Earth’s New Galactic Neighbor: a ‘Silent’ Black Hole

Silent. Invisible. Just 1,000 light-years from Earth. And there may be many, many others like it.

This artist’s impression shows the orbits of the objects in the HR 6819 triple system. This system is made up of an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third star in a wider orbit (also in blue). (ESO / L. Calçada)

(CN) — Astronomers did not think anything was staring back at them 1,000 light-years away, but the data suggested only one thing: a black hole.

During the study of a pair of stars in a neighboring star system, astronomers found the third object quietly waiting for them to take notice.

Researchers from the European Southern Observatory say this black hole is unique, because it does not react violently with its environment: it is silent, practically invisible and the closest black hole to Earth. They detail their findings in a study published Wednesday in the journal Astronomy & Astrophysics.

The implications behind the discovery mean we can search for more silent, nearly invisible black holes within our solar system.

Researchers were studying a double-star system, referred to as HR 6819, about 1,000 light-years from Earth, when they discovered the black hole. One star travels in a wide orbit, while the second star is locked in a narrow orbit. Through their analysis they found a gravitational pull from an object with a mass at least four times than that of our sun, according to ESO scientist Thomas Rivinius, who led the study.

There are several likely scenarios that could play out over the next couple of million years, according to Rivinius. The black hole will exhaust all the hydrogen from the orbiting star and could expand or cool down.

“Or it may grow so quickly, the black hole gets into the expanding atmosphere of the supergiant,” Rivinius said in an email. “This is called ‘common envelope.’ If that happens, the black hole may end up with the entire secondary inside.”

In the astronomers’ observations, the star locked in the narrow orbit traveled around an unseen object every 40 days, while the second star remained farther out.

“The observations needed to determine the period of 40 days had to be spread over several months,” said co-author Dietrich Baade, emeritus astronomer at ESO, in a statement accompanying the study.

Other black holes that have appeared in the Milky Way are not as shy as the object discovered by the ESO astronomers. Most are noisy as they emit powerful X-rays. Yet the possibility of other silent black holes is now a reality and the study’s findings can now provide other researchers details on what exactly they can look for in their own observations.

“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” Rivinius said in the statement.

Rivinius said the study’s findings includes the first such triple system.

“The formation of a (black hole) is a violent process, and most models would not have predicted a triple could survive that but would rather fly apart,” Rivinius said by email. “In fact, we see many stars which we think are the disrupted pieces of such systems.”

Neither binoculars nor a telescope is necessary to view HR 6819, according to researchers, because the star system is visible to the naked eye in the Southern Hemisphere.

Researchers observed the system with the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.

Researchers have already set their sights on another system, called LB-1, which is another double-star system that could potentially be a triple-star system.

“The (black hole) in HR 6819 probably is the closest known (black hole) to the sun, and together with LB-1, suggests a population of quiet (black holes),” write the study authors.

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