A star in the globular star cluster NGC 3201 behaves very strangely. It is thrown back and forth at speeds of several hundred thousand kilometers per hour, and the pattern is repeated every 167 days. Astronomers believe that this star is orbiting an invisible black hole, four times the mass of our sun. If so, it is the first inactive black hole that is found in a globular cluster, and the first one found by directly observing its gravitational pull.
Astronomers said that this important discovery is related not only to our understanding of the formation of these star clusters and black holes, but also to the origins of gravitational wave events, which are believed to occur during the fusion of two black holes in this mass range.
Our home galaxy, the Milky Way, is known to have more than 150 globular star clusters. They are symmetrical spheres of tens of thousands of ancient stars, orbiting outside the flat disk of the galaxy and believed to have formed at the beginning of the galaxy's history, around the same time that the Milky Way was forming.
It is believed that NGC 3201 is more than 10 billion years old (in contrast to the age of our sun of around four and a half million years). It is located in our sky in the direction of the southern constellation of Vela the Sails, at a distance of about 16,300 light years from Earth. The estimated mass of the entire cluster is approximately 254,000 times that of our sun.
Astronomers studied this cluster using the MUSE instrument on ESO's Very Large Telescope in Chile. Lead author Benjamin Giesers of Georg-August-Universität Göttingen in Germany noticed the strange behavior of a particular star in the group. He said:
I was orbiting something that was completely invisible, that it had a mass of more than four times the sun, this could only be a black hole! The first one found in a globular cluster when directly observing its gravitational attraction.
The relationship between black holes and globular clusters is important but mysterious. Due to their large masses and great ages, it is believed that these clusters have produced a large number of stellar mass black holes, created when massive stars within them exploded and collapsed during the long life of the cluster.
The ESO MUSE instrument provides astronomers with a unique ability to measure the movements of thousands of distant stars at the same time. With this new finding, the team was able to detect for the first time an inactive black hole in the heart of a globular cluster, one that is currently not swallowing matter and is not surrounded by a shiny gas disk. They could estimate the mass of the black hole through the movements of a star trapped in its enormous gravitational attraction.
From its observed properties, it was determined that the star was about 0.8 times the mass of our sun, and the mass of its mysterious equivalent was calculated about 4.36 times the mass of the sun, almost certainly a black hole.
Recent detections of radio and X-ray sources in globular clusters, as well as the detection of gravitational wave signals in 2016 by the fusion of two stellar Larger black holes suggest that these relatively small black holes may be more common in the globular clusters of what was thought.
Until recently, it was assumed that almost all black holes would disappear from globular clusters after a short time and that systems like this one should not even exist! But clearly this is not the case.
In short: astronomers observed the strange behavior of a star in the cluster of globular clusters NGC 3201 and concluded that it must be orbiting a black hole of four solar masses.
Source: "A Candidate Separated from Black Hole of Stellar Mass in Globular Cluster NGC 3201", by B. Giesers et al., To appear in the Monthly Notices of the Royal Astronomical Society.