A star twirling in a tight orbit around the centre of our galaxy has finally pinned down the existence of a monstrous black hole.



The size of the orbit and estimated mass of the star S2 rules out all but the most exotic possibilities for the relatively small but massive object lurking at the heart of the Milky Way.



Previous observations of X-ray radiation, as well as the movement of other central stars, have strongly suggested that the compact radio source known as Sagittarius A* is a supermassive black hole. But other explanations, such as a cluster of smaller black holes, neutron stars or a ball of heavy neutrinos, could not be ruled out until now.

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