A class of black holes of intermediate mass is expected but has never been detected, The suggestion that these beasts might lurk behind powerful X-ray sources in nearby galaxies is now strengthened. Black holes are known to exist in two mass regimes:those between two and ten times the mass of the Sun, known as stellar-mass black holes and formed from the collapse of the most massive stars; and those between a million and a billion times the mass of the Sun, the 'super massive' black holes. Such behemoths, including the one at the centre of the Milky Way, are the engines powering quasars and active galactic nuclei. Between the two extremes are intermediate-mass black holes, although these have never been detected unambiguously. Observations from NASA's Chandra space observatory have stoked the debate over their existence. The images show extremely luminous, compact X-ray sources lurking in and around star clusters in nearby galaxies -- sources that might be associated with intermediate-mass black holes.
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