 2004 and applied a technique to map the concentration of mercury on the star's surface.
An undulating high-concentration band of mercury circles the star's equator, the group reports online today in Nature Physics. The concentration of mercury in some locations varied as much as a hundredfold between measurements.
Team member Oleg Kochukhov, an astrophysicist at Uppsala University in Sweden, compares the mercury with thin planetary cirrus clouds. Stellar radiation would lift the mercury atoms, Adelman says, while stellar wind particles and variations in the star's rotational speed between the equator and poles might influence the clouds' rise and fall.
Adelman adds that weather may exist on other stars like Alpha Andromedae, but the mercury makes atmospheric changes easy to spot. "In most stars," he says, "you don't have such markers."