Weather observed on a star for the first time

 作者:独孤汕碟     |      日期:2019-02-28 07:07:01
By Jeff Hecht Weather – caused by the same forces as the weather on Earth – has been seen on a star for the first time, reveal observations of mercury clouds on a star called Alpha Andromedae. Previously, astronomers had thought that any structures on stars were caused by magnetic fields. Sunspots, for example, are relatively cool regions on the Sun where strong magnetic fields prevent energy from flowing outwards. But now, seven years of painstaking observations of Alpha Andromedae show that stars do not need magnetic fields to form clouds after all. Lying about 100 light years away, it is one of a class of stars unusually rich in mercury and manganese. Earlier observations of similar stars had revealed uneven distributions of mercury, but all of them had strong magnetic fields. The relatively massive stars do not mix gases in their atmospheres, which less massive stars, like the Sun, do. So the balance between the pull of gravity and the push of radiation pressure concentrates some heavy elements at certain atmospheric levels. At that point, their magnetic fields were thought to continue the separation process, sequestering some chemicals in particular regions. But researchers led by Oleg Kochukhov of Uppsala University in Sweden have found that this last step is not necessary to create chemical clouds on a star. They observed the mercury concentration in Alpha Andromedae – which does not have a detectable magnetic field – for seven years with 1.2- and 6-metre telescopes, detecting the mercury by its signature absorption line in the violet end of the spectrum. They resolved details on the spinning star’s surface by looking at how rapidly the clouds were turning towards or away from Earth. That revealed that the mercury concentration varies by as much as a factor of 10,000 across its surface, and the pattern of concentration changes as well. The evidence for changes in the mercury distribution over time “look very convincing”, comments Gregg Wade of the Royal Military College of Canada in Kingston, who discovered in 2006 that the star lacked a magnetic field. But exactly what causes the clouds to change over time is unclear. Kochukhov and colleagues say the changes “may have the same underlying physics as the weather patterns on terrestrial and giant planets”. The mercury clouds are on the brighter and larger member of a close pair of stars that orbit each other every 97 days. “The second star may create tides on the surface of the main star, much like the Moon creates tides on Earth, which drives evolution of the mercury cloud cover,” Kochukhov told New Scientist. But he adds that other explanations are possible. So for now, the weather on stars, as on Earth, remains hard to fathom. Journal reference: Nature Physics (doi: