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Weighing Black Holes the Easy Way
By Kenneth Silber
Staff Writer
posted: 12:48 pm ET
16 September 1999
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black_holes_measureScientists have developed a new method for measuring the mass of black holes, objects so dense that not even light can escape their gravity. Moreover, the technique is "very easy," says one of the researchers, Insu Yi of the Korea Institute for Advanced Study. Conventionally, scientists have estimated the mass of black holes by monitoring the invisible objects' gravitational effects on nearby stars. But such methods require numerous observations and entail considerable uncertainties, since the precise movements of distant stars are difficult to determine. The new technique involves measurements of gas that is being sucked into (and heated by) a black hole. As the gas is heated, it produces X-ray and radio emissions, which indicate how much matter is being drawn into the black hole -- which in turn allows an estimate of the black hole's mass. However, the method applies only to large black holes that attract gas in a particular pattern, known as ADAF, for "advection-dominated accretion flows." Such black holes appear to be common in the centers of galaxies. Yi, working with Stephen Boughn of Haverford College, weighed 10 large black holes with this method, and got results that generally agreed with conventional techniques. Yi spoke to space.com from the 5th Compton Seminar, an astronomical conference in New Hampshire where he was presenting the findings. The new technique is "more efficient" than conventional measurements, says Yi. However, he notes that the accuracy of the respective techniques depends on the data available about particular black holes. He expects the new method to be "complementary" rather than a replacement, but he hopes that it will be "widely adopted."
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