We’ve caught another glimpse of a shy cosmic monster. Middle-weight black holes are elusive, but our best evidence of one so far comes from inside one of the Milky Way’s brightest star clusters – and its existence suggests similar objects may have seeded the supermassive black holes occupying the centres of giant galaxies.
Black holes come in two main weight classes. Stellar-mass black holes form when a large star collapses, and are roughly ten times the mass of the sun. In contrast, supermassive black holes, such as Sagittarius A* at the Milky Way’s centre, typically have millions or billions of solar masses.
And black holes in between? Good luck finding one: most claims for their existence have diminished in the light of new evidence.
Now, Bülent Kızıltan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues have used a new technique to uncover an intermediate-mass black hole. It’s at the centre of the bright globular cluster 47 Tucanae, which is 15,000 light years from Earth. The cluster’s many stars radiate a combined luminosity 500,000 times more than the sun’s and have a total mass 700,000 times greater.
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