Editing Cygnus X-1

{{short description|Galactic X-ray source in the constellation Cygnus that is very likely a black hole}}
{{Other uses}}
{{Sky|19|58|21.6756|+|35|12|05.775|6000}}
{{featured article}}
{{Starbox begin
| name=Cygnus X-1}}
{{Starbox image
 | image=
   {{Location mark
     | image=Cygnus IAU.svg
     | float=center | width=250 | position=right
     | mark=Red circle.svg | mark_width=10 | mark_link=Cyg X-1
     | x%=62.0 | y%=61.0
   }}
 | caption=Location of Cygnus X-1 (circled)
}}
{{Starbox observe
| epoch=J2000
| constell=[[Cygnus (constellation)|Cygnus]]<ref name=constellation/>
| ra={{RA|19|58|21.67574}}<ref name=GaiaDR3/>
| dec={{DEC|+35|12|05.7845}}<ref name=GaiaDR3/>
| appmag_v=8.95<ref name=SIMBAD/>
}}
{{Starbox character
| class=O9.7Iab<ref name=SIMBAD/>
| b-v=+0.81<ref name=lob647/>
| u-b=−0.30<ref name=lob647/>
| variable=[[Ellipsoidal variable]]
}}
{{Starbox astrometry
| radial_v={{val|-2.70|3.2}}<ref name=SIMBAD/>
| prop_mo_ra={{val|-3.812|0.015}}
| prop_mo_dec={{val|-6.310|0.017}}
| pm_footnote=<ref name=GaiaDR3/>
| parallax=0.4439
| p_error=0.0159
| parallax_footnote=<ref name=GaiaDR3/>
| absmag_v={{val|-6.5|0.2}}<ref name=apj321/>
}}
{{Starbox detail
 | source = <ref name=Ramachandran2025/>
 |component1=Cygnus X-1
 | mass=13.8 to {{val|17.5|2.0|1.0}}
}}
{{Starbox detail|no_heading=y
| component1=HDE 226868
| mass = {{val|29|6|3}}
| radius={{val|22.9|1.5|2.5}}
| luminosity={{val|320000|82000|65000|fmt=commas}}
| temperature={{val|28500|1000|fmt=commas}}
| gravity={{val|3.17|0.10}}
| metal=
| rotation=
| age_myr=4.8-7.6<ref name=wong2012/>
}}
{{Starbox catalog
| names= {{odlist|V=V1357 Cygni|BD=+34°3815|HD=226868, HDE 226868|HIP=98298|SAO=69181}}<ref name=SIMBAD/>
}}
{{Starbox reference
|Simbad=Cyg+X-1
}}
{{Starbox end}}

'''Cygnus&nbsp;X-1''' (abbreviated '''Cyg&nbsp;X-1''')<ref name=science3656/> is a galactic [[Astrophysical X-ray source|X-ray source]] in the [[constellation]] [[Cygnus (constellation)|Cygnus]] and was the first such source widely accepted to be a [[black hole]].<ref name=esa20041105/><ref name=glister2011/> It was discovered in 1964 during a [[Sub-orbital spaceflight|rocket flight]] and is one of the [[Signal strength|strongest]] X-ray sources detectable from Earth, producing a peak X-ray [[flux density]] of {{val|2.3|e=-23|u=[[Watt|W]]/([[Meter|m]]<sup>2</sup>⋅[[Hertz|Hz]])}} ({{val|2.3|e=3|u=[[jansky]]}}).<!-- Units are taken from the cited source. See the Jansky article before changing. --><ref name=lewin_vanderklis2006/><ref name=usno2010/> It remains among the most studied [[astronomical object]]s in its class. The compact object is now estimated to have a mass about 21.2 times the [[Solar mass|mass of the Sun]]<ref name="SCI-20210218" /><ref name="NYT-20210218" /> and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole.<ref name=encyclopedia/> If so, the radius of its [[event horizon]] has {{val|300|ul=km}} "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1&nbsp;ms.<ref name=harko20060628/>

