Aided by their large masses and great ages, globular star clusters - dense collections of tens of thousands ancient stars orbiting most galaxies - are thought to have produced a large number of stellar-mass black holes as massive stars within them explode and collapse over the long lifetime of such clusters. In this case, the black hole is classed as "inactive" and it becomes even more invisible than normal, so another method of detection is required to find them. This particular star cluster is over 10.2 billion years old. "Due to their large masses and great ages, these clusters are thought to have produced a large number of stellar-mass black holes - created as massive stars within them exploded and collapsed over the long lifetime of the cluster", ESO officials stated. The estimated mass of the whole cluster is about 254,000 times that of our sun. The MUSE instrument of the Very Large Telescope revealed that the star was behaving oddly.
This important discovery impacts on our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events, the researchers said.
About 150 globular star clusters surround our galaxy.
Astronomers analyzed the star's motion and worked out that it's circling an invisible point in space, which can mean only one thing: It's orbiting a small black hole.
"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.
A view from the Hubble Space Telescope of the central region of the star cluster NGC 3201, found in the southern constellation Vela.
The discovery of this invisible black hole shows just how little we understand about the universe around us, and what may or may not be possible-or even commonplace-outside our own stellar neighborhood.
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Artist's impression of the black hole binary system in NGC 3201. Due to their large masses and great ages, these clusters are thought to have produced a large number of stellar-mass black holes - created as massive stars within them exploded and collapsed over the long lifetime of the cluster.
However, not all black holes interact with hot matter surrounding them, therefore radiation will not always be emitted. They could estimate the black hole's mass through the movements of a star caught up in its enormous gravitational pull.
The star isn't very different from the sun: it's 0.8 times its mass, but far older.
"Until recently, it was assumed that nearly all black holes would disappear from globular clusters after a short time and that systems like this should not even exist", Dr. Giesers said.
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.
Now astronomers need to discover more of these black holes to find out if the one in NGC 3201 is the norm or the loneliest black hole in a globe of stars. Chart via ESO, IAU and Sky & Telescope.
An global team of astronomers led by Georg-August-Universität Göttingen researcher Benjamin Giesers has discovered a star in the globular cluster NGC 3201 that is behaving very strangely.