Cygnus X-1 (HDE 226868)
Coordinates, VLBA/Gaia distance (2.22 kpc), 21.2-M☉ stellar black hole mass, 5.60-day O-supergiant binary, and observing guide for Cygnus X-1.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Optical companion magnitude (HDE 226868) | 8.95 mag | 8.84 to 9.03 mag | Optical V-band photometry (0.06-mag ellipsoidal modulation over 5.60-day orbit) | definitive | General Catalogue of Variable Stars (GCVS) & AAVSO VSX |
| Trigonometric distance (VLBA + Gaia DR3) | 2220 pc | +140 / −130 pc | VLBA radio parallax (Miller-Jones et al. 2021) + Gaia DR3 (7,240 ± 450 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Black hole dynamical mass | 21.2 M☉ | ± 2.2 M☉ | Revised 2.22 kpc distance + spectroscopic/ellipsoidal orbit (O-supergiant = 40.6 +7.7/−7.1 M☉) | definitive | SIMBAD Astronomical Database (CDS) |
| Binary orbital period | 5.599829 d | ± 0.000016 d | Radial-velocity & ellipsoidal photometric ephemeris | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Cygnus X-1 (HDE 226868 X-Ray Black Hole) at RA 19h 58m 21.67s, Dec +35° 12′ 05.8″ within Cygnus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Cygnus X-1, discovered during a suborbital sounding-rocket flight from White Sands in June 1964 and optically linked in 1971–1972 by Louise Webster, Paul Murdin, and Tom Bolton to the 8.95-magnitude blue supergiant star HDE 226868, was the first astronomical source widely accepted as a black hole (and the subject of the famous 1974 scientific wager between Stephen Hawking and Kip Thorne). Located 2.22 ± 0.14 kiloparsecs (7,240 light-years) away in Cygnus, it remains one of the brightest persistent hard X-ray sources in the sky.
Every 5.5998 days, the 40.6-solar-mass O9.7 Iab blue supergiant HDE 226868 and the unseen compact object orbit their common center of mass separated by just 0.24 AU (half of Mercury's distance from the Sun). Because the compact companion weighs 21.2 ± 2.2 solar masses—more than seven times the ~3-solar-mass Tolman–Oppenheimer–Volkoff upper limit for a neutron star—general relativity leaves no alternative to a stellar-mass black hole.
Fierce stellar winds from the tidally distorted O-supergiant are captured into a multi-million-kelvin accretion disk around the black hole, where X-ray continuum-fitting and iron K-alpha reflection lines show that the Cygnus X-1 black hole is spinning at >99% of the maximum relativistic Kerr spin limit (a* > 0.9985).
Practical Visual Observing Guide
- Unlike most black holes, Cygnus X-1's binary companion HDE 226868 shines at visual magnitude 8.95—making it an easy target in any 60–80 mm amateur telescope or 15×70 binoculars!
- To find HDE 226868, locate 3.9-magnitude Eta Cygni (η Cygni) halfway along the neck of the Northern Cross between Sadr (γ Cyg) and Albireo (β Cyg), then move just 0.5° (30 arcminutes) west-northwest to a close pair of 9th-magnitude stars; the northeastern member of that 1-arcminute pair is HDE 226868 / Cygnus X-1.
Catalog Cross-Identifications & Aliases
Common Aliases: Cyg X-1 · HDE 226868 · V1357 Cygni · HIP 98298
| Catalog Authority | Identifier |
|---|---|
| GAIADR3 | 2059383668236814720 |
| HIP | HIP 98298 |
| HD | HDE 226868 |
| SIMBAD | NAME Cyg X-1 |
| WIKIDATA | Q182485 |
Related Objects, Comparisons & Guides
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Northern Summer Milky Way Observing Tour
Summer observing guide from the Summer Triangle (Vega, Altair, Deneb) and Hercules Keystone down the Milky Way rift to the Sagittarius Galactic Center.
Proper Motion, Radial Velocity & Redshift
How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
Scientific Sources & Dataset Provenance
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- Gravitational-Wave Transient Catalog (GWTC-3) (O1–O3b Cumulative Catalog): Abbott, R., et al. (LIGO-Virgo-KAGRA) 2023, PRX, 13, 041039 (GWTC-3: Compact Binary Coalescences) — LIGO Scientific, Virgo, and KAGRA Collaborations
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg