New "Impossible" Black Hole Challenges Astronomers
RandomFactor writes:
15,000 light years from earth, astronomers have discovered a black hole, designated LB-1, that is 70 times the mass of the Sun - over twice the mass scientists thought was possible to exist in the Milky Way.
Scientists generally believe black holes come in two broad categories.
The more common stellar black holes -- up to 20 times more massive than the Sun -- form when the centre of a very big star collapses in on itself.
Supermassive black holes are at least a million times bigger than the Sun and their origins are uncertain.
It is estimated our galaxy contains 100 million stellar black holes, but they aren't believed to get this large.
LB-1 is twice as massive as anything scientists thought possible, said Liu Jifeng, a National Astronomical Observatory of China professor who led the research.
"Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution," he added.
The mass of LB-1 falls into a black hole mass-gap caused by what are referred to as pair-instability supernovae where the star's explosion does not leave behind a stellar remnant.
researchers believed that typical stars in the Milky Way shed most of their gas through stellar winds, preventing the emergence of a black hole the size of LB-1, Liu said.
"Now theorists will have to take up the challenge of explaining its formation," he said in a statement.
LB-1 still has its binary companion and the star is in a highly circular orbit. The most obvious formation possibility, capture of a second black hole similar to GW150914 where two black holes merged, would have caused a highly elliptical orbit that would not have had a chance to settle down due to the star's age.
One possibility, however, could be a fallback supernova, in which material ejected from the dying star falls immediately back into it, resulting in the direct formation of a black hole. This is theoretically possible under certain conditions, but no direct evidence for it currently exists.
Perhaps LB-1, the researchers noted in their paper, could be this direct evidence.
However it formed, LB-1 has suddenly become one of the most interesting objects in the Milky Way, and a flurry of follow-up observations are likely to ensue.
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Journal Reference:
Liu, J., Zhang, H., Howard, A.W. et al. A wide star-black-hole binary system from radial-velocity measurements$.Nature 575,618-621 (2019) doi:10.1038/s41586-019-1766-2
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