A surprisingly enormous black hole has been found in our galaxy

Astronomers have found the largest stellar black hole ever spotted, dubbed Gaia-BH3. It has a mass 33 times that of the sun, dwarfed only by supermassive black holes and those that formed through mergers of other black holes.

At about 2000 light years away, Gaia-BH3 is also the second-nearest black hole to Earth ever discovered. George Seabroke at University College London and his colleagues found this stellar black hole, meaning it formed from a star that had reached the end of its life, using the Gaia space telescope.

No light can escape a black hole, so most of them are found by spotting the glow of the hot material orbiting them and falling in. However, BH3 is dormant, not devouring any material. Instead, the researchers found it by noticing the strange motion of a star that seemed to be orbiting a patch of empty space.

This star itself is unusual, too – it is made almost entirely of hydrogen and helium. Most stars contain at least some heavier elements, which formed in the hearts of massive stars and were distributed throughout space by supernovae. But the first generations of stars would have had very low amounts of heavy elements. The composition of BH3’s partner star suggests that the enormous star that eventually collapsed to form BH3 was also one of these primitive objects, which may have evolved differently from the way massive stars do today. This would explain how the black hole got so huge. Its size would be difficult to account for if it had evolved more like regular stars do.

Finding such a massive black hole wasn’t a complete surprise – experiments that hunt for gravitational waves, which are ripples in space-time caused by the motions of heavy objects, have found signs of them in other galaxies.

“From these gravitational wave measurements, we should be expecting to see such black holes in our own galaxy, but we hadn’t until now,” says Seabroke. And this should just be the beginning, he says: “The star is extremely bright, and generally if you find something this bright, you expect to find many more fainter.”