  The evolution of massive stars is influenced by the mass lost to stellar
winds over their lifetimes. These winds limit the masses of the stellar
remnants (such as black holes) that the stars ultimately produce. We use radio
astrometry to refine the distance to the black hole X-ray binary Cygnus X-1,
which we find to be $2.22^{+0.18}_{-0.17}$ kiloparsecs. When combined with
previous optical data, this implies a black hole mass of $21.2\pm2.2$ solar
masses, higher than previous measurements. The formation of such a high-mass
black hole in a high-metallicity system constrains wind mass loss from massive
stars.
