Physics
Black holes have long been a fascinating subject for those that study the sky above us. Despite hundreds of years of observation, questions still remain around the operation, and even existence, of these stellar sinkholes. Professor Juri Poutanen and his colleagues from the University of Turku have combined the use of state of the art observational technology with cutting edge theoretical research to try and answer some of these issues
The mystery of black holes
It was in 1783 that a Cambridge don, John Michell, first postulated the existence of a star so massively dense that light would be unable to escape from its gravitational field. However, scientific understanding of black holes and the physics behind their formation and subsequent emission of radiation has progressed relatively slowly since they were first brought to the attention of the astronomy world. Progress has been hindered by the fact that black holes are still only a theoretical concept, and studies being limited to observational, rather than experimental, form. Now, recent advances in technology have
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allowed scientists to supplement theoretical work with increasingly in- depth observational studies.
Stellar-mass black holes Poutanen and his team began their work by studying stellar black holes. These black holes are characterised by their relatively small mass of three to twenty solar mass units and by being formed by the collapse of a massive star. Stellar black holes are studied in binary systems in which the black hole has a companion star from which matter is drawn into an orbit, forming an accretion disc. As the gas orbits the black hole, it is subject to
gravitational and frictional forces, and is consequently
condensed and heated,
resulting in the emission of X-rays that can be observed. However, thanks to recent observational developments, X-ray emission is not the only type of radiation to be detected emanating from the accretion disc of a black hole. “For thirty to forty years people have been studying black holes, mostly via X-ray emissions,” says Poutanen, “But about ten years ago we saw observations that indicated optical and infrared emissions from black holes, emitted simultaneously with the X-rays”. Analysing emissions from the accretion region of binary black hole systems is
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