Cygnus X-1 contains a 21-solar mass black hole—Implications for massive star winds

The evolution of massive stars is influenced by the mass lost to stellarwinds over their lifetimes. These winds limit the masses of the stellarremnants (such as black holes) that the stars ultimately produce. Weused radio astrometry to refine the distance to the black hole x-raybinary Cygnus X-1, which we found to be 2.22_‒0.17^+0.18 kiloparsecs.When combined with archival optical data, this implies a black hole massof 21.2 ± 2.2 solar masses, which is higher than previousmeasurements. The formation of such a high-mass black hole in ahigh-metallicity system (within the Milky Way) constrains wind mass lossfrom massive stars.