Astronomers have used a star's peculiar behavior to find a black hole hiding in a giant star cluster. The black hole couldn’t have been found in any other way because it is absolutely inactive, which makes this the first detection of a stellar-mass black hole in a gravitational cluster and the first one employing a star's movement.
As reported in the Monthly Notices of the Royal Astronomical Society, the object is located in cluster NGC 3201 and is estimated to weigh over four times our Sun. Astronomers were studying the motion of stars in the cluster and noticed that one star, in particular, was moving forward and backward at a speed of several hundred thousand kilometers per hour, repeating this curious motion every 167 days.
“It was orbiting something that was completely invisible, which had a mass more than four times the Sun – this could only be a black hole! The first one found in a globular cluster by directly observing its gravitational pull,” lead author Benjamin Giesers, from the Georg-August-Universität Göttingen in Germany, said in a statement.
This observation has important consequences. Astronomers have reason to believe that globular clusters can be formidable factories of stellar-mass black holes, which are created by the many massive stars that inhabit a globular cluster over its long life. But observational evidence had been lacking. This led researchers to suggest that black holes might escape the cluster after their formation. This research is one of the latest examples showing that this is not the case.
“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!" said Giesers. "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.”
Recent detections of radio waves and X-rays suggest that NGC 3201’s black hole is a common occurrence. And gravitational wave physicists have speculated that some of the black hole mergers we have witnessed so far happened in the heart of globular clusters, where these stellar black holes can grow to the sizes witnessed in the gravitational waves observations.