A star in the cluster NGC 3201 that is behaving in an extremely strange manner has been discovered by astronomers using ESO’s MUSE instrument on the Very Large Telescope in Chile.
It appears to be orbiting an invisible black hole with almost four times the mass of the Sun - the first such inactive stellar-mass black hole discovered in a globular cluster and the first found by directly sensing its gravitational pull. This significant discovery impacts on the understanding of the development of these black holes, star clusters, and the origins of gravitational wave events.
Globular star clusters are considered to be huge spheres of tens of thousands of stars orbiting most galaxies. They are found among the oldest known stellar systems in the Universe and date back to almost the beginning of galaxy growth and evolution. Presently, more than 150 are known to belong to the Milky Way.
One particular cluster, known as NGC 3201 and located in the southern constellation of Vela (The Sails), has presently been analyzed with the help of the MUSE instrument on ESO’s Very Large Telescope in Chile. A global team of astronomers have discovered that one of the stars  in NGC 3201 is behaving very strangely — it is being thrown backwards and forwards at speeds of several hundred thousand kilometers per hour, with the pattern that keeps repeating every 167 days .
Key author Benjamin Giesers (Georg-August-Universität Göttingen, Germany) was fascinated by the star’s behavior: “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.”
The relationship between globular clusters and black holes is a vital but mysterious one. Due to their great ages and large masses, these clusters are assumed to have produced a huge number of stellar-mass black holes — developed as massive stars within them exploded and then collapsed over the extended lifetime of the cluster .
ESO’s MUSE instrument offers astronomers with an exceptional potential to measure the motions of thousands of distant stars in a simultaneous manner. With this new discovery, the team has for the very first time succeeded in detecting an inactive black hole at the heart of a globular cluster — one that is not presently swallowing matter and is not surrounded by a glowing disk of gas. The black hole’s mass could be estimated through the movements of a star caught up in its enormous gravitational pull .
From its observed properties the star was established to be almost 0.8 times the mass of the Sun, and the mass of its secretive counterpart was calculated at about 4.36 times the Sun’s mass — almost definitely a black hole .
Latest detections of X-ray and radio sources in globular clusters, and also the 2016 detection of gravitational-wave signals generated by the blending of two stellar-mass black holes, indicate that these relatively small black holes could be more common in globular clusters than earlier thought.
Giesers concludes: “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! 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.”
 The star found is a main sequence turn-off star, meaning it is at the end of the main sequence phase of its life. Having exhausted its primary hydrogen fuel supply it is now on the way to becoming a red giant.
 A large survey of 25 globular clusters around the Milky Way is currently being conducted using ESO’s MUSE instrument with the support of the MUSE consortium. It will provide astronomers with the spectra of 600 to 27 000 stars in each cluster. The study includes analysis of the “radial velocity” of individual stars — the speed at which they move away from and toward the Earth, along the line of sight of the observer. With radial velocity measurements the orbits of stars can be determined, as well as the properties of any massive object they may be orbiting.
 In the absence of continuous star formation, as is the case for globular clusters, stellar-mass black holes soon become the most massive objects present. Generally, stellar-mass black holes in globular clusters are about four times as massive as the surrounding low-mass stars. Recent theories have concluded that black holes form a dense nucleus within the cluster, which then becomes detached from the rest of the globular material. Movements at the center of the cluster are then thought to eject the majority of black holes, meaning only a few would survive after a billion years.
 Stellar-mass black holes — or collapsars — are formed when massive stars die, collapsing under their own gravity and exploding as powerful hypernovae. Left behind is a black hole with most of the mass of the former star, which can range from a few times the mass of our Sun to several tens of times as massive.
 As no light is able to escape black holes because of their tremendous gravity, the primary method of detecting them is through observations of radio or X-ray emissions coming from hot material around them. But when a black hole is not interacting with hot matter and so not accumulating mass or emitting radiation, as in this case, the black hole is “inactive” and invisible, so another method of detection is required.
 Because the non-luminous object in this binary system cannot be directly observed there are alternative, although much less persuasive, explanations for what it could be. It is perhaps a triple star system made up of two tightly bound neutron stars, with the observed star orbiting around them. This scenario would require each tightly bound star to be at least twice the mass of our Sun, a binary system that has never been observed before.