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A distant star in the cluster NGC 3201 that's exhibiting odd behavior may be drawing attention to a secret. A team of international astronomers thinks it's behaving that way because the star is orbiting a black holethat has four times the mass of the sun.

The team saw the star moving backward and forward at speeds of several hundred kilometers per hour, in a cycle that repeats every 167 days. [The Strangest Black Holes in the Universe]

"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," Benjamin Giesers, lead author on the new research and an astrophysicist with the University of Göttingen in Germany, said in a statement.

 Artist's impression of a black hole binary system in the star cluster NGC 3201, where a distant star may orbit a black hole with four times the sun's mass.
Artist's impression of a black hole binary system in the star cluster NGC 3201, where a distant star may orbit a black hole with four times the sun's mass.
Credit: L. Cal├žada/ESO

Unlike active black holes, the black hole in this system — if it truly exists — isn't swallowing matter or expelling gas. That's why it's so hard to spot. The star is roughly 0.8 times the mass of the sun. Because of the star's movements around the invisible black hole, the researchers estimate that the black hole's mass is about 4.36 times the mass of the sun. 

If confirmed, this would be the first black hole that astronomers have ever been found in a globular cluster — a densely packed group of ancient stars — by looking at the black hole's gravitational pull on another object. The find was spotted using the European Southern Observatory's MUSE instrument on the Very Large Telescope in Chile.

A view from the Hubble Space Telescope of the central region of the star cluster NGC 3201, found in the southern constellation Vela. A star found orbiting a black hole is shown within the blue circle.
A view from the Hubble Space Telescope of the central region of the star cluster NGC 3201, found in the southern constellation Vela. A star found orbiting a black hole is shown within the blue circle.
Credit: ESA/NASA

ESO officials said in the statement that this find will help astronomers understand how globular clusters and black holes are formed, as well as the origins of gravitational waves — ripples in space-time caused by huge gravitational interactions.

A view from the Hubble Space Telescope of the central region of the star cluster NGC 3201, found in the southern constellation Vela. A star found orbiting a black hole is shown within the blue circle.
A view from the Hubble Space Telescope of the central region of the star cluster NGC 3201, found in the southern constellation Vela. A star found orbiting a black hole is shown within the blue circle.
Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin

"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," ESO officials stated.

"Recent detections of radio and X-ray sources in globular clusters, as well as the 2016 detection of gravitational-wave signals produced by the merging of two stellar-mass black holes, suggest that these relatively small black holes may be more common in globular clusters than previously thought," they added.

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