Physicists at MIT and Caltech have discovered the first “black hole triple,” a formation more than 8,000 light years from Earth that experts say expands the current understanding of how black holes form.
Black holes typically appear in a binary system, comprising of a black hole and a secondary object, such as a star or another black hole, but the research team says this latest discovery shows two stars orbiting the black hole, MIT said in a statement Wednesday.
The pair of stars are orbiting from vastly different distances, researchers said. One star is spiraling close to the black hole, completing an orbit every six and a half days or so, while the second star is at a much greater distance, orbiting the black hole once every 70,000 years.
The discovery raises questions about how the black hole was formed, as black holes typically follow the violent explosion of a dying star, releasing enough energy to knock away any loosely bound objects in the outskirts of its orbit, MIT said. The presence of this outer star indicates the black hole was born through a “gentler, direct collapse,” where the dying star caves in on itself and does not release as much energy.
“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” study author Kevin Burdge, a Pappalardo Fellow in the MIT Department of Physics, said in a statement. “This system is super exciting for black hole evolution, and it also raises questions of whether there are more triples out there.”
The research team’s findings were published Wednesday in Nature.
This particular black hole, identified as V404 Cygni, has been one of the most well-studied since it was first confirmed as a black hole in 1992, MIT said. Previous research had determined the black hole had an inner, closely orbiting star, but a second “blob” of light seen nearby had not been fully investigated until now.
The discovery came as the research team from MIT and Caltech was looking for evidence of new black holes in the Milky Way galaxy by going through Aladin Lite, a repository of astronomical observations, aggregated from telescopes in space and around the world, MIT said.
Burdge, who said his research includes developing high-speed cameras for large telescopes, said he and fellow MIT astronomer Erin Kara were curious whether taking a “high-speed movie” of a black hole could help identify new ones.
Burdge said he found that V404 Cygni had been filmed in such a way about 20 years ago. When he pulled up some images of the black hole, he said he was surprised when he noticed the second blob of light, which he determined was most likely coming from a far-off star, MIT said.
Burdge said this black hole has been documented in more than 1,300 scientific papers. He searched those papers for any mention of the word “triple” and found none.
“I assumed someone must have noticed [the second star], but no,” he said in a phone interview Wednesday.
Burdge said black holes are typically studied by x-ray and radio astronomers, but those methods won’t show most of the stars around a black hole. He said this discovery came about simply because he noticed the outer star in a picture and looked into it.
“This one is satisfying because it’s kind of a simple discovery,” Burdge said. “It’s just looking at a picture, and I think it reminds a lot of astronomers that there’s more to the job than just analyzing complicated data. You shouldn’t forget to do the simple things, like just look with your own eyes at some pictures and see what you find.”
To determine whether the outer star was linked to the black hole and inner star, the research team looked to a satellite called Gaia that tracked the motion of stars in the galaxy since 2014 and found that the two stars were moved exactly in tandem, the odds of which are about one in 10 million, MIT said.
“It’s almost certainly not a coincidence or accident,” Burdge said. “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.”
Burdge calculated that the outer star is about 3,500 astronomical units from the black hole (one astronomical unit is equal to the distance between Earth and the sun), or about 100 times the distance between Pluto and the sun, MIT said.
The outer star likely would have been knocked away if the black hole had formed from a typical supernova, but its presence and movement indicates the black hole took shape through a different process, which Burdge believed may have been a direct collapse
Burdge ran tens of thousands of simulations to see how the two stars would be affected by a black hole formed by a supernova or by a direct collapse.
“The vast majority of simulations show that the easiest way to make this triple work is through direct collapse,” Burdge said.
Researchers found that the outer star is in the process of becoming a red giant, which occurs toward the end of a star’s life, according to MIT.
The team determined the star and the black hole are about 4 billion years old. For comparison, the Earth formed about 4.5 billion years ago, and the universe is about 13.8 billion years old, according to NASA.
“We’ve never been able to do this before for an old black hole,” Burdge said in the statement. “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.”
Nick Stoico can be reached at nick.stoico@globe.com.