Cygnus&nbsp;X-1 belongs to a high-mass [[X-ray binary]] system, located about  7,000 [[light-year]]s away,<ref name="SCI-20210218" /> that includes a blue [[Supergiant star|supergiant]] [[variable star]] designated '''HDE&nbsp;226868''',<ref name=ziolkowski2014/> which it orbits at about 0.2&nbsp;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.<ref name=apj304_371/> Matter in the inner disk is heated to millions of degrees, generating the observed X-rays.<ref name=nayashin_dove1998/><ref name=mnras325_3_1045/> A pair of [[relativistic jet]]s, arranged [[perpendicular]]ly to the disk, are carrying part of the energy of the infalling material away into interstellar space.<ref name=mdsai76_600/>

This system may belong to a [[stellar association]] called Cygnus&nbsp;OB3, which would mean that Cygnus&nbsp;X-1 is about 5&nbsp;million years old and formed from a [[wiktionary:progenitor|progenitor]] star that had more than {{val|40|u=[[solar mass]]es}}. 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.<ref name=science300_5622_1119/>

Cygnus&nbsp;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.<ref name=episode/> Hawking conceded the bet in 1990 after observational data had strengthened the case that there was indeed a [[black hole]] in the system.<ref name=su20040227/>

==Discovery and observation==
Observation of X-ray emissions allows [[astronomer]]s to study celestial phenomena involving gas with temperatures in the millions of degrees. However, because X-ray emissions are blocked by [[Atmosphere of Earth|Earth's atmosphere]], observation of [[X-ray astronomy|celestial X-ray sources]] is not possible without lifting instruments to altitudes where the X-rays can penetrate.<ref name=herbert2002/><ref name=apj611_2_1084/> Cygnus&nbsp;X-1 was discovered using [[X-ray astronomy detector|X-ray instruments]] that were carried aloft by a [[Sub-orbital spaceflight|sounding rocket launched]] from [[White Sands Missile Range]] in [[New Mexico]]. As part of an ongoing effort to map these sources, a survey was conducted in 1964 using two [[Aerobee]] suborbital rockets. The rockets carried [[Geiger counters]] to measure X-ray emission in [[wavelength]] range 1–{{val|15|u=[[Ångström|Å]]}} across an 8.4° section of the sky. These instruments swept across the sky as the rockets rotated, producing a map of closely spaced scans.<ref name=science3656/>

As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg&nbsp;XR-1 (later Cyg&nbsp;X-1) in the constellation Cygnus. The [[Celestial coordinate system|celestial coordinates]] of this source were estimated as [[right ascension]] 19<sup>h</sup>53<sup>m</sup> and [[declination]] 34.6°. It was not associated with any especially prominent [[radio astronomy|radio]] or [[light|optical]] source at that position.<ref name=science3656/>

Seeing a need for longer-duration studies, in 1963 [[Riccardo Giacconi]] and [[Herbert Gursky|Herb Gursky]] proposed the first orbital satellite to study X-ray sources. [[NASA]] launched their [[Uhuru (satellite)|Uhuru]] satellite in 1970,<ref name=heasarc20030626/> which led to the discovery of 300 new X-ray sources.<ref name=giacconi20021208/> Extended Uhuru observations of Cygnus&nbsp;X-1 showed fluctuations in the X-ray intensity that occurs several times a second.<ref name=apj166_L1/> This rapid variation meant that the X-ray generation must occur over a compact region no larger than ~{{val|e=5|u=km}} (roughly the size of [[Jupiter]]),<ref>This is the distance light can travel in a third of a second.</ref> as the [[speed of light]] restricts communication between more distant regions.

In April–May 1971, Luc Braes and George K. Miley from [[Leiden Observatory]], and independently Robert M.&nbsp;Hjellming and Campbell Wade at the [[National Radio Astronomy Observatory]],<ref name=apj168_L91/> detected radio emission from Cygnus&nbsp;X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2&nbsp;+35 1910 = HDE&nbsp;226868.<ref name=nature232_5308_246/><ref name=vrsb9_100_173/> On the [[celestial sphere]], this star lies about half a [[Degree (angle)|degree]] from the [[apparent magnitude|4th-magnitude]] star [[Eta Cygni]].<ref name=bernard_stecker1999/> It is a supergiant star that is by itself incapable of emitting the observed quantities of X-rays. Hence, the star must have a companion that could heat gas to the millions of degrees needed to produce the radiation source for Cygnus&nbsp;X-1.

