Astronomer Jifeng Liu of the National Astronomical Observatory of China: "LB-1 is twice as massive as what we thought possible. Now theorists will have to take up the challenge of explaining its formation." Credit:

Astronomers have found a stellar-mass black hole clocking in at around 70 times the mass of the Sun — but according to current models of stellar evolution, its size is impossible.

The Milky Way is estimated to contain 100 million stellar black holes but 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, AFP reported.

“Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution,” he added.

Scientists generally believe that there are two types of black holes.

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. LB-1 is 15,000 light years from Earth, according to the journal Nature.

But 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.

Astronomers are still only beginning to grasp “the abundance of black holes and the mechanisms by which they form,” David Reitze, a physicist at the California Institute of Technology who was not involved in the discovery, told AFP.

Stellar black holes are usually formed in the aftermath of supernova explosions, a phenomenon that occurs when extremely large stars burn out at the end of their lives.

“LB-1’s large mass falls into a range “known as the ‘pair instability gap’ where supernovae should not have produced it,” Reitze said.

“That means that this is a new kind a black hole, formed by another physical mechanism!” LB-1 was discovered by an international team of scientists using China’s sophisticated LAMOST telescope.

Way back in 1783, English natural scientist John Michell (the first person to propose the existence of black holes) suggested that black holes may be detectable if they were orbited by something that does emit light — such as a companion star — which would be tugged around the resulting binary system’s mutual centre of gravity, Science Alert reported.

This is now known as the radial velocity method, and it’s one of the main ways we search for and confirm the existence of hard-to-see exoplanets as they exert a small gravitational influence on their stars. And it can also be used to find other invisible things — such as black holes.

Liu and his colleagues were using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China to search for these wobbly stars, and got a hit on a main-sequence blue giant star.

But it took follow-up observations using the powerful Gran Telescopio Canarias in Spain and the Keck Observatory in the US to reveal the amazing nature of what the scientists had found.


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