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J Liu, H Zhang, AW Howard, Z Bai, Y Lu, R Soria, S Justham, X Li, Z Zheng, T Wang, K Belczynski, J Casares, W Zhang, H Yuan, Y Dong, Y Lei, H Isaacson, S Wang, Y Bai, Y Shao, Q Gao, Y Wang, Z Niu, K Cui, C Zheng, X Mu, L Zhang, W Wang, A Heger, Z Qi, S Liao, M Lattanzi, WM Gu, J Wang, J Wu, L Shao, R Shen, X Wang, J Bregman, R Di Stefano, Q Liu, Z Han, T Zhang, H Wang, J Ren, J Zhang, J Zhang, X Wang, A Cabrera-Lavers, R Corradi, R Rebolo, Y Zhao, G Zhao, Y Chu and X Cui
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 Sun1-4. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star-black-hole binaries5,6. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. Here we report radial-velocity measurements taken over two years of the Galactic B-type star, LB-1. We find that the motion of the B star and an accompanying Hα emission line require the presence of a dark companion with a mass of [Formula: see text] solar masses, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories.
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