Unlike typical binaries, this system consists of a central black hole absorbing a nearby star and a second, more distant star orbiting it every 70,000 years. This unusual structure suggests that the black hole formed from a direct collapse rather than a supernova explosion, challenging dominant theories on the origin of black holes and highlighting the potential of newly undiscovered ternary systems.

Discovery of triple black hole

Most black holes discovered so far are part of a pair called a binary system. In these pairs, the black hole tightly orbits another object, such as a star, a dense neutron star, or even another black hole. The black hole’s intense gravity binds them together, forming a tight orbital bond.

Now scientists have made a surprising discovery that expands our understanding of black holes, the types of objects they can interact with, and how they form.

In a new study published Oct. 24 in the journal Nature, physicists from the Massachusetts Institute of Technology and the California Institute of Technology report observing a “triple black hole” for the first time. This unique system consists of a central black hole that orbits the black hole every 6.5 days, absorbing a nearby smaller star, similar to known binary systems. However, it is noteworthy that a second, much more distant star also revolves around the black hole. According to the researchers, this distant companion orbits the black hole every 70,000 years.

The fact that a black hole appears to gravitationally hold an object at such a distance raises questions about the origin of the black hole. Black holes are thought to be formed by the powerful explosion of a dying star; In this process, known as a supernova, the star releases enormous amounts of energy and light in a final explosion before collapsing into an invisible black hole.

The consequences of a soft formation process

However, the team’s findings suggest that if the newly observed black hole were the result of a typical supernova, the energy it would release before collapsing would blow up loosely bound objects around it. The second star, the outer star, shouldn’t still be hanging around.

Instead, the team suspects that the black hole formed as a result of a milder “direct collapse” process; In this process, the star collapses in on itself, forming a black hole without a dramatic afterglow. Such a soft origin is unlikely to disturb weakly attached distant objects.

Because the new triple system contains a very distant star, it suggests that the black hole in the system was born from a lighter direct collapse. Although astronomers have been observing more violent supernovae for centuries, the team says the new triple system may be the first evidence of a black hole forming from this gentler process.

“We believe that most black holes are formed by massive star explosions, but this discovery helps disprove that,” says study author Kevin Burdge, a Pappalardo Research Fellow in MIT’s Department of Physics. “This system is extremely interesting from the perspective of the evolution of black holes and also raises the question of whether triplets still exist.”

The study was co-authored by Erin Cara, Claude Cañizares, Dipto Chakrabarty, Anna Froebel, Sarah Millholland, Sol Rappaport, Rob Simcoe, and Andrew Vanderburgh of MIT and Karim El-Badry of the Caltech Institute.

Research on the formation and evolution of black holes

The discovery of the triple black hole happened almost by chance. Physicists found this while browsing through Aladin Lite, which contains astronomical observations collected from telescopes in space and around the world. Astronomers can use an online tool to search for images of the same part of the sky taken by different telescopes tuned to different energies and wavelengths of light.

The team searched the Milky Way galaxy for signs of new black holes. Out of curiosity, Burge looked at an image of V404 Cygni, a black hole about 8,000 light-years from Earth that was one of the first objects confirmed to be a black hole in 1992. Since then, V404 Cygni has become one of the most studied black holes and has been documented in more than 1,300 scientific papers. However, none of these studies reported what Burge and colleagues observed.

Examining optical images of V404 Cygni, Burge saw what appeared to be two points of light surprisingly close together. The first point was what others identified as a black hole and an inner star orbiting closely together. The star is so close that it sheds some of its material onto the black hole and emits light that Burge can see. But the second cluster of light was something scientists had not yet discovered. Burge determined that this second light most likely came from a very distant star.

“The fact that we can see two separate stars at such a great distance means that the stars must be very far apart,” says Burge, who estimates that the outer star is 3,500 AU (ao) away from the black hole (1 AU is the distance between the Earth and the Sun). In other words, the outer star is 3500 times farther from the black hole than the Earth is from the Sun. This is also equal to 100 times the distance between Pluto and the Sun.

Examination of tandem movement and the origin of the system

The question that came to mind then was whether the outer star was related to the black hole and its inner star. To answer this question, researchers turned to Gaia, a satellite that has been precisely tracking the movements of all the stars in the galaxy since 2014. The team analyzed the motions of inner and outer stars over the last 10 years of Gaia data and found that the stars move exactly together compared to other nearby stars. They calculated that the odds of such a tandem were about one in 10 million.

“This is definitely not a coincidence or coincidence,” Burge says. “We see two stars moving behind each other because they are connected by a weak gravitational string. So it must be a triple system.”

So how can this system be created? If the black hole had originated from a typical supernova, the powerful explosion would have ejected the outer star long ago.

“Imagine pulling a kite, but instead of a strong string, you’re pulling a net,” says Burge. “If you pull too hard, the net will break and you will lose the kite. “Gravity is like a loosely tied rope and it’s very weak, and if you do anything dramatic with the inner binary star, you’ll lose the outer star.”

Modeling results and determining the age of the system

But to really test this idea, Burge ran simulations to see how such a triple system could evolve and protect the outer star.

He introduced three stars at the beginning of each simulation (the third was a black hole before it was a black hole). He then ran tens of thousands of simulations, each with slightly different scenarios of how the third star might collapse into a black hole and how that would affect the motion of the other two stars. For example, he modeled a supernova by changing the amount and direction of the energy it emits. He also modeled direct collapse scenarios in which the third star collapses in on itself, creating a black hole without emitting any energy.

“The vast majority of simulations show that the easiest way to do this trifecta is through direct collapse,” says Burge.

In addition to providing clues to the origin of the black hole, the outer star also revealed the age of the system. Physicists observed that the outer star was in the process of becoming a red giant, a phase that occurs at the end of a star’s life. Based on this stellar transit, the team determined that the outer star is approximately 4 billion years old. Considering that neighboring stars were born around the same time, the team concludes that the age of the triple black hole is 4 billion years.

“We’ve never achieved this before for an old black hole,” says Burge. “We now know that V404 Cygni is part of a trio, that it may have formed by direct collapse, and that thanks to this discovery, it formed approximately 4 billion years ago.”