Free Astronomy Magazine March-April 2018

31 MARCH-APRIL 2018 SPACE CHRONICLES 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. In the absence of continuous star for- mation, as is the case for globular clusters, stellar-mass black holes soon become the most massive ob- jects 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 de- tached 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. ESO’s MUSE instrument provides as- tronomers 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 es- timate the black hole’s mass through the movements of a star caught up in its enormous gravita- tional pull. From its observed prop- erties 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. Recent detections of radio and X- ray sources in globular clusters, as well as the 2016 detection of grav- itational-wave signals produced by the merging of two stellar-mass black holes, suggest that these rel- atively 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 gravi- tational effects of a stellar-mass black hole in a globular cluster. This finding helps in understand- ing the formation of globular clus- ters and the evolution of black holes and binary systems — vital in the context of understanding grav- itational wave sources.” !