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Cygnus X-1An intense galactic X-ray source and the first to yield good observational evidence for the existence of a black hole of stellar mass. Identification of Cygnus X-1 in 1971 with the luminous supergiant star HDE 226868 was followed by the discovery of a 5.6-day binary period. Analysis of the optical observations showed the mass of the ‘unseen’ companion star (the X-ray source) to lie in the range 6–15 solar masses, well above the limit allowed for a neutron star (≤ 3 solar masses) or white dwarf (≤ 1.4 solar masses). Support for the view that Cygnus X-1 involves gas accretion from the optical supergiant onto a black hole is provided by the rapid (millisecond) and irregular variability of the X-ray emission.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
Cygnus X-1[′sig·nəs ‚eks ′wən]
A source of x-rays whose intensity varies in an irregular manner, associated with a weak variable radio source and a ninth-magnitude spectroscopic binary star, designated HDE226868, that consists of a blue supergiant and an invisible companion, which may be a black hole. Abbreviated Cyg X-1.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.