Posts Tagged ‘HR 6819’

Dark Black Hole Next Door

May 6, 2020

HR 6819, is a double star system, about 1,000 light-years away, or roughly 9.5 thousand, million, million kilometers, in the Constellation Telescopium. On the scale of the galaxy, it’s next door. Usually Black Holes are detected by the sparks of material furiously accelerated as it falls towards the hole (“Accretion” BH). Copious X-rays are emitted. However, this phenomenon arises typically when two stars are in close orbit, one goes supernova, then implode into a Black Hole, and material keeps transferring from the other star towards the hole.

If the stars are far enough, and one collapses into a Black Hole, there is no reason for a transfer of material. The Black Hole arises because, to our knowledge, gravity will overwhelm any force we know of, if there is enough mass M in a tight enclosure. Notice the “if”. In truth we are not absolutely sure that Black Hole equation, passed a point will behave as General Relativity supposes, because we could only be sure if we are sure we have all of physics figured out. Highly unlikely…

A basic trick used all over astronomy to evaluate the masses of stars and planets, as long as something rotates around them. Gravitational constant has been put equal to unity, to simplify.

But one thing that is clear, and it was already clear to Laplace in the Eighteenth Century, if there is enough mass concentrated, particles of light won’t come out. Laplace waxed lyrical on the subject, until he realized that, thanks to Young’s and others’ work, it looked like light was, after all, a wave, not a particle as Laplace had assumed, following Newton. It was clear how to hold a particle down, with a concentrated mass… But not a wave. So Laplace, assuming now light was a wave, removed Black Holes from late editions of his book!  We can see that the wave-particle perplexity was already causing trouble centuries ago…

The HR 6819 Black Hole can be characterized from its interaction with the two stars of HR 6819 – one that orbits the hole, and the other that orbits this inner pair.

HR 6819 can be seen with just the naked eye from the southern sky. No telescope or binoculars are needed. The 2.2m telescope at La Silla Observatory in Chile reveals the inner of the two visible stars to be orbiting an unseen object every 40 days.

Considering the speed of the orbiting star, and its radius, the invisible object is found to be around four solar masses, more than twice the mass at which a Black Hole is unavoidable in a dark object (a star can have up to hundreds of times the mass of the Sun, but only because raging thermonuclear fire keeps it inflated).

Stars at the end of their lives with a bit more than 1.4 solar masses will implode the Black Hole way. Some think there maybe 10^8 of them in the Milky Way… Which has an area of roughly 10^10 light years… this makes it likely there are other really black Black Holes of the same type closer than that…

Patrice Ayme

***

From the source:

A naked-eye triple system with a nonaccreting black hole in the inner binary,⋆⋆

Abstract:

Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple. A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit. The radial-velocity semi-amplitude of 61.3 km s−1 of the inner star and its minimum (probable) mass of 5.0 M (6.3 ± 0.7 M) imply a mass of the unseen object of ≥4.2 M (≥5.0 ± 0.4 M), that is, a black hole (BH). The spectroscopic time series is stunningly similar to observations of LB-1. A similar triple-star architecture of LB-1 would reduce the mass of the BH in LB-1 from ∼70 M to a level more typical of Galactic stellar remnant BHs. The BH in HR 6819 probably is the closest known BH to the Sun, and together with LB-1, suggests a population of quiet BHs. Its embedment in a hierarchical triple structure may be of interest for models of merging double BHs or BH + neutron star binaries. Other triple stars with an outer Be star but without BH are identified; through stripping, such systems may become a source of single Be stars.


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