All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. We report here radial-velocity measurements taken over two years of the Galactic B-type star, LB-1

Among the myriad discoveries presented in the second data release of the Gaia mission, there was an enigmatic color-magnitude diagram (CMD) of Milky Way halo stars, showing a striking double (blue/red) sequence. The blue sequence was linked to a major merger that our Galaxy experienced early in its history (Gaia-Enceladus). The origin of the red sequence was unclear, and it was generally associated, because of its chemical composition, with the Milky Way thick disk. However, the lack of accurate ages precluded a clear understanding of its nature. We compared this double-sequenced observed