CYGNUS X-1 = HDE 226868

Cygnus X-1 is centered in the picture. The visible star, companion to the black hole X-ray source, is HDE 226868, a ninth magnitude class O (O7.5) supergiant. Just to the right of center is the fourth magnitude (3.89) K0 giant Eta Cygni. The hot fifth magnitude (4.93) blue B2.5 dwarf 28 Cygni hovers at the upper left edge. NGC 6871 is a large open cluster at the heart of the loose, unbound Cyg OB3 association, to which Cyg X-1 seems to belong. Sh2-101 is a small, diffuse, reddish, ionized interstellar nebula. Note the dark, cold interstellar dust clouds that block the background stars that surround Cyg X-1, which dim the light of the bright supergiant by 3 1/2 magnitudes. Were the dust not there, the star would be fifth magnitude and readily visible to the naked eye.