[[Betty Louise Turtle|Louise Webster]] and [[Paul Murdin]], at the [[Royal Observatory, Greenwich|Royal Greenwich Observatory]],<ref name=nature235_2_37/> and [[Tom Bolton (astronomer)|Charles Thomas Bolton]], working independently at the [[University of Toronto]]'s [[David Dunlap Observatory]],<ref name=nature235_2_271/> announced the discovery of a massive hidden companion to HDE&nbsp;226868 in 1972. Measurements of the [[Doppler shift]] of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters.<ref name=luminet1992/> Based on the high predicted mass of the object, they surmised that it may be a [[black hole]], as the largest possible [[neutron star]] cannot exceed three times the [[solar mass|mass of the Sun]].<ref name=aaa305_871/>

With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus&nbsp;X-1 was most likely a black hole.<ref name=rolston1997/><ref name=apl16_1_9/> More precise measurements of Cygnus&nbsp;X-1 demonstrated variability down to a single [[millisecond]]. This interval is consistent with [[turbulence]] in a disk of accreted matter surrounding a black hole—the [[accretion disk]]. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.<ref name=apj189_77/>

[[File:Cygnus x1 xray.jpg|right|thumb|X-ray image of Cygnus&nbsp;X-1 taken by a balloon-borne telescope, the [[High-Energy Replicated Optics]] (HERO) project]]
Cygnus&nbsp;X-1 has since been studied extensively using observations by orbiting and ground-based instruments.<ref name=SIMBAD/> The similarities between the emissions of X-ray binaries such as HDE&nbsp;226868/Cygnus&nbsp;X-1 and [[active galactic nuclei]] suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated [[Relativistic jet|jets]].<ref name=mnras372_3_1366/> For this reason, Cygnus&nbsp;X-1 is identified among a class of objects called [[microquasar]]s; an analog of the [[quasar]]s, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE&nbsp;226868/Cygnus&nbsp;X-1 may lead to further insights into the mechanics of [[active galaxy|active galaxies]].<ref name=brainerd20050720/>

==Binary system==
The [[black hole]] and [[blue supergiant]] star form a [[binary star|binary system]] in which they orbit around their [[center of mass]] every 5.599829 days.<ref name=aaa343_861/> From the perspective of Earth, the compact object never goes behind the other star; in other words, the system does not [[eclipse]]. However, the [[orbital inclination|inclination of the orbital plane]] to the [[Line-of-sight propagation|line of sight]] from Earth remains uncertain, with predictions ranging from 27° to 65°. A 2007 study estimated the inclination as {{val|48.0|6.8|s=°}}, which would mean that the [[semi-major axis]] is about {{val|0.2|u=[[Astronomical Unit|AU]]}}, or 20% of the distance from Earth to the Sun. The [[orbital eccentricity]] is thought to be only {{val|0.018|0.002}}, meaning a nearly circular orbit.<ref name=orosz2011/><ref name=apj200/> The system is expected to merge into a single [[black hole]] in five billion years, possibly generating [[gravitational wave]]s during the proccess.<ref name=Ramachandran2025/>

[[File:V1357CygLightCurve.png|thumb|right|A [[Photometric_system#Photometric_letters|blue-band]] [[light curve]] for Cygnus X-1, adapted from Kemp ''et al.'' (1987)<ref name="Kemp"/>]]
The HDE&nbsp;226868/Cygnus&nbsp;X-1 system shares a common motion through space with an association of massive stars named Cygnus&nbsp;OB3, which is roughly 7,000&nbsp;[[light-year]]s distant. This implies that HDE&nbsp;226868, Cygnus&nbsp;X-1 and this [[stellar association|OB association]] may have formed at the same time and location. If so, then the age of the system is about {{val|5|1.5|u=million years}}. The motion of HDE&nbsp;226868 with respect to Cygnus&nbsp;OB3 is {{val|9|3|ul=km/s}}, a typical value for random motion within a stellar association. HDE&nbsp;226868 is about {{val|60|u=parsecs}} from the center of the association and could have reached that separation in about {{val|7|2|u=million years}}—which roughly agrees with estimated age of the association.<ref name=science300_5622_1119/> The distance to Cyg X-1 is calculated by trigonometric parallax as {{Convert|1860|+/-|120|pc|ly|abbr=off|lk=on}},<ref name=apj742_2/> and by radio astrometry as {{Convert|2220|+/-|170|pc|ly}}.<ref name="SCI-20210218" />

With a [[galactic latitude]] of 4° and [[galactic longitude]] 71°,<ref name=SIMBAD/> this system lies inward along the same [[Orion Arm|Orion Spur]], in which the Sun is located within the [[Milky Way]],<ref name=apj167_L15/> near where the spur approaches the [[Carina–Sagittarius Arm|Sagittarius Arm]]. Cygnus&nbsp;X-1 has been described as belonging to the Sagittarius Arm,<ref name=goebel/> though the structure of the Milky Way is not well established.

===Compact object===
From various techniques, the mass of the compact object appears to be greater than the maximum mass for a [[neutron star]]. Stellar evolutionary models suggest a mass of {{val|20|5|u=solar masses}},<ref name=MNRAS358_3_851/> while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object yielded a more precise value of {{val|14.8|1|u=solar masses}}. In all cases, the object is most likely a black hole<ref name=orosz2011/><ref name=esa070516/>—a region of space with a [[gravity|gravitational field]] that is strong enough to prevent the escape of [[electromagnetic radiation]] from the interior. The boundary of this region is called the [[event horizon]] and has an effective radius called the [[Schwarzschild radius]], which is about {{val|44|u=km}} for Cygnus&nbsp;X-1. Anything (including [[matter]] and [[photon]]s) that passes through this boundary is unable to escape.<ref name=mit20060109/>  Measurements published in 2021 yielded an estimated mass of {{val|21.2|2.2|u=solar masses}}.<ref name="SCI-20210218" /><ref name="NYT-20210218" /> A 2025 study estimate a mass of 17.5 [[solar mass]]es, which may be just an upper limit, with another method yielding {{solar mass|13.8}}.<ref name=Ramachandran2025/>

Evidence of just such an event horizon may have been detected in 1992 using [[ultraviolet]] (UV) observations with the [[High Speed Photometer]] on the [[Hubble Space Telescope]]. As self-luminous clumps of matter spiral into a black hole, their radiation is emitted in a series of pulses that are subject to [[gravitational redshift]] as the material approaches the horizon. That is, the [[wavelength]]s of the radiation steadily increase, as predicted by [[general relativity]]. Matter hitting a solid, compact object would emit a final burst of energy, whereas material passing through an event horizon would not. Two such "dying pulse trains" were observed, which is consistent with the existence of a black hole.<ref name=pasp113/>

[[File:Chandra image of Cygnus X-1.jpg|thumb|right|[[Chandra X-ray Observatory]] image of Cygnus&nbsp;X-1]]

The spin of the compact object is not yet well determined. Past analysis of data from the space-based [[Chandra X-ray Observatory]] suggested that Cygnus&nbsp;X-1 was not rotating to any significant degree.<ref name=mgmgr10/><ref name=roy_watzke2003/> However, evidence announced in 2011 suggests that it is rotating extremely rapidly, approximately 790 times per second.<ref name="rapid rotation"/>

====Formation====
The largest star in the Cygnus&nbsp;OB3 association has a mass 40 times that of the Sun. As more massive stars evolve more rapidly, this implies that the progenitor star for Cygnus&nbsp;X-1 had more than 40 solar masses. Given the current estimated mass of the black hole, the progenitor star must have lost over 30 solar masses of material. Part of this mass may have been lost to HDE&nbsp;226868, while the remainder was most likely expelled by a strong stellar wind. The [[helium]] enrichment of HDE&nbsp;226868's outer atmosphere may be evidence for this mass transfer.<ref name=mnras341_2_385/> Possibly the progenitor may have evolved into a [[Wolf–Rayet star]], which ejects a substantial proportion of its atmosphere using just such a powerful stellar wind.<ref name=science300_5622_1119/>

