'Lonely' Black Hole Hidden in Giant Globular Cluster Revealed by Weird, Boomeranging Star

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The star appears to be orbiting an invisible black hole with about four times the mass of the sun. ESO/L. Calçada

Astronomers using the Very Large Telescope in Chile have discovered a star in a massive globular cluster that appears to orbit an invisible black hole four times the size of our sun.

The star, in cluster NGC 3201 (in the southern constellation Vela) led the researchers to the black hole through what they described in a new paper published in the scientific journal Monthly Notices of the Royal Astronomical Society as "curious" behavior. The star appeared to be flinging itself back and forth—at speeds of several hundred thousand kilometers per hour—on a loop that repeated every 167 days, according to a European Space Organization press release.

"It was orbiting something that was completely invisible," lead author Benjamin Giesers, a PhD student at Göttingen University's Institute for Astrophysics, said according to a Göttingen press release. "This could only be a black hole!"

The discovery marks both the first inactive black hole ever found at the heart of a giant globular star cluster and the first ever found by following the trail of its gravitational pull instead of relying on X-ray or radio sources. It should help astronomers better understand not only the formation of giant star clusters like NGC 3201, but the formation of black holes themselves.

An "inactive" black hole refers to one that's no longer surrounded by a luminous disc of gas because it's no longer sucking in matter. Led by astronomers from the University of Göttingen in Germany, an international team of researchers made the discovery using the ESO's Multi Unit Spectroscopic Explorer, an instrument that helps the Very Large Telescope pick up objects that can't be studied with traditional imaging surveys.

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The star appears to be orbiting an invisible black hole with about four times the mass of the sun. ESO/L. Calçada

An "inactive" black hole refers to one that's no longer surrounded by a luminous disc of gas because it's no longer sucking in matter. Led by astronomers from the University of Göttingen in Germany, an international team of researchers made the discovery using the ESO's Multi Unit Spectroscopic Explorer, an instrument that helps the Very Large Telescope pick up objects that can't be studied with traditional imaging surveys.

A globular star cluster is an spherical region densely packed with tens of thousands of stars; there are more than 150 known globular clusters in the Milky Way, according to the ESO press release. They're some of the universe's oldest recorded stellar systems of any kind, which is why they're useful for understanding galaxy formation and evolution, according to the ESO. Scientists think globular clusters produce a lot of stellar-mass black holes because over time, so many of the stars the clusters contain explode and collapse.

"Until recently, it was assumed that almost all black holes would disappear from globular clusters after a short time and that systems like this should not even exist!" Giesers said, according to the ESO press release. "But clearly this is not the case—our discovery is the first direct detection of the gravitational effects of a stellar-mass black hole in a globular cluster. This finding helps in understanding the formation of globular clusters and the evolution of black holes and binary systems—vital in the context of understanding gravitational wave sources."

'Lonely' Black Hole Hidden in Giant Globular Cluster Revealed by Weird, Boomeranging Star | Tech & Science