Cygnus X-1
Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1965 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3×10−23 W/(m2⋅Hz) (2.3×103 jansky). 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 Sun and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. 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.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 19h 58m 21.67574s |
| Declination | +35° 12′ 05.7845″ |
| Apparent magnitude (V) | 8.95 |
| Characteristics | |
| Spectral type | O9.7Iab |
| U−B color index | −0.30 |
| B−V color index | +0.81 |
| Variable type | Ellipsoidal variable |
| Astrometry | |
| Radial velocity (Rv) | −2.70±3.2 km/s |
| Proper motion (μ) | RA: −3.812±0.015 mas/yr Dec.: −6.310±0.017 mas/yr |
| Parallax (π) | 0.4439 ± 0.0149 mas |
| Distance | 7,300 ± 200 ly (2,250 ± 80 pc) |
| Absolute magnitude (MV) | −6.5±0.2 |
| Details | |
| Mass | 21.2 M☉ |
| Radius | 20–22 R☉ |
| Luminosity | 3–4×105 L☉ |
| Surface gravity (log g) | 3.31±0.07 cgs |
| Temperature | 31000 K |
| Rotation | every 5.6 days |
| Age | 5 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Cygnus X-1 belongs to a high-mass X-ray binary system, located about 2.22 kiloparsecs from the Sun, that includes a blue supergiant variable star designated HDE 226868, which it orbits at 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. Matter in the inner disk is heated to millions of degrees, generating the observed X-rays. A pair of relativistic jets, arranged perpendicularly to the disk, are carrying part of the energy of the infalling material away into interstellar space.
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.
Cygnus X-1 was the subject of a friendly scientific wager between physicists Stephen Hawking and Kip Thorne in 1975, with Hawking, hoping to lose, betting that it was not a black hole. He conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole in the system. As of 2004, this hypothesis lacked direct empirical evidence but was generally accepted based on indirect evidence.