A surprising discovery challenges existing models of black hole creation and reveals the age of the stellar system
Many of the black holes discovered to date appear as part of a binary system. These binary systems include a black hole and a secondary object - such as a star, compressed neutron star or another black hole - that surround each other due to the strong gravity of the black hole and form a tight orbital system. Now, a surprising discovery is expanding the understanding of black holes, the objects that can be part of the system around the black hole that created the system.
In a study published in the journal Nature, astrophysicists from MIT and Caltech report that they have observed a triple system around a black hole for the first time. The new system includes a central black hole that swallows a small star that orbits it every 6.5 days - a configuration similar to many binary systems. But surprisingly, another star is also seen circling the black hole, but at a greater distance. The researchers estimate that the distant star completes a revolution around the black hole once every 70,000 years.
The fact that the black hole manages to hold a gravitational force on such a distant body raises questions about the origin of the black hole itself. The accepted hypothesis is that black holes form from the violent explosion of a dying star - a process known as a supernova, in which a star releases enormous amounts of energy and light in a final explosion before collapsing into an invisible black hole.
However, the research team's discovery suggests that if the observed black hole had formed from a typical supernova, the energy that would have been released before its collapse would have been so powerful that it would have blown away any loose objects. The second and more distant star was not supposed to stay in place. The researchers hypothesize that the black hole was created by a less violent process of "direct collapse," where a star simply collapses in on itself and forms a black hole without a dramatic eruption. Such a less violent process would not interfere with loose and distant objects.
The triple system, which includes a very distant star, indicates that the black hole was born through a less violent direct collapse than a supernova. While astronomers have observed violent supernovae for centuries, the researchers say the new triple system may be the first sight of a black hole created by a less violent process.
"Until now, we thought that most black holes were formed from violent stellar explosions, but this discovery calls that into question," said paper author Kevin Burdge, a Pappalardo Fellow in the Department of Physics at MIT. "This system is very exciting for the study of the evolution of black holes, and it also raises questions about whether there are other triple systems like this."
joint movement
The discovery of the trio of black holes happened almost by chance. The astrophysicists discovered it while looking for signs of new black holes in the Milky Way galaxy in the Aladin Lite observatory, which brings together astronomical observations from telescopes in space and around the world. Out of curiosity, Bridge examined an image of V404 Cygni—a black hole located about 8,000 light-years from Earth and one of the first objects ever to be definitively identified as 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 articles. However, none of these studies reported what Bridge and his colleagues observed.
When he examined the optical images of V404 Cygni, Bridge saw what appeared to be two spots of light, surprisingly close together. The first blob was the black hole and the inner star that surrounds it. The star is so close that it loses some of its material to the black hole, creating the light Bridge saw. The second spot, however, was something the scientists hadn't studied in depth until now. Bridge stated that the second light was probably coming from a very distant star.
"The fact that we can see two separate stars at such a distance means that the stars must be very far apart," said Budge, who calculated that the exoplanet is 3,500 AU from the black hole (1 AU is the distance from the Earth to the Sun). . In other words, the exoplanet is 3,500 times farther from the black hole than the Earth is from the Sun, which is equal to 100 times the distance between Pluto and the Sun.
The question that arose next was whether the outer star is related to the black hole and the inner star. To answer this, the researchers used the Gaia satellite, which has been tracking the movements of all the stars in the galaxy with great precision since 2014. The team analyzed the movements of the inner and outer stars over the past decade of Gaia data and found that the stars move in complete coordination with each other compared to other nearby stars. They calculated that the chance of such coordinated movement is about one in 10 million.
"It's almost certainly not a coincidence," Budge said. "We see two stars moving together because they are bound by a weak gravitational force. So it must be a tripartite system."
Pulling strings
How, then, was the system created? If the black hole was formed from a typical supernova, the violent outburst would have blown the outer star away long ago.
"If you imagine that you are pulling a kite, and instead of a strong string you are pulling with a spider's web," explained Berdge. "If you pull too hard, the cold will break and the kite will be lost. Gravity is like this weak thread - if you do something dramatic to the inner double, you'll lose the outer star."
To test this idea, Bridge performed simulations in which he examined how or to answer the question of how such a system could have developed and maintained the outer planet. At the start of each simulation, he inserted 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 became a black hole and affected the motions of the other two stars. For example, he ran a simulation of a supernova, changing the amount and direction of energy it released. He also ran simulations of direct collapse scenarios, in which the third star simply collapsed in on itself to form a black hole, without releasing any energy.
"Most simulations show that the simplest way to create this triple system is through a direct collapse," Berdge said.
In addition to clues about the origin of the black hole, the exoplanet also revealed the age of the system. The researchers expected that the outer star was in the process of becoming a red giant - a stage that occurs at the end of a star's life. Based on the star's transition to this phase, the team determined that the age of the outer star is about 4 billion years. Given that nearby stars form in the same time period, the team concludes that the black hole trio system is also about 4 billion years old.
"We've never been able to do this before for an old black hole," Berdge says. "Now that we know V404 Cygni is part of a ternary system, it probably formed from a direct collapse, and it formed about 4 billion years ago, thanks to this discovery."
The research was supported in part by the National Science Foundation (NSF).
More of the topic in Hayadan:
- The quantum mechanics of black holes
- Stars engulfed by a black hole release energy long after the process begins
- Exotic black holes may be a byproduct of dark matter
- At least ten thousand Milky Way black holes surround the central black hole
- Support for the cosmic version of the birth of the solar system