Strange star reveals black hole in cluster

An invisible black hole has been discovered in a star cluster because astronomers were able to observe how its gravitational pull affects the motions of a nearby star.

Colour-composite image of the globular cluster NGC 3201, obtained with the WFI instrument on the ESO/MPG 2.2-m telescope at La Silla. Globular clusters are large aggregates of stars, that can contain up to millions of stars. They are among the oldest objects observed in the Universe and were presumably formed at about the same time as the Milky Way Galaxy, in the early phase after the Big Bang. This particular globular cluster is located about 16 000 light-years away towards the Southern Vela constellation. The data were obtained as part of the ESO Imaging Survey (EIS), a public survey being carried out by ESO and member states, in preparation for the VLT First Light. The original image and astronomical data can be retrieved from the EIS Pre-Flames Survey Data Release pages, where many other nice images are also available.

Composite image of the globular cluster NGC 3201, obtained with the WFI instrument on the ESO/MPG 2.2-m telescope at La Silla. Image Credit: ESO

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Astronomers have detected what appears to be an invisible black hole within a globular cluster, by observing how its gravitational pull is flinging a star back and forth.

The discovery marks the first ever inactive stellar-mass black hole found in a globular cluster, and the first to be found by directly detecting its gravitational pull.

Globular star clusters are some of the oldest objects in the Universe and can contain up to millions of stars.

They are thought to have produced a large number of stellar-mass black holes during their long lifetimes.

NGC 3201 is one such cluster, and astronomers using the Very Large Telescope found that one of its stars is being flung back and forward at speeds of several hundred thousand kilometres per hour.

This pattern repeats every 167 days.

Often, black holes can be detected because of the light they give off as they suck material in.

This process generates enormous amounts of heat energy, causing the infalling material to glow.

But the black hole in this study is inactive, meaning it is not currently swallowing material and is therefore not glowing.

The astronomers who carried out the study detected the black hole because of the strange motion of the nearby star.

They were also able to use the movements of the star to calculate it is about 0.8 times the mass of the Sun, and that the suspected black hole is about 4.36 times the Sun’s mass.

“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.” says lead author Benjamin Giesers of the Georg-August-Universität Göttingen, Germany.

“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.

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“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.”