Milky Way’s largest stellar black hole also second-closest to Earth, scientists discover

Astronomers have detected the largest stellar black hole with mass 33 times that of the Sun in the Milky Way galaxy. It is also the second closest black hole to Earth, sitting just 2,000 light years away from the planet.

When a star with more than eight times the Sun’s mass runs out of fuel, it explodes as a supernova and its core collapses to form a stellar black hole.

So far, studies have suggested that there are around 50 suspected or confirmed stellar-mass black holes in the Milky Way, but there could be as many as 100 million in our galaxy alone, according to NASA.

The new discovered stellar black hole “Gaia BH3” has dethroned Cygnus X-1, which is 21 times as massive as the Sun, to become the most massive black hole of stellar origin in the Milky Way. Further, such black holes, on average, are only about 10 times as massive as the Sun. 

“No one was expecting to find a high-mass black hole lurking nearby, undetected so far,” Pasquale Panuzzo, an astronomer from the National Centre for Scientific Research (CNRS) at the Observatoire de Paris – PSL, France, said in a statement. “This is the kind of discovery you make once in your research life,” he added.

Nearly all stellar black holes were found because they exist in binaries or pair up with a companion star. The more massive of the two likely evolves into a black hole, with its companion orbiting it. Gaia BH3 was observed causing an odd ‘wobbling’ motion on the companion star orbiting it. 

The discovery was made when researchers were analysing data from the European Space Agency’s Gaia mission, launched in 2013 with the goal of building the largest, most precise three-dimensional map of our galaxy by surveying nearly two billion objects.

The team next used data from ground-based observatories, including from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO’s VLT located in Chile’s Atacama Desert. 

These observatories studied key properties of the companion star, which allowed astronomers to precisely measure the mass of BH3.

The study also found clues for another question: What explains the large masses for black holes of stellar origin. Stars with an initial mass larger than 30 times that of the Sun are predicted to lose most of their mass during their evolution. They are expected to produce black holes which are below 20 times as massive as the solar body.

Researchers have hypothesised that black holes with such large masses are forged from stars that are metal poor or have very few elements heavier than hydrogen and helium in their chemical composition. 

These metal-poor stars, the researchers explained, may lose less mass over their lifetimes and hence have more material left over to produce high-mass black holes after their death. But there is a dearth of evidence directly linking metal-poor stars to high-mass black holes until now, they added.

Through this study, the team studied the companion star to conclude that it was very metal-poor, suggesting that star that collapsed to form BH3 was also low on its metallic content, adding weight to this theory.

“Although we cannot exclude that this black hole is the result of the merger of two less massive black holes, this discovery strongly supports the scenario where high-mass black holes are remnants of low-metallicity stars,” the researchers wrote in their paper.

Apart from stellar black holes, our galaxy plays host to a supermassive black hole (which sits at the centre of the galaxy and are hundreds of thousands of billion times the Sun’s mass). The supermassive black hole in Milky Way is Sagittarius A*, about 4 million times the mass of the Sun and is 26,000 light years away.

Scientists are also on the hunt for intermediate black holes, which are thought to be one hundred to hundreds of thousands of times or tens of thousands of times the Sun’s mass, and primordial ones, which are therorised to have formed in the first second after the birth of the universe, with masses ranging from 100,000 times less than a paperclip to 100,000 times more than the Sun, according to NASA.