Black hole buried in giant star cluster

Science and Technology

Using the European Space Organisation (ESO)’s Very Large Telescope (Cerro Paranal, Chile), astronomers have discovered a star in a massive globular cluster, which appears to orbit an invisible black hole that is four times the size of our Sun.

This stellar mass is ‘the first black hole found in a globular cluster and the first found by directly detecting its gravitational pull’.

The star (0.8 times the mass of our Sun), located in the southern constellation Vela (cluster NGC 3201), exhibits an ‘odd behaviour’ as it seems to fling itself back and forth at speeds of several thousand kilometres per hour, on a loop that repeats every 167 days.

Video Source: ESO/ESA/NASA/Digitized Sky Survey 2/N. Risinger (skysurvey.org). 

Benjamin Giesers (Georg-August-Universität Göttingen, Germany), lead author, was intrigued by such 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 globular star clusters and black holes is very important but remains mysterious. Due to their large masses and great ages, these clusters are believed to have produced a large number of stellar-mass black holes, created as massive stars exploded and collapsed within them.

Artist’s impression of the black hole binary system in NGC 3201. (Source: ESO/L. Calçada)
Artist’s impression of the black hole binary system in NGC 3201. (Source: ESO/L. Calçada)

This is the first time that scientists have been able to detect an inactive black hole lying at the centre of a globular cluster that is not currently swallowing matter nor surrounded by a glowing gas disc. The black hole’s mass was estimated based on the movements of a star caught up in its gravitational pull.

Giesers commented that “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”.

This discovery helps understand “the formation of these star clusters, black holes, and the origins of gravitational wave ends”.