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Cygnus X-1 - Wikipedia

Cygnus X-1

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Cygnus X-1 (abbreviated Cyg X-1)1 is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole.23 It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3 × 10 W/(m ⋅ Hz) (2.3 × 10 jansky).45 It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 times the mass of the Sun67 and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole.8 If so, the radius of its event horizon has 300 km “as upper bound to the linear dimension of the source region” of occasional X-ray bursts lasting only for about 1 ms.9

Cygnus X-1 is a high-mass X-ray binary system located about 7,000 light-years away,6 that includes a blue supergiant variable star.10 The supergiant and black hole are separated by about 0.2 AU, or 20% of the distance from Earth to the Sun. A stellar wind from the star provides material for an accretion disk around the X-ray source.11 Matter in the inner disk is heated to millions of degrees, generating the observed X-rays.1213 A pair of relativistic jets, arranged perpendicularly to the disk, are carrying part of the energy of the infalling material away into interstellar space.14

This system may belong to a stellar association called Cygnus OB3, which would mean that Cygnus X-1 is about 5 million years old and formed from a progenitor star that had more than 40 solar masses. The majority of the star’s mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.15

Cygnus X-1 was the subject of a friendly scientific wager between physicists Stephen Hawking and Kip Thorne in 1975, with Hawking—betting that it was not a black hole—hoping to lose.16 Hawking conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole in the system.17

Printed 2026-06-28.

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Footnotes

  1. Bowyer, S.; Byram, E. T.; Chubb, T. A.; Friedman, H. (1965). “Cosmic X-ray Sources”. Science. 147 (3656): 394–398. Bibcode:1965Sci…147..394B. doi:10.1126/science.147.3656.394. PMID 17832788. S2CID 206565068. ↩

  2. “Observations: Seeing in X-ray wavelengths”. ESA. 2004-11-05. Retrieved 2008-08-12. ↩

  3. Glister, Paul (2011-11-29). “Cygnus X-1: A Black Hole Confirmed”. Centauri Dreams: Imagining and Planning Interstellar Exploration. Retrieved 2024-09-06. ↩

  4. Lewin, Walter; Van Der Klis, Michiel (2006). Compact Stellar X-ray Sources. Cambridge University Press. p. 159. ISBN 0-521-82659-4. ↩

  5. “2010 X-Ray Sources”. The Astronomical Almanac. U.S. Naval Observatory. Archived from the original on 2010-03-28. Retrieved 2009-08-04. gives a range of 235– 1320 μJy at energies of 2– 10 kEv, where a Jansky (Jy) is 10 Wm Hz. ↩

  6. Miller-Jones, James C. A.; Bahramian, Arash; et al. (2021-03-05). “Cygnus X-1 contains a 21–solar mass black hole—Implications for massive star winds”. Science. 371 (6533): 1046–1049. arXiv:2102.09091. Bibcode:2021Sci…371.1046M. doi:10.1126/science.abb3363. ISSN 0036-8075. PMID 33602863. S2CID 231951746. ↩ ↩2

  7. Overbye, Dennis (18 February 2021). “A Famous Black Hole Gets a Massive Update – Cygnus X-1, one of the first identified black holes, is much weightier than expected, raising new questions about how such objects form”. The New York Times. Retrieved 21 February 2021. ↩

  8. The Illustrated Encyclopedia of the Universe. New York, NY: Watson-Guptill. 2001. p. 175. ISBN 0-8230-2512-8. ↩

  9. Harko, T. (June 28, 2006). “Black Holes”. University of Hong Kong. Archived from the original on February 10, 2009. Retrieved 2008-03-28. ↩

  10. Ziolkowski, Janusz (2014). “Masses of the components of the HDE 226868/Cyg X-1 binary system”. Monthly Notices of the Royal Astronomical Society: Letters. 440: L61. arXiv:1401.1035. Bibcode:2014MNRAS.440L..61Z. doi:10.1093/mnrasl/slu002. S2CID 54841624. ↩

  11. Gies, D. R.; Bolton, C. T. (1986). “The optical spectrum of HDE 226868 = Cygnus X-1. II — Spectrophotometry and mass estimates”. The Astrophysical Journal. 304: 371–393. Bibcode:1986ApJ…304..371G. doi:10.1086/164171. ↩

  12. Nayakshin, Sergei; Dove, James B. (November 3, 1998), “X-rays From Magnetic Flares In Cygnus X-1: The Role Of A Transition Layer”, arXiv:astro-ph/9811059 ↩

  13. Young, A. J.; Fabian, A. C.; Ross, R. R.; Tanaka, Y. (2001). “A Complete Relativistic Ionized Accretion Disc in Cygnus X-1”. Monthly Notices of the Royal Astronomical Society. 325 (3): 1045–1052. arXiv:astro-ph/0103214. Bibcode:2001MNRAS.325.1045Y. doi:10.1046/j.1365-8711.2001.04498.x. S2CID 14226526. ↩

  14. Gallo, Elena; Fender, Rob (2005). “Accretion modes and jet production in black hole X-ray binaries”. Memorie della Società Astronomica Italiana. 76: 600–607. arXiv:astro-ph/0509172. Bibcode:2005MmSAI..76..600G. ↩

  15. Mirabel, I. Félix; Rodrigues, Irapuan (2003). “Formation of a Black Hole in the Dark”. Science. 300 (5622): 1119–1120. arXiv:astro-ph/0305205. Bibcode:2003Sci…300.1119M. doi:10.1126/science.1083451. PMID 12714674. S2CID 45544180. ↩

  16. “Inside Einstein’s Mind”. Nova. Season 42. Episode 23. 25 Nov 2015. Event occurs at 43:54. PBS. Kip Thorne: Stephen Hawking had a terribly deep investment in it actually being a black hole, and so he made the bet against himself as an insurance policy, so at least he would get something out of it, if Cygnus X-1 turned out not to be a black hole. ↩

  17. “Galaxy Entree or Main Course?”. Swinburne University. February 27, 2004. Retrieved 2008-03-31. ↩

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