If the progenitor star had exploded as a [[supernova]], then observations of similar objects show that the remnant would most likely have been ejected from the system at a relatively high velocity. As the object remained in orbit, this indicates that the progenitor may have collapsed directly into a black hole without exploding (or at most produced only a relatively modest explosion).<ref name=science300_5622_1119/>

====Accretion disk====
[[File:Cygx1 spectrum.jpg|right|thumb|A [[Chandra X-ray Observatory|Chandra]] X-ray spectrum of Cygnus&nbsp;X-1 showing a characteristic peak near {{val|6.4|ul=keV}} due to [[ion]]ized [[iron]] in the accretion disk, but the peak is gravitationally red-shifted, broadened by the [[Doppler effect]], and skewed toward lower energies<ref name=chandra20060830/>]]

The compact object is thought to be orbited by a thin, flat disk of accreting matter known as an [[accretion disk]]. This disk is intensely heated by friction between ionized gas in faster-moving inner orbits and that in slower outer ones. It is divided into a hot inner region with a relatively high level of ionization—forming a [[Plasma (physics)|plasma]]—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius,<ref name=mnras325_3_1045/> or about 15,000&nbsp;km.

Though highly and erratically variable, Cygnus&nbsp;X-1 is typically the brightest persistent source of [[hard X-ray]]s—those with energies from about 30 up to several hundred kiloelectronvolts—in the sky.<ref name=apj611_2_1084/> The X-rays are produced as lower-energy photons in the thin inner accretion disk, then given more energy through [[Compton scattering]] with very high-temperature [[electron]]s in a geometrically thicker, but nearly transparent [[stellar corona|corona]] enveloping it, as well as by some further reflection from the surface of the thin disk.<ref name=apj484_1_375/> An alternative possibility is that the X-rays may be Compton-scattered by the base of a jet instead of a disk corona.<ref name=asr38_12_2810/>

The X-ray emission from Cygnus&nbsp;X-1 can vary in a somewhat repetitive pattern called [[quasi-periodic oscillations]] (QPO). The mass of the compact object appears to determine the distance at which the surrounding plasma begins to emit these QPOs, with the emission radius decreasing as the mass decreases. This technique has been used to estimate the mass of Cygnus&nbsp;X-1, providing a cross-check with other mass derivations.<ref name=apj678_2_1230/>

Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus&nbsp;X-1.<ref name=science297_5583_947/><ref name=wen1998/> The [[pulsar|pulsations from neutron stars]] are caused by the neutron star's rotating magnetic field, but the [[no-hair theorem]] guarantees that the magnetic field of a black hole is exactly aligned with its rotation axis and thus is static. For example, the X-ray binary [[V&nbsp;0332+53]] was thought to be a possible black hole until pulsations were found.<ref name=apj2le288_L45/> Cygnus&nbsp;X-1 has also never displayed X-ray bursts similar to those seen from neutron stars.<ref name=ag44_6_77/> Cygnus&nbsp;X-1 unpredictably changes between two X-ray states, although the X-rays may vary continuously between those states as well. In the most common state, the X-rays are "hard", which means that more of the X-rays have high energy. In the less common state, the X-rays are "soft", with more of the X-rays having lower energy. The soft state also shows greater variability. The hard state is believed to originate in a corona surrounding the inner part of the more opaque accretion disk. The soft state occurs when the disk draws closer to the compact object (possibly as close as {{val|150|u=km}}), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus&nbsp;X-1 transitions back to the hard state.<ref name=apj626_2_1015/>

The spectral transition of Cygnus X-1 can be explained using a two-component [[advection|advective]] flow solution, as proposed by Chakrabarti and Titarchuk.<ref name="CHTI_1"/> A hard state is generated by the inverse Comptonisation of seed photons from the Keplarian disk and likewise synchrotron photons produced by the hot electrons in the centrifugal-pressure–supported boundary layer ([[CENBOL]]).<ref name="CHM_1"/>

