Gaia BH2: The Quiet Black Hole in the Milky Way’s Hidden Population

We combine Gaia astrometry, ground-based spectroscopy, and TESS asteroseismology to study Gaia BH2, a red-giant star orbited by a dormant black hole of about 8.9 solar masses in a 1,277-day orbit—the widest known black-hole binary. The star’s oscillations provide an independent mass, confirming the black hole, while its alpha-enhanced chemistry hints at an ancient origin. This system implies a vast hidden population of quiet, wide binaries and previews how Gaia DR4/DR5 will map the Milky Way’s missing mass.

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