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BLACK holes are surrounded by layers of gas that resemble the Sun鈥檚 atmosphere, an astronomer has discovered. The finding suggests that both the Sun and black holes may be governed by similar processes.

Nan Zhang of the University of Alabama in Huntsville studied two black holes 10 000 and 40 000 light years away. Both are ringed by gaseous accretion discs pulled from companion stars and huge jets of material spew from their poles at close to the speed of light. Earlier observations had shown that hot, tenuous envelopes of gas called coronas lie above and below the discs of gas.

Now the Chandra X-ray Observatory, which was launched in July 1999, has revealed that the black holes have a third intermediate layer in their atmosphere. This 鈥渨arm layer鈥� reaches about 17 million 掳C, compared with 100 million 掳C for the hot coronas and 3 million 掳C for the thicker and colder disc. The Sun has three similar layers: the visible photosphere, the warmer but much thinner chromosphere which surrounds it, and the hot but tenuous corona. On average, all three layers are 500 times cooler than their black hole counterparts.

鈥淎round the Sun, magnetic fields are responsible for heating the corona,鈥� says Zhang. He believes similar processes could be at work around the black holes, which spin thousands of times a minute-fast enough to create their own strong magnetic fields. 鈥淲e believe these magnetic fields are pulling material out of the accretion disc and putting energy into the warm layer.鈥