For the first time ever, the NASA/ESA (European Space Agency) Hubble Space Telescope has provided direct evidence for a lone black hole drifting through space by precise mass measurements of the phantom object. Until now, measurements of black holes have been inferred statistically or through interactions with binary systems or the core of galaxies. This meant that large black holes are only found with companion stars, unlike in this case.
This wandering black hole is about 5,000 light years away from Earth, in the Carina-Sagittarius arm of our galaxy. But with its discovery, astronomers estimate that the nearest isolated stellar-mass black hole might be as close as 80 light-years away. To put that into perspective, Proxima Centauri, the closest star to earth, is about 4.2 light-years away.
Stellar black holes are formed when massive stars collapse upon themselves. The collapse of such a star causes a supernova. The remaining core is then crushed by gravity into becoming a black hole. Since the self-detonation of such stars is not perfectly symmetrical, these black holes might get a “kick” which could propel them in one direction at great speeds.
Telescopes can’t really photograph such black holes because they don’t emit light. But since they warp space, they deflect, distort and amplify any starlight that lines up exactly behind it. Ground-based telescopes look for such brightening which could be a tell-tale sign of a massive object passing between us and the star. This phenomenon is called microlensing. The Hubble follows up with its own observations.
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The Hubble was used to measure the exact amount of microlensing by the black hole since it is capable of the sort of precision required for such measurements. The star’s image was offset from what should have been its position by about a milliarcsecond. This is the cosmic equivalent of measuring the height of an adult human lying on the surface of the moon from Earth.
But one of the teams reporting on the cosmic object proposes the possibility of the object having a slightly lower mass range than stellar black holes. They estimated that the mass of the invisible object may be between 1.6 times and 4.4 times the mass of our sun. At the lower end of that range, this object would be a neutron star but at the higher end, it would be a black hole.