The X-ray flux from Cygnus&nbsp;X-1 varies periodically every 5.6&nbsp;days, especially during [[Conjunction (astronomy)|superior conjunction]] when the orbiting objects are most closely aligned with Earth and the compact source is the more distant. This indicates that the emissions are being partially blocked by circumstellar matter, which may be the stellar wind from the star HDE&nbsp;226868. There is a roughly 300-day periodicity in the emission, which could be caused by the [[precession]] of the accretion disk.<ref name=apj531_1_546/>

====Jets====
[[File:Tulip and Cygnus X1.png|right|thumb|The picture shows a curved bow shock structure resulting from the Cygnus X-1 accretion disk jet interacting with a dense interstellar cloud]]
As accreted matter falls toward the compact object, it loses [[gravitational energy|gravitational potential energy]]. Part of this released energy is dissipated by [[Astrophysical jet|jets]] of particles, aligned [[perpendicular]] to the accretion disk, that flow outward with [[Special relativity|relativistic]] velocities (that is, the particles are moving at a significant fraction of the [[speed of light]]). This pair of jets provide a means for an accretion disk to shed excess energy and [[angular momentum]]. They may be created by [[magnetic field]]s within the gas that surrounds the compact object.<ref name=science300_5627/>

The Cygnus&nbsp;X-1 jets are inefficient radiators and so release only a small proportion of their energy in the [[electromagnetic spectrum]]. That is, they appear "dark". The estimated angle of the jets to the line of sight is 30°, and they may be [[Precession|precessing]].<ref name=apj626_2_1015/> One of the jets is colliding with a relatively dense part of the [[interstellar medium]] (ISM), forming an energized ring that can be detected by its radio emission. This collision appears to be forming a [[nebula]] that has been observed in the [[Visible spectrum|optical wavelengths]]. To produce this nebula, the jet must have an estimated average power of 4–{{val|14|e=36|u=[[erg]]/s}}, or {{val|9|5|e=29|ul=W}}.<ref name=mnras376_3_1341/> This is more than 1,000 times the power emitted by the Sun.<ref name=apj418_457/> There is no corresponding ring in the opposite direction because that jet is facing a lower-density region of the [[Interstellar medium|ISM]].<ref name=nature436_7052_819/>

In 2006, Cygnus&nbsp;X-1 became the first stellar-mass black hole found to display evidence of [[gamma-ray]] emission in the very high-energy band, above {{val|100|ul=GeV}}. The signal was observed at the same time as a flare of hard X-rays, suggesting a link between the events. The X-ray flare may have been produced at the base of the jet, while the gamma rays could have been generated where the jet interacts with the stellar wind of HDE&nbsp;226868.<ref name=apjl665_1_L51/>

===HDE&nbsp;226868===
[[File:Cygnus X-1.png|right|thumb|An artist's impression of the HDE&nbsp;226868–Cygnus&nbsp;X-1 binary system]]
HDE&nbsp;226868 is a supergiant star with a [[spectral class]] of O9.7&nbsp;Iab,<ref name=SIMBAD/> which is on the borderline between class-O and class-B stars. It has an estimated surface temperature of 31,000&nbsp;[[Kelvin|K]]<ref name=eas030610/> and mass approximately 20–40 times the [[solar mass|mass of the Sun]]. Based on a stellar evolutionary model, at the estimated distance of 2,000&nbsp;parsecs, this star may have a radius equal to about 15–17<ref name=orosz2011/> times the [[solar radius]] and has approximately 300,000–400,000 times the [[solar luminosity|luminosity of the Sun]].<ref name=MNRAS358_3_851/><ref name=iorio2007/> For comparison, the compact object is estimated to be orbiting HDE&nbsp;226868 at a distance of about 40 solar radii, or twice the radius of this star.<ref name=apj620_1_398/>

