Scientists have for the first time discovered an invisible, inactive black hole – about four times the mass of the Sun – by observing strange star orbiting it. This is the first such inactive stellar-mass black hole found in a globular star cluster, researchers said. The discovery impacts our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events.
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 3201, situated in the southern constellation of Vela, has been studied using the MUSE instrument on European Southern Observatory (ESO)’s Very Large Telescope in Chile.
Researchers found that one of the stars 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. “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,” said Benjamin Giesers from ESO.
The relationship between black holes and globular clusters is an important but mysterious one. Due to 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. 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. 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.