Odd behavior of star reveals lonely black hole hiding in giant star cluster

Globular star clusters are huge spheres of tens of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the Universe and date back to near the beginning of galaxy growth and evolution. More than 150 are currently known to belong to the Milky Way.

One particular cluster, called NGC 3201and situated in the southern constellation of Vela (The Sails), has now been studied using the MUSE instrument on ESO's Very Large Telescope in Chile. An international team of astronomers has found that one of the stars [1] in NGC 3201 is behaving very oddly -- it is being flung backwards and forwards at speeds of several hundred thousand kilometres per hour, with the pattern repeating every 167 days [2].

Lead author Benjamin Giesers was intrigued by the star's behaviour: "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 black holes and globular clusters is an important but mysterious one. Because of 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 [3][4].

ESO's MUSE instrument provides astronomers with a unique ability to measure the motions of thousands of far away stars at the same time. With this new finding, the team have for the first time been able to detect an inactive black hole at the heart of a globular cluster -- one that is not currently swallowing matter and is not surrounded by a glowing disc of gas. They could estimate the black hole's mass through the movements of a star caught up in its enormous gravitational pull [5].

From its observed properties the star was determined to be about 0.8 times the mass of our Sun, and the mass of its mysterious counterpart was calculated at around 4.36 times the Sun's mass -- almost certainly a black hole [6].

Recent detections of radio and X-ray sources in globular clusters, as well as the 2016 detection of gravitational-wave signals produced by the merging of two stellar-mass black holes, suggest that these relatively small black holes may be more common in globular clusters than previously 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."

Notes

[1] 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.

[2] 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.

[3] 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 centre of the cluster are then thought to eject the majority of black holes, meaning only a few would survive after a billion years.

[4] 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.

[5] 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.

[6] 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.