The largest stellar black hole in the Milky Way galaxy has been found by the European Space Agency’s Gaia mission. And it is extraordinarily close to Earth.
Named as Gaia BH3, it is 33 times the mass of our Sun, more than 50% bigger than the next biggest stellar black hole Cygnus X-1 which is 21 solar masses.
Larger black holes exist in our galaxy, but none of these were formed by the collapse of a massive star’s core.
For example, Sagittarius A* – the supermassive black hole at the centre of the Milky Way – is estimated to be about 4.2 million times that of the Sun. Astronomers have also found several intermediate-mass black holes ranging from a few hundred to tens of thousands of times the mass of the Sun. These are too large to have formed from of the collapse of a star. How they formed, however, remains a mystery.
Like all black holes, Gaia BH3’s gravity is so immense that light cannot escape its pull, meaning it cannot be observed directly. It was spotted in Gaia’s data because of the ‘wobbling’ that it caused on a star orbiting the object.
BH3 is remarkably close – it is the second closest black hole to Earth that has been confirmed. It is 2,000 light-years away in the constellation Aquila. The black hole’s discovery is detailed in a paper published in the journal Astronomy & Astrophysics.
“No one was expecting to find a high-mass black hole lurking nearby, undetected so far,” says first author and Gaia collaboration member, Pasquale Panuzzo from the Observatoire de Paris, part of France’s National Centre for Scientific Research (CNRS). “This is the kind of discovery you make once in your research life.”
The astronomers confirmed the existence of the black hole using ground-based observatories.
Similarly large stellar black holes have been spotted outside the Milky Way using different methods.
Astrophysicists believe that such large stellar black holes form from stars with small amounts of elements heavier than hydrogen and helium. These so-called “metal-poor” stars remain larger as they near the end of their lives meaning they can produce high-mass black holes when they die.
Direct evidence linking metal-poor stars to massive stellar black holes has been lacking until now.
Stars in pairs tend to have similar compositions. The team was able to determine that the star which formed BH3 must have been metal poor because its companion star is low in heavy elements as predicted.
