In solving the mysteries of the universe, astronomers have spotted a massive black hole in the Milky Way galaxy with a mass of about 68 suns, it is much much heavier than other stellar-mass black holes in and around the Milky Way, scientists say. That’s not just a record, it’s also a conundrum. According to theory, black holes in our galaxy that form from the explosive deaths of massive stars — as this one likely did — shouldn’t be heavier than about 25 suns.
This black hole named ‘LB-1’ is roughly 15,000 light-years from Earth according to the report published in Nature.
The Milky Way is expected to contain stellar black holes that number around 100 million, that scientists imagined possible, but this LB-1 is twice as large as the largest stellar-mass black hole, according to lead researcher and astronomer Jifeng Liu from the National Astronomical Observatory of China.
“Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution,” Liu said in a statement.
So far, researchers have arrived at two distinct kinds of black holes. The more common ones are stellar black holes, which can be up to 20 times as massive as our Sun (the oddity in question is approx 70x the mass of our Sun). These can form when the core of a massive star collapses in on itself.
The second kind of black hole, supermassive black holes, are at least a million times bigger than the Sun. Their origins? Unknown. Researchers think that the typical star in the Milky Way will shed most of their gas through the stellar wind — naturally preventing the formation of a black hole as massive as LB-1.
“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 [LB-1] is a new kind of black hole, formed by another physical mechanism!”
The observation was made by Liu and his team from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China, seeking out “wobbly stars” that might point to a nearby black hole (which otherwise appears invisible).
The star they spotted was around 35 million years old, clocking in at around eight times the mass of the Sun. It was also orbiting LB-1 every 79 days on what the researchers describe as a “surprisingly circular” orbit.
This circular orbit of LB-1’s companion star is puzzling because there’s no scenario that the scientists could imagine that fits both — the formation of the black hole and the existence of a companion star that’s orbiting it in a circular orbit.
If LB-1 formed when two black holes collided, after which it captured a star, the circular orbit of its companion would be highly unlikely. What’s expected is a very eccentric, elliptical orbit. Sure, time could smooth this orbit out and turn it “more circular”, but that would take longer than the star’s age (so… over 35 million years of time — again, an unlikely possibility, per the study).
A possibility that researchers are pursuing is that LB-1 is a “fallback supernova” — material was ejected from the dying star at some point, after which it fell immediately back into the star, directly producing a black hole. While this is a little-understood, theoretical possibility, scientists haven’t found any direct evidence for black holes formed this way till date.