Something strange was happening in the constellation of the Vela. In the globular cluster NGC 3201, a star behaved in a rather unusual way, moving back and forth with a speed of several hundred thousand kilometers per hour. Now, thanks to the MUSE instrument of the Very Large Telescope of the European Southern Observatory (ESO) in Chile, a team of astronomers has managed to unravel this mystery. The force behind this unusual stellar movement is an invisible black hole, with about four times the mass of the Sun. It is the first time that such an object has been discovered by the direct detection of its gravitational force.
It is also the first dormant stellar mass black hole of its kind to be detected in a globular cluster. And it is located about 16,300 light years distant from Earth.
This great discovery has a great impact on our understanding of the formation of star clusters, black holes, and the origins of famous gravitational wave events.
Globular star clusters are huge spheres of tens of thousands of stars that orbit most galaxies. Today we know that up to 150 belong to the Milky Way.
The astronomers, led by Benjamin Giesers of the Georgia Augusta University in Göttingen, Germany, discovered that one of the stars in NGC 3201 was moving back and forth at speeds of several hundred thousand kilometers per hour, with a pattern that repeated every 167 days.
This main sequence star off (at the end of the main sequence phase of its life) is on its way to becoming a red giant.
Intrigued by the fact that something invisible but very massive appeared to orbit, astronomers thought it could only be a black hole.
The MUSE instrument provided a unique way to measure the motions of thousands of distant stars at the same time, detecting, for the first time, a dormant black hole at the heart of a globular cluster.
Considering that this black hole is no longer swallowing matter and is not surrounded by a glowing disk of gas, scientists were able to estimate the mass of the massive black hole through the movements of the star captured by its colossal gravitational force. Thus, astronomers have shown that the black hole in question is about 4.36 times the mass of the Sun.
“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 will help us understand the formation of globular clusters and the evolution of black holes and binary systems, vital in the context of understanding gravitational wave sources. “, concludes Benjamin Giesers, leader of the work.
Reference: A detached stellar-mass black hole candidate in the globular cluster NGC 3201. Monthly Notices of the Royal Astronomical Society. Preprint 2018
