Evrim Yazgin
Cosmos science journalist
For the first time, a black hole has been discovered with two orbiting stars, leading astrophysicists to suggest a surprising way such systems could form.
Many black holes have been discovered in binary systems. These pairs involve a central black hole and another object such as a star, denser neutron star or another black hole.
The first triple system, described in a paper published in Nature, includes a black hole named V404 Cygni which was discovered in 1992. V404 Cygni is about 8,000 light-years from Earth and is one of the first objects to be confirmed as a black hole.
Orbiting V404 Cygni every 6.5 days is a small star. This star is so close, it is shedding material which is being consumed by the immense gravity of the black hole. But the team which found the star noticed a second “blob” of light next to the black hole.
They noticed this was a second star orbiting the black hole, but much further away.
“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,” says author Stephen Burdge from MIT.
The outer star is 100 times the distance between Pluto and the Sun. The second star orbits the black hole every 70,000 years, the team calculated.
“This system is super exciting for black hole evolution, and it also raises questions of whether there are more triples out there,” adds Burge.
“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” Burge explains. Such an explosion should send nearby objects flying into space.
“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 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.”
Burge’s team turned to computer simulations to test this idea and see under what conditions the second star would stay gravitationally tied to the black hole.
Instead of a violent explosion leading one of three stars to turn into a black hole, they tested a gentler “direct collapse” scenario 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.
In addition, the outer star helped the physicists determine the age of the black hole, since neighbouring stars are born around the same time.
“We’ve never been able to do this before for an old black hole,” Burdge says. “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.”
