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Twice as strong as the upper limit: discovered "impossible" black hole

Deep in the Milky Way, researchers find an unusually heavy stellar black hole – with 70 times the mass of our Sun. There should be nothing like it in theory – so how can this be?

About 15,000 light-years deep in the Milky Way, an international research team has detected an unexpectedly heavy black hole: the object with the catalog number LB-1 has about 70 times the mass of our sun. This is more than double the upper limit that astronomers have assumed so far for the so-called stellar black holes that arise from a dying star. The team led by Jifeng Liu of the Chinese Academy of Sciences presents its surprising discovery in the journal "Nature".

At the center of our domestic galaxy – as in most galaxies – is a supermassive black hole with millions of our sun. Furthermore, it is estimated that the Milky Way contains about 100 million stellar black holes. They arise when a very massive star collapses at the end of its existence under its own weight, because the back pressure of nuclear fusion has dried up. However, stars with the typical chemical composition of our galaxy should discharge much of their mass in the form of huge stellar winds before collapsing. Such massive remains as those observed now should not be left behind.

"It shouldn't exist"

"Black holes in this mass should not exist according to most current models of stellar evolution in our galaxy," Liu said in a statement of the Academy. "LB-1 is twice as massive as we thought possible, and now theorists face the challenge of explaining the origins."

Indeed, the Ligo gravitational observatory in the United States had previously detected signals from the fusion of stellar black holes up to 50 solar masses, albeit from galaxies to billions of light years away with various chemical assumptions. The observation of LB-1 now shows that there are apparently heavy black holes in our galaxy, the researchers point out. "This discovery forces us to carefully examine our star-forming models of the black hole," the Academy Director Ligo David Reitze of the University of Florida, who was not involved in the current work, quoted.

Tracked down with makeup

Since black holes usually do not emit light, researchers around Liu used a trick to track down LB-1: the black hole has a companion star that is eight times larger than our Sun and orbits the black hole in 79 days . It moves about its orbit about half the time towards us and the other half slightly away from us. This can be observed in the light of the star. Just like a police siren sounds brighter when the car gets closer and darker as it moves away, the star's light is a little bluer as it moves towards us and slightly shifts. redder when it moves away.

Most of the other known black holes have been discovered by incorporating matter that becomes extremely hot and then glows brilliantly before disappearing into the throat of the gravitational monster.