Which blue supergiant is the visible companion star in the Cygnus X-1 system? The visible companion is HDE 226868, an O-type supergiant orbiting the system’s black hole.
HDE 226868 is classified approximately as O9.7 Iab, placing it near the boundary between O- and B-type supergiants. It is extremely hot, with a surface temperature of about 31,000 kelvins, and is far more massive and luminous than the Sun. Its stellar wind supplies gas that the compact companion captures, heats, and emits as X-rays.
The star’s identity was pinned down after radio observations in 1971 accurately located Cygnus X-1. Astronomers then measured Doppler shifts in HDE 226868’s spectrum. Those shifts revealed an unseen companion with a mass too large to be a neutron star, making it one of the earliest strong black-hole candidates.
HDE 226868 is not itself the black hole. That is the key distinction in this question: the star is the optically visible member, while the compact object is detected indirectly through its gravitational influence and the X-rays produced by accreting stellar-wind material. The pair completes an orbit in roughly 5.6 days.