The surface of HDE&nbsp;226868 is being [[Tidal force|tidally]] distorted by the [[gravity]] of the massive companion, forming a tear-drop shape that is further distorted by rotation. This causes the optical brightness of the star to vary by 0.06&nbsp;magnitudes during each 5.6-day binary orbit, with the minimum magnitude occurring when the system is aligned with the line of sight.<ref name=caballero/> The "ellipsoidal" pattern of light variation results from the [[limb darkening]] and [[gravity darkening]] of the star's surface.<ref name=cox2001/>

When the spectrum of HDE&nbsp;226868 is compared to the similar star [[Alnilam]], the former shows an overabundance of [[helium]] and an underabundance of [[carbon]] in its atmosphere.<ref name=rmaa31_1_63/> The [[ultraviolet]] and [[H-alpha|hydrogen-alpha]] spectral lines of HDE&nbsp;226868 show profiles similar to the star [[P&nbsp;Cygni]], which indicates that the star is surrounded by a gaseous envelope that is being accelerated away from the star at speeds of about 1,500&nbsp;km/s.<ref name=aaa63_1/><ref name=apj506_1_424/>

Like other stars of its spectral type, HDE&nbsp;226868 is thought to be shedding mass in a [[stellar wind]] at an estimated rate of {{val|2.5|e=-6}} solar masses per year; or one solar mass every 400,000 years.<ref name=apj203_438/> The gravitational influence of the compact object appears to be reshaping this stellar wind, producing a focused wind geometry rather than a spherically symmetrical wind.<ref name=apj620_1_398/> X-rays from the region surrounding the compact object heat and ionize this stellar wind. As the object moves through different regions of the stellar wind during its 5.6-day orbit, the UV lines,<ref name=baas38_334/> the radio emission,<ref name=mnras302_1_L1/> and the X-rays themselves all vary.<ref name=apj583_1_424/>

The [[Roche lobe]] of HDE&nbsp;226868 defines the region of space around the star where orbiting material remains gravitationally bound. Material that passes beyond this lobe may fall toward the orbiting companion. This Roche lobe is believed to be close to the surface of HDE&nbsp;226868 but not overflowing, so the material at the stellar surface is not being stripped away by its companion. However, a significant proportion of the stellar wind emitted by the star is being drawn onto the compact object's accretion disk after passing beyond this lobe.<ref name=apj304_371/>

The gas and dust between Earth and HDE&nbsp;226868 results in a reduction in the apparent magnitude of the star, as well as a reddening of the hue—red light can more effectively penetrate the dust in the interstellar medium. The estimated value of the interstellar [[Extinction (astronomy)|extinction]] (''A<sub>V</sub>'') is 3.3&nbsp;[[Apparent magnitude|magnitudes]].<ref name=apj185_2_L113/> Without the intervening matter, HDE&nbsp;226868 would be a fifth-magnitude star,<ref name=sut_ir/> and thus visible to the unaided eye.<ref name=kaler/>

==Stephen Hawking and Kip Thorne==
[[File:2259 Black Hole 1280 English.jpg|thumb|NASA's "Galaxy of Horrors" poster for Cygnus X-1<ref name=devoured/>]]
Cygnus&nbsp;X-1 was the subject of a bet between physicists [[Stephen Hawking]] and [[Kip Thorne]], in which Hawking bet against the existence of black holes in the region. Hawking later described this as an "insurance policy" of sorts. In his book ''A Brief History of Time'' he wrote:<ref name=hawking1988/>
{{bquote|This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine ''[[Private Eye]]''. If black holes do exist, Kip will get one year of ''[[Penthouse (magazine)|Penthouse]]''. When we made the bet in 1975, we were 80% certain that Cygnus&nbsp;X-1 was a black hole. By now [1988], I would say that we are about 95% certain, but the bet has yet to be settled.}}
According to the updated tenth-anniversary edition of ''A Brief History of Time'', Hawking has conceded the bet<ref name=hawking1998/> due to subsequent observational data in favor of black holes. In his own book ''[[Black Holes and Time Warps]]'', Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in [[Russia]], finding the framed bet, and signing it.<ref name=thorne1994/> While Hawking referred to the bet as taking place in 1975, the written bet itself (in Thorne's handwriting, with his and Hawking's signatures) bears additional witness signatures under a legend stating "Witnessed this tenth day of December 1974".<ref name=wager/> This date was confirmed by Kip Thorne on the January&nbsp;10, 2018 episode of [[Nova (American TV series)|''Nova'']] on [[PBS]].<ref name=apocalypse/>

