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Patchy but hot.
Astronomers have found shifting cloud patterns over star alpha Andromedae similar to the weather dynamics on planets.

Oleg Kochukhov/Uppsala University

A Stellar Forecast

With temperatures in the thousands of degrees and clouds of mercury rolling across the sky, it might not be anybody's idea of a fine summer day. But a new discovery may give astronomers the chills. A team has found the first evidence that stars, just like some planets and moons, can experience a form of weather: clouds forming, dissipating, and roiling above their blazing-hot surfaces. The discovery should help solve some lingering mysteries about stellar evolution.

The clues to the discovery have been around for nearly a century. Some stars vary widely in the chemical composition of their atmospheres, even though the stars are about the same age and were all born within the same cloud of dust and gas. Over the past decade, telescopes and detectors have finally improved enough that astronomers can make detailed observations of these atmospheres. One of the first stars on which surface features could be distinguished was alpha Andromedae, located about 95 light-years away in the constellation Andromeda.

When astrophysicist Oleg Kochukhov of Uppsala University in Sweden and his team looked closely, they found that the composition of the star's atmosphere was changing over time as various places became enriched and depleted in mercury. The usual explanation, that the atmosphere was being disturbed by a magnetic field--in the same way that sunspots and flares are created on the sun--didn't fit, because alpha Andromedae lacks a strong magnetic field. Something else had to be causing the phenomenon.

After watching the star and its changing atmosphere for 7 years, Kochukhov says, the team determined that the mercury spots were actually clouds, and alpha Andromedae was experiencing weather. The data showed that the gaseous mercury clouds were following exactly the same kind of dynamics that govern weather on Earth and on the solar system's gas-giant planets, such as Jupiter and Saturn, the team reported online Sunday in Nature Physics.

"We're really seeing stellar weather," says astrophysicist Gregg Wade of the Royal Military College in Kingston, Ontario. He says stars with stable atmospheres like alpha Andromedae offer "great laboratories" for looking at both large-scale and small-scale dynamics of the atmospheres of stars. Just like weather on Earth, he says, stellar weather probably involves local manifestations of large-scale circulation patterns. But there's also a mystery. Wade says mercury is the only element in alpha Andromedae's atmosphere that can form clouds, and thus far astronomers aren't sure why.

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