Lonely black hole hiding in giant star cluster identified

Astronomers have discovered a star in a massive globular cluster that appears to be orbiting an inactive and invisible black hole about four times the mass of the Sun.

Using European Southern Observatory's (ESO) MUSE instrument on the Very Large Telescope in Chile, the astronomers observed that the star is in the cluster NGC 3201 situated in the southern constellation of Vela (The Sails).

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.

Astronomers discovered that one of the stars in a global cluster 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," lead author Benjamin Giesers from the Georg-August-Universitat Gottingen in Germany, said in a statement.

This important discovery impacts on our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events, the researchers said.

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.

"The new 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," Giesers said.

--IANS

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