An artist’s impression showing what the binary star system VFTS 243 – containing a black hole and a large luminous star orbiting each other – might look like if we observed it closely. (REUTERS)
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WASHINGTON — Astronomers in a galaxy next to our Milky Way have seen what they call a cosmic “needle in a haystack” — a black hole that is not only classified as dormant, but appears to have been born without the explosion of a dying star.
Researchers said Monday that it differs from all other known black holes in that it is “X-ray quiescent” — not emitting powerful X-rays indicative of engulfing nearby material with its strong gravitational force — and that it was not born in a stellar explosion called a supernova.
Black holes are extremely dense objects with a gravitational pull so intense that not even light can escape.
This one, with a mass at least nine times that of our sun, was detected in the Tarantula Nebula region of the Large Magellanic Cloud and is located about 160,000 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
An extremely luminous and hot blue star with a mass about 25 times that of the sun orbits this black hole in a stellar marriage. This so-called binary system is called VFTS 243. The researchers think that the companion star will eventually also become a black hole and could merge with the others.
Dormant black holes, believed to be relatively common, are difficult to detect because they interact very little with their environment. Numerous previously suggested candidates have been debunked with further research, including by members of the team that discovered it.
“The challenge is to find those objects,” said Tomer Shenar, an astronomy researcher at the University of Amsterdam, lead author of the study published in the journal Nature Astronomy. “We discovered a needle in a haystack.”
“It’s the first object of its kind discovered after astronomers searched for decades,” said astronomer and co-author Kareem El-Badry of the Harvard & Smithsonian Center for Astrophysics.
It is the first object of its kind discovered after decades of searching by astronomers.
–Kareem El-Badry, astronomer and co-author of the study
The researchers used six years of observations from the European Southern Observatory’s Very Large Telescope in Chile.
There are different categories of black holes. The smallest, like the newly discovered ones, are so-called stellar black holes, formed by the collapse of massive individual stars at the end of their life cycles. There are also intermediate black holes and the huge supermassive black holes that are at the center of most galaxies.
“Black holes are intrinsically dark objects. They don’t emit light. Therefore, to detect a black hole, we usually look to binary systems in which we see a luminous star moving around a second, undetected object,” study said. co-author Julia Bodensteiner, a postdoctoral researcher at the European Southern Observatory in Munich.
The collapse of massive stars in black holes is generally believed to be accompanied by a powerful supernova explosion. In this case, a star, perhaps 20 times the mass of our sun, blew some of its material into space during its death throes, then collapsed on its own without an explosion.
The shape of its orbit with its companion provides evidence of the lack of an explosion.
“The system’s orbit is almost perfectly circular,” Shenar said.
Had a supernova occurred, the force of the explosion would have kicked the newly formed black hole in a random direction, producing an elliptical rather than circular orbit, Shenar added.
Black holes can starve mercilessly, swallowing up any material – gas, dust and stars – that wanders within their gravitational pull.
“Black holes can only starve mercilessly if there’s something close enough to devour them. Usually we detect them when they receive material from a companion star, a process we call accretion,” Bodensteiner said.
Shenar added: “In so-called dormant black hole systems, the companion is far enough away that the material doesn’t accumulate around the black hole to heat up and emit X-rays. Instead, it is immediately swallowed by the black hole.”