A detached stellar-mass black hole candidate in the globular cluster NGC 3201

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 our Milky Way Galaxy.

Our home galaxy, the Milky Way, is know to have more than 150 globular star clusters.
Owing to their old ages and high masses, Galactic globular
clusters probably have produced a large number of stellarmass black holes during their lifetimes.

One particular cluster, called NGC 3201, has now been studied using the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope in Chile which provides astronomers with a unique ability to measure the motions of thousands of faraway stars at the same time

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.

In a study published in Monthly Notices of the Royal Astronomical Society astronomers announced the discovery of a black hole in the globular cluster NGC 3201. By noticing the exceptionally bizarre orbital behavior of a specific star in the cluster, the researchers were able to conclude that a black hole is lurking in the core of NGC 3201, located some 16,000 light years away.
They noticed that this particular star was being flung back and forth so quickly (over 200,000 miles per hour) that it must be orbiting an invisible black hole at least four times as massive as the Sun.
The discovery of the black hole which is not feeding on nearby material, and therefore invisible to direct observations is the first detection of a stellar mass black hole made purely by measuring its gravitational influence on other stars.

This finding helps in understanding the formation of globular clusters and the evolution of black holes and binary systems which are vital in the context of understanding gravitational wave sources.



Phedias Hadjicharalambous.
Cyprus Astronomy Organisation
International Dark-Sky Association