Astronomers have discovered closest black hole yet in trinary star system

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Artist’s impression showing orbits of the objects in the HR 6819 triple system. There is 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).
Enlarge / Artist’s impression showing orbits of the objects in the HR 6819 triple system. There is 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


Astronomers with the European Southern Observatory (ESO) have discovered a black hole that is the nearest such object yet found, just 1,000 light years away—close enough to be seen with the unaided eye. It is part of a triple star system, dubbed HR 6819, and the ESO scientists believe other members of this class of systems may also harbor black holes that previously were not a high priority for black hole searches. They announced their discovery in a new paper published in the journal Astronomy and Astrophysics.

Scientists think there are far more black holes in the Universe than we have discovered to date—probably hundreds of millions of them, given the age of our Universe—because we can't observe them directly; we can merely infer their presence by their effect on surrounding matter. A black hole's gravitational effects can influence the orbits of nearby stars, for example, or infalling matter can form an accretion disk of hot gas rapidly orbiting the black hole, emitting powerful X-rays. Or an unfortunate star will get too close to a black hole and be torn apart for its trouble, with the infalling remnants also accelerating and heating up to emit X-rays into space.

But the majority of black holes are actually quiet and hence very difficult to detect. This latest discovery offers useful clues about where at least some of the truly dark black holes might be hiding. “One will never get enough telescope time to do a thorough search like that on all objects,” ESO scientist Thomas Rivinius, a co-author on the paper, told Ars. “What you need to do is a staged approach to help you identify candidates, then thin out the candidates list, and only then have a close and detailed look at the remaining ones. Knowing what to look for should put us in a better position to find them.”

The ESO team had been conducting a study of double-star systems, and HR 6819 was included as part of their observational data-gathering since it appeared to be just such a system. But while reviewing their data, the astronomers found clear evidence of an unexpected third object in the system: a black hole that had previously eluded detection.

In a trinary star system, two of the stars orbit each other as a binary pair, while the third star orbits the pair at a greater distance. This ensures that the system is stable, since if the inner and outer orbits were the same size, one of the stars would eventually be ejected from the system. In the case of HR 6819, one of the two visible stars orbits an invisible object every 40 days, while the other visible star orbits farther away. By studying the orbit of the star in the inner pair, the team was able to infer the black hole's presence and also calculate its mass. “An invisible object with a mass at least four times that of the Sun can only be a black hole,” said Rivinius.

“We used to believe that single stars are the most usual ones,” said Rivinius. “In fact, at least for the really massive ones, single stars are probably the rarest.” That's because the greater a star's mass, the less likely it is to be alone, and Rivinius points out that even single massive stars could, in fact, be the survivors of multiple star systems that were “disrupted,” or have fainter companion stars we just can't detect. Trinary systems like HR 6819 are less common, but nor are they extremely rare. Physicists currently believe that the supernovae that give rise to black holes would actually disrupt the structure of multiples. “If a significant number of multiples, however, survive the supernovae, this changes the statistics,” said Rivinius.

“If such a system happens to be in the immediate neighborhood, it is likely common in other regions of the galaxy as well,” said Rivinius. His back-of-the-envelope calculation suggests that there could be 2,500 such systems. That's not going to clear up the large discrepancy between the black holes we've discovered and the number astronomers believe could be out there. “But considering so far we were not aware any such triple could exist, it is quite a step,” he added. The ESO team has already identified a second star system that might also be a trinary with a black hole, although more observational data is needed to confirm this.

The discovery that a black hole can be part of a trinary star system is also relevant because astronomers have suggested that such triple systems could be progenitors of binary systems with two black holes, or a black hole/neutron star pairing. When the partners in those binary systems merge, the violent event emits gravitational waves that can be detected by the LIGO collaboration.

“The problem with LIGO detections is that for two black holes in a normal, lonely binary, it takes a very long time to close in to each other, until they finally merge,” said Rivinius. “In fact, it takes longer than the current age of the Universe, and we really shouldn't see as many mergers as we do, if that was the only mechanism. But the closer they are already, the (much) faster it goes.”

It's known as the Lidov-Kozai mechanism. It occurs when, for example, a close binary inner pair has a circular orbit, but not in the same plane as the outer orbit. This causes the inner orbit to become more “eccentric,” according to Rivinius. “In short, it means the third body can help the two inner ones get close to each other, at least at times,” he said.

However, that is not going to be the case with the HR 6819 trinary system. “The two stars in HR 6819 are not massive enough to explode as a supernova and form a black hole,” said ESO's Dietrich Baade, another co-author. “Therefore, HR 6819 will never harbor two black holes, and it will never be a full equivalent of the progenitors of gravitational wave events. But it is a useful nearby proxy to investigate.”

DOI: Astronomy and Astrophysics, 2020. 10.1051/0004-6361/202038020  (About DOIs).

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