Astronomers have discovered what they believe to be the closest black hole to Earth. It is part of a triple-star system designated HR 6819 located 1120 light-years away in the southern constellation of Telescopium. The star system is close enough and bright enough to be visible to the naked eye. The next closest black hole (part of binary star system 1A 0620-00) lies 3457 light-years away in the constellation of Monoceros.
HR 6819 was originally classified as a binary star system. The outermost companion is a Be star with an unknown orbital period. The innermost “star,” however, was discovered to be a binary system itself through radial velocity measurements in 2020. This innermost binary is comprised of a B3 III blue giant star orbiting a 4.2 solar-mass invisible companion every 40.3 days.
By definition, black holes are invisible. They also seem to defy common sense. While every black hole has a finite mass, its volume is always zero. Or to put it another way, black holes are infinitely dense. Without physical dimensions, the effective radius of a black hole is merely a reference to its event horizon, the spherical (or ovoid in the case of spinning black holes) boundary where nothing, not even light, can escape its gravity.
Being invisible, we can only infer their existence by studying their interactions with other celestial bodies within their gravitational influence. In multiple-star systems with close enough orbits, black holes can be detected via X-ray emissions produced when matter from a companion star falls into the black hole.
Or, in the case of HR 6819, we can infer the existence of a black hole by the lack of emissions, particularly visible light. If the newly discovered 4.2 solar-mass companion was another star, it should be visible from this distance.
Additionally, while both neutron stars and black holes are post-collapse remnants of supernovae, this invisible companion’s large size (4.2 solar-masses) eliminates the possibility of it being the former since the theoretical upper size limit for a neutron star is believed to be 2.16 solar-masses (post-collapse).
Thus, by process of elimination, a black hole seems to be the most likely explanation.
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