On the Distance to Cygnus X-1 (HDE 226868)

Abstract

From U, B, V photometry of 104 stars in a field of radius ~ 30′ centered on the X-ray binary star Cyg X-1 (HDE 226868), we have studied the color excess E(B–V) as a function of distance. Spectral types were observed de novo for 42 of these stars. We conclude that HDE 226868 cannot be nearer than 1 kpc, and is probably at a distance of 2.5 kpc or more. The primary component is therefore a luminous OB star of mass ~30 M⊙, and the X-ray component has a minimum mass ~6 M⊙.