A surprisingly big black hole might have swallowed a star from the inside out, and scientists are baffled
▻http://theconversation.com/a-surprisingly-big-black-hole-might-have-swallowed-a-star-from-the-
NAOC, Chinese Academy of Sciences
About 15,000 light years away, in a distant spiral arm of the Milky Way, there is a black hole about 70 times as heavy as the Sun.
This is very surprising for astronomers like me. The black hole seems too big to be the product of a single star collapsing, which poses questions for our theories of how black holes form.
Our team, led by Professor Jifeng Liu at the National Astronomical Observatories, Chinese Academy of Sciences, has dubbed the mysterious object LB-1.
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LB-1 is the first major result of our search with LAMOST. We saw a star eight times bigger than the Sun, orbiting a dark companion about 70 times as heavy as the Sun. Each orbit took 79 days, and the pair are about one and a half times as far away from each other as Earth and the Sun.
We measured the star’s motion by slight changes in the frequency of the light we detected coming from it, caused by a Doppler shift as the star was moving towards Earth and away from it at different times in its orbit.
We also did the same for a faint glow coming from hydrogen gas around the black hole itself.
How was LB-1 formed? It is unlikely that it came from the collapse of a single massive star: we think that any big star would lose more mass via stellar winds before it collapsed into a black hole.
One possibility is that two smaller black holes may have formed independently from two stars and then merged (or they may still be orbiting each other).
Another more plausible scenario is that one “ordinary” stellar black hole became engulfed by a massive companion star. The black hole would then swallow most of the host star like a wasp larva inside a caterpillar.
The discovery of LB-1 fits nicely with recent results from the LIGO-Virgo gravitational wave detectors, which catch the ripples in spacetime caused when stellar black holes in distant galaxies collide.
The black holes involved in such collisions are also significantly heavier (up to about 50 solar masses) than the sample of active black holes in the Milky Way. Our direct sighting of LB-1 proves that these overweight stellar black holes also exist in our galaxy.
