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    Astronomers observe the black hole that could have formed easily

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    The conventional wisdom among astronomers is that black holes – those exceptionally dense objects with gravity so strong that not even light can escape – are formed in the violent explosion, called a supernova, of a massive dying star. But some, it seems, can be born in a gentler way.

    Researchers have identified a black hole that appears to have been created by the collapse of the core of a large star in its death throes, but without the usual explosion. It was observed gravitationally bound to two ordinary stars.

    Black holes they have previously been observed orbiting another star or black hole in what are called binary systems. But this is the first known instance 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).

    An artist’s view of a massive black hole merging into a cluster, possibly like GW190521, an observation of gravitational waves by LIGO/Virgo. (credit: Wikimedia Commons)

    This black hole, called V404 Cygni, has been studied extensively 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 has shown that it has two instead companions.

    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 as massive as the sun. This star orbits the black hole every 6-1/2 days at a distance of only one-seventh of that separating the Earth from the Sun.

    The black hole appears to be siphoning off material from this star, which has swelled in what is called a red giant phase as part of its natural aging process.

    Detecting black holes

    The researchers detected 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 that which separates Earth from Sun.

    The reason why researchers suspect a gentle birth process for the black hole is simple. The triple system would break up, they said, if the star that became a black hole exploded.

    A black hole is thought to form when a large star exhausts the nuclear fuel in its core and collapses inward under its own gravitational pull, setting off a huge explosion that blasts its outer layers into space. The resulting crushed core forms the black hole.

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    But some astronomers have proposed another path to black hole formation called “direct collapse” where the star collapses after using up all its fuel but doesn’t explode.

    “We call these events a ‘failed supernova.’ “Basically, gravitational collapse acts too quickly 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 expel any matter,” he said. Massachusetts Institute of Technology astronomer Kevin Burdge, lead author of the study published in the journal Nature.

    The researchers estimated that the members of this triple system first formed about 4 billion years ago as ordinary stars.

    “The triple system could not have survived if the black hole was born with a natal shock, so this finding tells us that at least some black holes form without a shock – involving a quiet implosion rather than an explosive supernova.” , added Caltech astronomer and study co-author Kareem El-Badry.

    This system will not have three members forever, given that the black hole consumes its nearest neighbor. This suggests that some known binary systems with a black hole and an ordinary star may have initially formed as a triple system, only for the black hole to swallow one of its partners.

    “People have actually predicted that black hole binaries could form mostly through triple evolution, but there’s never been any direct evidence until now,” El-Badry said.



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