Black holes have a reputation as ravenous monsters that tear apart stars and planets, but most of them are very placid—they're all but invisible out there in the emptiness of space. That can make black holes tough to identify, but astronomers have just spotted a big one. The newly discovered black hole is the largest stellar black hole ever found in our galaxy, and it's in Earth's celestial backyard.
The object in question has been dubbed Gaia BH3, recognizing the key contribution of the ESA's Gaia space observatory. Gaia's mission is to map the locations of stars in the Milky Way to create the most accurate 3D space catalog ever. Astronomers from the Observatoire de Paris at France's National Centre for Scientific Research scoured Gaia data looking for evidence of new black holes, and they hit the jackpot with Gaia BH3. It tips the scales at 33 solar masses, which dwarfs the next largest, Cygnus X-1, at 21 solar masses.
Gaia BH3 is a quiet black hole that is not currently devouring any stars, making it invisible in the electromagnetic spectrum. However, it has a companion star (known as 2MASS J19391872+1455542) in a long, stable orbit. The team, led by astronomer Pasquale Panuzzo, used Gaia data to identify a slight "wobble" in the star's orbit. This telltale signal told the scientists that the star was orbiting a monster black hole. The team was particularly intrigued because this object is in our neighborhood at just 2,000 light-years away. There's only one known to be closer: another Gaia discovery called BH1. It's about nine solar masses and sits 1,500 light-years away.
Astronomers followed this analysis by conducting new observations of Gaia BH3 using ground-based instruments like the European Southern Observatory’s Very Large Telescope (VLT). This confirmed the black hole's impressive mass—for a stellar black hole. Black holes coming from the collapse of a single star have masses up to several dozen times the Sun. Thus, Gaia BH3 is unusually large for a stellar black hole. However, supermassive black holes are a different animal. Sagittarius A* in the center of the Milky Way is about 4 million times more massive than the Sun.
This is not the largest stellar black hole ever spotted, but the others are all in different galaxies. Astronomers have suggested that these beefy black holes are formed from metal-poor stars that lose less mass over their lives. That means the eventual black hole ends up with more mass. The European Southern Observatory says data from BH3 supports this. The Ultraviolet and Visual Echelle Spectrograph instrument on the VLT revealed the companion star has low metallicity, which means the black hole progenitor star probably did as well.
There is a great deal of uncertainty around how large (but not supermassive) black holes can form. Having BH3 so nearby could help advance our understanding of intermediate-mass black holes.
