By james art 
By Will Dunham | Reuters
The conventional wisdom among astronomers is that black holes—those extremely dense objects with gravity so powerful that even light cannot escape—form in the violent explosion, called a supernova, of a massive dying star. But some, it turns out, may have been born in a gentler way.
Researchers have identified a black hole that appears to have formed through the collapse of the core of a large star in its death throes, but without the usual explosion. It has been observed gravitationally bound to two ordinary stars.
Black holes have previously been spotted orbiting another star or another black hole in so-called binary systems. But this is the first known case of a triple system with a black hole and two stars.
This system is located about 7,800 light-years from Earth in the constellation Cygnus. A light year is the distance light travels in one year, 5.9 trillion miles (9.5 trillion km).
This black hole, called V404 Cygni, has been extensively studied since it was confirmed in 1992. It was previously thought to orbit just one other star, but data from the European Space Agency’s Gaia space observatory showed that it has two companions instead .
Researchers said the black hole, with an estimated mass nine times that of our sun, is in the process of eating one of its companions, a star about seven-tenths more massive than the sun. This star orbits the black hole every 6-1/2 days at a distance of only about one-seventh of that which separates the Earth and the sun.
The black hole appears to be sucking material out of this star, which has bloated into what is known as a red giant phase as part of the natural aging process.
The researchers found another star about 1.2 times more massive than the sun, gravitationally bound to these two but quite far away, orbiting them every 70,000 years at a distance 3,500 times greater than that separating Earth and the sun.
The reason why researchers suspect that a black hole has a mild birth process is simple. The ternary system would have collapsed, they said, if the star turned black hole had exploded.
A black hole is thought to form when a large star runs out of nuclear fuel in its core and collapses inward under its own gravitational pull, setting off a massive explosion that blows its outer layers into space. The resulting crushed core forms the black hole.
But some astronomers have proposed another path to black hole formation, called “direct collapse,” in which the star buckles after using up all its fuel but doesn’t explode.
“We call these events ‘failed supernova.’ “Basically, gravitational collapse just happens too fast for the supernova to trigger, and instead you get an implosion — which sounds super dramatic and awesome, but it’s ‘gentle’ in the sense that you don’t eject any matter,” said MIT Astronomer Kevin Burge, lead author of the study, published in the journal Nature.
Researchers estimate that the members of this ternary system formed about 4 billion years ago as ordinary stars.
“The ternary system could not survive if the black hole was born with a natal kick, so this finding tells us that at least some black holes form without a kick – suggesting a quiet implosion rather than an explosive supernova,” added the astronomer from Caltech and study co-author Karim El-Badri.
This system will not have three members forever, given that the black hole is absorbing its nearest neighbor. This suggests that some known binary systems with a black hole and an ordinary star may have initially formed as a ternary system, only to have the black hole swallow one of its partners.
“People have actually predicted that black hole binaries could form mostly through ternary evolution, but there’s never been any direct evidence before,” El-Badri said.
Editor’s note: Reporting by Will Dunham Editing by Rosalba O’Brien
