- Black holes form when the core of a very massive star collapses in on itself at the end of its life, gravitationally crushing all its huge mass into a tiny point in space.
- It’s logical that black holes would form alongside supernova explosions. As the outer layers of a star blast violently outward, the inner part of the star collapses violently inward.
- Now astronomers have found a black hole that’s part of a triple system. It suggests a history where no supernova occurred, and where the black hole formed by the more “gentle” method of direct collapse.
Black hole in a triple star system
In theory, all you need to create a black hole is a large mass in a small space. A large-enough mass in a small-enough space will inevitably collapse to become a black hole. It’ll be forced to collapse by its own self-gravity. But typically we think of something as triggering that collapse. The star runs out of fuel and explodes as a supernova, for example, crushing the star inward, sparking the collapse, until gravity can take over and do the rest. In the real space of our Milky Way galaxy, astronomers have seen stellar-mass black holes – objects of, say, 5 to 50 times our sun’s mass – in double systems. They’re possible to see in double systems because the presence of a 2nd star means astronomers can see the hole’s effect on the star, often in the form of an accretion disk swirling around the hole. Now, for the first time, astronomers have confirmed a black hole in a triple star system. They say it’s evidence that no supernova took place in this system and that instead the black hole formed by direct collapse.
So here, these astronomers say, is the first stellar-mass black hole ever found that apparently formed without the aid of a supernova. The system is called V404 Cygni, and it’s about 8,000 light-years away.
Researchers the Massachusetts Institute of Technology (MIT) and Caltech announced the find on October 23, 2024. They are (somewhat misleadingly) calling it a “black hole triple.” But it’s not three black holes. It’s one black hole in a triple system, with two stars.
The researchers, led by lead author Kevin Burdge in the MIT Department of Physics, published their peer-reviewed discovery in Nature on October 23, 2024. There is also a preprint version of the paper available on arXiv.
1st known ‘black hole triple’
So this is the first known stellar-mass black hole, in a system with three objects instead of just two. The closer star orbits so near the black hole that its “year” is only 6.5 Earth-days long. The black hole is consuming that star, and there’s evidence for an accretion disk of soon-to-be-eaten star-stuff, swirling around the hole. A 2nd star is orbiting much farther out than the first one. The 2nd star takes the star 70,000 years to complete one orbit!
Black holes, of course, are known for their powerful gravitational pull. And gravity is directly dependent on mass; the greater the mass, the stronger the pull. But gravity inversely dependent on distance; that is, the greater the distance, the weaker the pull. And a star orbiting every 70,000 years would be really far from the black hole. So – so if that black hole were once a massive star, and if that star exploded as a supernova – how did the hole hold on to the 3rd, more distant star? That mystery is making astronomers question how the black hole first originated.
Scientists think that black holes tend to form violently. This happens when a dying star explodes in a supernova. The star erupts in one last massive burst of energy and light, then collapses into a much smaller black hole. It isn’t really a hole, but a small object so dense that even light cannot escape it. This renders it invisible to human eyes, hence the name black hole.
A surprising discovery
V404 Cygni is a well-known and studied black hole, first confirmed in 1992. Yet what Burdge and his colleagues found had never been observed before. The researchers used the Aladin Lite sky atlas, an online repository of astronomical observations from telescopes in space and all around the world, for their study. They were actually looking for new black holes in the Milky Way galaxy.
The researchers decided to review images of V404 Cygni as well. They noticed two blobs of light close to each other. Astronomers already knew about the first blob, which was the material around the black hole and an orbiting star. But astronomers had never studied the second blob before. The research team said it was probably another star, much farther away than the first one. They calculated that the outer star was 3,500 astronomical units (AU) away from the black hole. 1 AU is the distance between the Earth and sun. That’s also about 100 times the distance of Pluto from the sun. Burdge said:
The fact that we can see two separate stars over this much distance actually means that the stars have to be really very far apart.
Astrophysicists from Caltech and MIT report that they have observed for the first time a “black hole triple”—a system of three stars, one of which is a black hole.
— Caltech (@Caltech) October 23, 2024
2nd star in tandem with the 1st
But was this second star actually gravitationally associated with the other star and black hole? Or was it completely separate? Using the Gaia satellite, the researchers determined that the second star moved exactly in tandem with the first star and black hole. So they were all part of the same system. Gaia’s mission has been to track the motions of all stars in the Milky Way since 2014. With calculated odds of about one in 10 million, that couldn’t be a coincidence, the researchers noted. Burdge said:
It’s almost certainly not a coincidence or accident. We’re seeing two stars that are following each other because they’re attached by this weak string of gravity. So this has to be a triple system.
A more gentle formation?
But there’s a problem with knowing how V404 Cygni formed. If it was the result of a typical supernova, then the second star shouldn’t even be there. The incredible force of the explosion should have “kicked it away” completely from the exploding star. So why is it still orbiting the black hole, albeit at a great distance?
The researchers think that it formed through a gentler process, which they call “direct collapse.” Instead of the star ending its life in a fiery explosion, it simply collapses in on itself. This can also theoretically form a black hole, but without the fireworks. The V404 Cygni system may be the first direct evidence of that formation process. As Burdge said:
We think most black holes form from violent explosions of stars, but this discovery helps call that into question. This system is super exciting for black hole evolution, and it also raises questions of whether there are more triples out there.
Like pulling a kite
Burdge likened the process to pulling a kite:
Imagine you’re pulling a kite, and instead of a strong string, you’re pulling with a spider web. If you tugged too hard, the web would break and you’d lose the kite. Gravity is like this barely bound string that’s really weak, and if you do anything dramatic to the inner binary, you’re going to lose the outer star.
The researchers tested thousands of computer simulations, all beginning with three stars (the third star being the one that will become the black hole). For the supernova, the simulations varied the amount of energy the blast would give off, and in what direction. They also compared the supernova versus direct collapse scenarios.
Direct collapse was the clear winner. As Burdge noted:
The vast majority of simulations show that the easiest way to make this triple work is through direct collapse.
Black hole triple is almost as old as our solar system
The simulations not only showed how the V404 Cygni system came to be. They also revealed its age. The outer distant star is in the process of becoming a red giant. That means it is around 4 billion years old. Our own solar system is about 4.5 billion years old. The rest of the V404 Cygni system is also likely to be the same age, since other nearby stars are also thought to have first formed about the same time as the outer star in the system. As Burdge concluded:
We’ve never been able to do this before for an old black hole. Now we know V404 Cygni is part of a triple, it could have formed from direct collapse, and it formed about 4 billion years ago, thanks to this discovery.
In 2021, astronomers also identified eight concentric rings of material around V404 Cygni, only seen in X-rays and not visible light. The astronomers used NASA’s Chandra X-ray Observatory and the Neil Gehrels Swift Observatory (Swift) to detect the rings.
Bottom line: Physicists say they have discovered the 1st-known black hole in a triple system. Two stars orbit with the black hole, 1 very close and the other farther away.
Source: The black hole low-mass X-ray binary V404 Cygni is part of a wide triple
Source (preprint): The black hole low mass X-ray binary V404 Cygni is part of a wide hierarchical triple, and formed without a kick
Via MIT
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