Astronomers have discovered the largest stellar black hole ever found in the Milky Way, with a mass 33 times that of the Sun, dubbed Gaia-BH3. The presence of an unusual companion star may help explain how this black hole became so massive. The findings were recently published in Astronomy & Astrophysics.
Artistic representation of a high-mass stellar black hole and its companion star's orbit. Image credit: ESO/L. Calçada
Gaia-BH3, located approximately 2000 light-years away from Earth, is the second-closest black hole discovered to date. George Seabroke and colleagues from University College London discovered this stellar black hole using the Gaia space telescope. It formed from the end stages of a dying star.
Most black holes are detected by observing the glowing matter orbiting and falling into them since no light can escape a black hole. However, BH3 is dormant, not consuming any matter. Instead, researchers spotted it by noticing the peculiar motion of a star, which seemed to be orbiting around an empty space.
The star itself is also unusual—it is composed almost entirely of hydrogen and helium. Most stars have at least some heavier elements, formed at the cores of massive stars and distributed throughout space by supernovae. But first-generation stars had low heavy-element content. The composition of BH3's companion star suggests that the massive star that eventually collapsed into BH3 was one of these primordial objects, with an evolution possibly different from today's high-mass stars. This explains why the black hole became so massive. If its evolution had been more typical of regular stars, its size would be hard to account for.
The discovery of such a massive black hole isn't entirely surprising—experiments searching for gravitational waves have already detected signs of them in other galaxies. Gravitational waves are ripples in spacetime caused by the movement of massive objects.
"From the measurements of these gravitational waves, we should have expected to see such black holes in our own galaxy, but we hadn't seen them until now," says Seabroke, suggesting this is just the beginning. "This star is very bright, and usually, if you find something that bright, you'll find darker things."
