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Remember that black hole just 1,000 light years from Earth? Scientists queue up to say it may not exist after all

This year is just one disappointment after another, eh?

The black hole thought to be nearest to Earth – a mere 1,000 or so light years away – may not exist at all, more than a dozen scientists have warned.

The astronomers have written papers, listed below, concluding that the nearby star system HR 6819 does not contain a black hole at all, let alone the closest one to our fragile planet, contrary to a previous study:

  • Is HR 6819 a triple system containing a black hole? – An alternative explanation [PDF] by J. Bodensteiner, T. Shenar, L. Mahy, M. Fabry, P. Marchant, M. Abdul-Masih, G. Banyard, D. M. Bowman, K. Dsilva, A. J. Frost, C. Hawcroft, M. Reggiani, and H. Sana of the Institute of Astronomy, at KU Leuven in Belgium.
  • A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1 [PDF] by Kareem El-Badry and Eliot Quataert of the Department of Astronomy and Theoretical Astrophysics Center, University of California Berkeley, Berkeley, in the US.
  • The nearest discovered black hole is likely not in a triple configuration [PDF] by Mohammadtaher Safarzadeh and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in the US, and Silvia Toonen of the Institute of Gravitational Wave Astronomy, University of Birmingham, England.

Back in May, a paper [PDF] written by folks at the European Southern Observatory (ESO) with the help of others, and published in Astronomy & Astrophysics, said HR 6819 consisted of two bright blue suns – one a B-type star, and the other a Be-type – and a black hole.

Multi-star systems aren’t particularly special, though this one appeared to be – as it would seem to be home to the nearest black hole to Earth. The ESO team said they clocked the hole by analyzing the motion of the blue stars in the system. One appeared to be stationary while the other seemed to be orbiting a third mystery object. The boffins declared the void a black hole, and calculated its mass to be about 4.2 solar masses.

“The basic argument is the same as with planet detecting, [something known as the] 'radial velocity method',” Thomas Rivinius, an ESO astronomer and lead author of the May study, told The Register, explaining how his team came to its conclusion.

“We see one object rushing back and forth within two months, up to 60 kilometres per second. Using both Kepler's and Newton's laws, this gives the smallest possible mass for the counter-body in this orbital motion, but it depends on whether the object we see is a normal star or a highly exotic object.


There's a black hole lurking within 1,000 light years of Earth – and you can see stars circling it with the naked eye


“We know there are two luminous objects in the system. One of the two we see is a normal Be star, that is not disputed. The other might be a normal B star as well. In that case, the fact it has a high radial velocity amplitude must mean that there is a third object and that this third body is a black hole.”

But the other experts don’t agree. After studying the same data, at least some of them believe the blue stars are actually orbiting each other, and there is no black hole. “Rivinius et al found that the Be star was stationary,” Kareem El-Badry, an astronomy PhD student at the University of Berkeley, California, told El Reg.

“In that case, you need a black hole, since the B star needs to be orbiting something. But we showed, by disentangling the composite spectra, that the B star is actually orbiting the Be star. We've shown that all the properties of the system can be explained by a model without a black hole, so there's no reason to posit that one exists."

The team at KU Leuven appeared to support this idea in their paper, too. Rivinius, however, is not convinced they are right, though he acknowledged that both the Berkeley and KU Leuven analysis carry some merit. In order for the opposing view to be correct, it requires the B-type star to have a very low mass and strange properties. It has to be a so-called stripped helium star – the leftover remnants of a sun that has been eaten away by its larger companion solar body.

“It would have been a supergiant just before, and very recently dumped most of its mass onto the Be star, and is now contracting rapidly to become a subdwarf,” Rivinius said of the B-type sun. "Such a contraction phase lasts maybe a few ten-thousand years, and no other object is known to be in such a phase. In scientific terms, it would be the far more extraordinary discovery, as black holes are in fact quite common.”

He did note, however, that although unlikely, this scenario isn't impossible given the data. The best way to settle the argument once and for all is to observe the system using interferometry.

The ESO team has submitted a proposal to use the interferometer instrument on its Very Large Telescope, and if approved, they can begin their study as early as October. Rivinius isn’t hopeful.

“Whether it will be possible to observe HR 6819 this season, before it goes behind the Sun, is unfortunately not certain given the current state of South America," he said, referring to the surge in COVID-19 coronavirus cases on the continent. "Currently, both observing and proposal submission is suspended. We may have to wait until next year.” ®

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