Scientists have found that many black holes exist in pairs with other objects, like stars or neutron stars, orbiting each other due to the black hole’s gravity. Recently, researchers from MIT and Caltech made an exciting discovery of a “black hole triple.” This system has one black hole pulling in a small star every 6.5 days, similar to other pairs.
However, a second star is much farther away and takes about 70,000 years to orbit the black hole. This finding helps us understand more about black holes and their formation.
The presence of a distant star orbiting the newly discovered black hole raises questions about how it formed. Black holes are typically created from supernova explosions, where a dying star releases a lot of energy before collapsing. This explosion usually pushes away nearby objects.
However, the researchers believe this black hole may have formed through a gentler process called “direct collapse,” where a star collapses in on itself without a big explosion. This would explain why the distant star is still nearby, as this method wouldn’t disrupt objects farther away.
Three black holes on a collision course
The presence of the distant star in the new triple system suggests that the black hole likely formed through a gentler process called direct collapse rather than a violent supernova. While astronomers have long studied supernovae, this finding may provide the first evidence of a black hole forming through this quieter method.
Astronomers discovered this new system by chance while looking through Aladin Lite, a repository of astronomical observations aggregated from telescopes in space and around the world.
The research team was searching for new black holes in the Milky Way when Kevin Burdge, a Pappalardo Fellow in the MIT Department of Physics, took a closer look at V404 Cygni, a well-known black hole about 8,000 light years away. Although this black hole has been studied extensively, Burdge noticed something new in the images.
He saw two blobs of light close together: one was the black hole and a nearby star that was losing material to it, which is why it shone. The second blob, which hadn’t been examined closely before, appeared to be a distant star orbiting the black hole. This observation could help expand our understanding of black hole systems.
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The fact that we can see two separate stars over this much distance means that the stars have to be very far apart,” says Burdge, who calculated that the outer star is 3,500 astronomical units (AU) away from the black hole (1 AU is the distance between the Earth and sun).
“In other words, the outer star is 3,500 times farther away from the black hole than the Earth is from the sun. This equals 100 times the distance between Pluto and the sun.”
The researchers wanted to find out if the distant star was connected to the black hole and its nearby star. To do this, they used data from Gaia, a satellite that has been tracking star movements since 2014. By analyzing the motions of both stars over the past ten years, they found that they moved together, unlike nearby stars. The chances of this happening by chance are about one in 10 million.
Solving Newton’s elusive three-body problem
Burdge noted that this close movement likely isn’t a coincidence. This suggests that the two stars are linked by a weak gravitational pull, confirming that this is indeed a triple system. How, then, could the system have formed? If the black hole arose from a typical supernova, the violent explosion would have kicked away the outer star long ago.
“Imagine you’re pulling a kite, and instead of a strong string, you’re pulling with a spider web,” Burdge says. “If you tugged too hard, the web would break, and you’d lose the kite. Gravity is like this feeble and barely bound string, and if you do anything dramatic to the inner binary, you’re going to lose the outer star.”
However, to really test this idea, Burdge carried out simulations to see how such a triple system could have evolved and retained the outer star.
At the start of each simulation, he introduced three stars (the third being the black hole before it became a black hole). He then ran tens of thousands of simulations, each with a slightly different scenario for how the third star could have become a black hole and subsequently affected the motions of the other two stars. For instance, he simulated a supernova, varying its amount and direction of energy. He also simulated scenarios of direct collapse, in which the third star simply caved in on itself to form a black hole without giving off any energy.
The vast majority of simulations show that the easiest way to make this triple work is through direct collapse,” Burdge says.
Scientists uncover new properties of black holes
The outer star in the triple system has provided important clues about the system’s age. Researchers found that this star is becoming a red giant, a phase that happens near the end of a star’s life. This observation led them to estimate that the outer star is about 4 billion years old. Since stars in the same region are usually born around the same time, they conclude that the entire black hole triple system is about 4 billion years old.
Burdge noted that this is the first time they’ve determined the age of an old black hole, linking it to the idea that V404 Cygni likely formed through direct collapse about 4 billion years ago.
Journal Reference:
- Burdge, K.B., El-Badry, K., Kara, E. et al. The black hole low-mass X-ray binary V404 Cygni is part of a wide triple. Nature (2024). DOI: 10.1038/s41586-024-08120-6