This dense cluster of stars is thought to be spiralling around a medium-sized black hole.© Gemini Observatory/AURAA second black hole lurks at the centre of our Galaxy, according to astronomers who have watched a cluster of stars spinning around it.
Just three years ago, astronomers confirmed that the Milky Way revolves around a supermassive black hole1, called Sagittarius A *, which is about 2.6 million times more massive than the Sun.
But now a much smaller black hole, just 1,300 times our Sun's mass, has been found orbiting about three light years away from its supermassive cousin.
“This is the first intermediate-mass black hole found in our Galaxy.”
Jean-Pierre Maillard
Institute of Astrophysics in Paris, France
Jean-Pierre Maillard, an astronomer from the Institute of Astrophysics in Paris, France, led a team that looked at a very bright area of the galactic core called IRS 13, previously thought by astronomers to be a single object.
Using infrared observations from the Gemini Observatory at the summit of Mauna Kea on the Big Island of Hawaii, they discovered that IRS 13 is in fact a rotating cluster of seven stars, just 0.065 light years across.
Adding data from the Hubble Space Telescope and the Chandra X-ray Observatory, they calculated from the movement of the seven stars that they must be orbiting an intermediate-mass black hole, called IRS 13E, which spirals around Sagittarius A * at about 280 kilometres per second.
"This is the first intermediate-mass black hole found in our Galaxy," Maillard told news@nature.com. The team reports its discovery in the latest issue of Astronomy and Astrophysics2.
The IRS 13 cluster has also been seen emitting strong X-rays, another tell-tale sign that it hides a black hole. Smaller X-ray emissions throughout our Galaxy suggest that there may be many mini-black holes closer to Earth that are just one or two times the mass of our Sun, but this has yet to be confirmed.
Extreme gravity
The seven stars could be the remnant of a massive star cluster that has been stripped down by the supermassive Galactic Centre, says Maillard. The observations may help to confirm the idea that supermassive black holes, believed to nestle at the heart of many galaxies, boost their mass by sucking up smaller black holes and stars from within the galaxy.
It may also explain why so many massive stars are found in the area. As the extreme gravity around Sagittarius A * prevents stars forming from clouds of dust and gas, it is possible that these stars form farther out in the Galaxy before being dragged into place by intermediate black holes.
Judging from their size and colour, the seven stars are likely to be fairly short-lived, adds Maillard, and so are likely to burn out before they spiral to their doom.
Institute of Astrophysics in Paris, France
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