The Rule of Threes

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A lot of black holes across the universe are part of a binary system, which means they include the celestial body and a secondary object – such as a star or another black hole.

But a new study on a far-away system has shown that black holes don’t always come in pairs.

Researchers at MIT and Caltech recently identified for the first time a “black hole triple” system about 8,000 light years from Earth.

Known as V404 Cygni, it consists of a central black hole consuming a nearby star while a second star orbits from an astonishing distance, roughly every 70,000 Earth years.

The research team discovered the cosmic anomaly while sifting through the star-tracking data from the European Space Agency’s Gaia mission, which has precisely mapped stars’ motions since 2014.

For decades, astronomers had initially dismissed the far-flung star as an unrelated background object, but the recent analysis showed that it is gravitationally bound to the black hole system, which forms an intricate cosmic trio.

“The tertiary star has actually been hiding in plain sight for more than 30 years … they just all assumed the star was a chance alignment,” co-author Kareem El-Badry said in a statement for Caltech.

Kevin Burdge, a physicist at MIT and the study’s lead author, described the gravitational bond between the black hole and the distant star as a “weak string of gravity,” akin to a kite being pulled by a spider web.

“If you tugged too hard, the web would break, and you’d lose the kite,” he told MIT News.

The study also provided some new theories about how V404 Cygni formed.

Typically, black holes are thought to form from the violent explosion of a dying star, known as a supernova, which would likely eject any loosely bound companions.

But the recent findings suggested that this black hole was born through “direct collapse,” where the star implodes without an explosion, leaving surrounding objects undisturbed.

The authors noted that the study opens a new chapter in black hole research, including questions about how common triple systems are.

“Triples open up evolutionary pathways that are not possible for pure binaries,” explained El-Badry. “People have actually predicted before that black hole binaries might form mostly through triple evolution, but there was never any direct evidence until now.”