== In popular culture ==
Cygnus X-1 is the subject of a [[Cygnus X-1 (song series)|two-part song series]] by [[Canadians|Canadian]] [[progressive rock]] band [[Rush (band)|Rush]]. The first part, "Book I: The Voyage", is the last song on the 1977 album ''[[A Farewell to Kings]]''. The second part, "Book II: Hemispheres", is the first song on the following 1978 album, ''[[Hemispheres (Rush album)|Hemispheres]]''. The lyrics describe an explorer aboard the spaceship ''[[Rocinante]]'', who travels to the black hole, believing that there may be something beyond it. As he moves closer, it becomes increasingly difficult to control the ship, and he is eventually drawn in by the pull of gravity.<ref name=rush/>

In the 1979 Disney live-action science fiction film ''[[The Black Hole (1979 film)|The Black Hole]]'', the scientific survey ship captained by Dr. Hans Reinhardt to study the black hole of the film's title is the ''Cygnus'', presumably (although never stated as such) named for the first-identified black hole, Cygnus X-1.<ref name=essays/>

== See also ==
{{Portal|Astronomy|Physics}}
* [[X-ray binary]]
* [[List of nearest black holes]]
* [[Stellar black hole]]
* [[Cygnus Molecular Nebula Complex]]

==References==
{{reflist|colwidth=30em|refs=

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}}

==External links==
{{Commons category|Cygnus X-1}}
*{{cite web
 | date=June 10, 2005 | url=http://sci.esa.int/integral/32709-artist-s-impression-of-cygnus-x-1/
 | title=Artist's impression of Cygnus&nbsp;X-1
 | publisher=ESA | access-date=2008-03-24 }}
*{{cite web
 | date=April 1, 1996
 | url=http://www.oa.uj.edu.pl/research/cygx1.html
 | title=Cygnus&nbsp;X-1, the black hole
 | publisher=Astronomical Observatory of the Jagiellonian University
 | access-date=2008-03-24 }}
*{{Cite web |last1=Cyrmon |first1=W. |last2=Aigner |first2=C. |last3=Bruckmueller |first3=E. |last4=Kernegger |first4=R. |display-authors=1 |date=December 18, 2002 |title=Black Hole in Cygnus |url=http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/austria_cygnus_1/ |url-status=dead |archive-url=https://web.archive.org/web/20150915105044/http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/austria_cygnus_1/ |archive-date=2015-09-15 |access-date=2008-03-29 |publisher=ESA}}
*{{Cite APOD
 | title=Possible Jet Blown Shells Near Microquasar Cygnus&nbsp;X-1 | date=June 8, 2009
 | access-date=2009-06-08 }}
*{{WikiSky|z=8}}
*[http://www.constellation-guide.com/cygnus-x-1/ Cygnus X-1 at Constellation Guide]
* [https://arxiv.org/abs/1506.00007 NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk] Michael Parker, 29 May 2015
* [http://www.nustar.caltech.edu/image/nustar120628a NuSTAR's First View of High-Energy X-ray Universe] NASA/JPL-Caltech June 28, 2012

{{s-start}}
{{s-ach|rec}}
{{s-bef|before=''None'' <br /> <small> Cyg X-1 is the first black hole discovered </small>}}
{{s-ttl|title=[[List of nearest black holes|Least distant black hole]]|years=1972&mdash;1986}}
{{s-aft|after=[[V616 Monocerotis]]}}
{{s-end}}

{{Stars of Cygnus}}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Hipparcos objects|098298]]
[[Category:O-type supergiants]]
[[Category:Cygnus (constellation)]]
[[Category:Henry Draper Catalogue objects|226868]]
[[Category:Stellar black holes]]
[[Category:X-ray binaries]]
[[Category:Objects with variable star designations|Cygni, V1357]]
[[Category:Durchmusterung objects]]
[[Category:Rotating ellipsoidal variables]]