Gaia BH2

Gaia BH2 is a binary system consisting of a red giant and a stellar-mass black hole. Gaia BH2 is located about 3,800 light years away in the constellation of Centaurus, making it as of 2024 the third-closest known black hole system to Earth. Gaia BH2 is the second black hole discovered from Gaia DR3 astrometric data.
The black hole and red giant orbit the system barycentre every 1,277 days, or around 3.5 years, with a moderate eccentricity of 0.518. The black hole's mass is around, which means its Schwarzschild radius should be about. The red giant has a mass of and a radius of. Its temperature is estimated at. The star is enriched in alpha elements, thus is believed to have undergone mass transfer with another star.

Discovery

Gaia BH2 was originally discovered as a black hole binary candidate in 2022, found via astrometric observations with Gaia, along with Gaia BH1. At that time it was not clear if Gaia BH2 did definitely harbour a black hole, but it was the only plausible candidate in the Gaia data other than Gaia BH1. Later radial velocity observations confirmed this black hole system and refined its orbital parameters.