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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 A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...
. It was discovered in 1964 during a rocket flight and is one of the
strongest "Strongest" is a song recorded by Norwegian singer and songwriter Ina Wroldsen. The song was released on 27 October 2017 and has peaked at number 2 in Norway. "Strongest" is Wroldsen's first solo release on Syco Music Syco Music is a defunct ...
X-ray sources detectable from Earth, producing a peak X-ray flux density of (). It remains among the most studied
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
s in its class. The compact object is now estimated to have a mass about 21.2 times the
mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...
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 "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms. Cygnus X-1 belongs to a high-mass
X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...
system, located about 2.22 kilo
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...
s from the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, that includes a blue
supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars sp ...
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 An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
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 jet An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets beco ...
s, arranged
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
ly 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 . 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 Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Richard P. Fe ...
in 1975, with Hawking 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 A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...
in the system. This hypothesis lacks direct empirical evidence but has generally been accepted from indirect evidence.


Discovery and observation

Observation of X-ray emissions allows
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either obse ...
s to study celestial phenomena involving gas with temperatures in the millions of degrees. However, because X-ray emissions are blocked by Earth's atmosphere, observation of celestial X-ray sources is not possible without lifting instruments to altitudes where the X-rays can penetrate. Cygnus X-1 was discovered using X-ray instruments that were carried aloft by a 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 The Aerobee rocket was one of the United States' most produced and productive sounding rockets. Developed by the Aerojet Corporation, the Aerobee was designed to combine the altitude and launching capability of the V-2 with the cost effective ...
suborbital rockets. The rockets carried
Geiger counters A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental p ...
to measure X-ray emission in wavelength range 1– 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. As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg XR-1 (later Cyg X-1) in the constellation Cygnus. The celestial coordinates of this source were estimated as right ascension 19h53m and
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
34.6°. It was not associated with any especially prominent radio or optical source at that position. Seeing a need for longer-duration studies, in 1963 Riccardo Giacconi and Herb Gursky proposed the first orbital satellite to study X-ray sources. NASA launched their Uhuru satellite in 1970, which led to the discovery of 300 new X-ray sources. Extended Uhuru observations of Cygnus X-1 showed fluctuations in the X-ray intensity that occurs several times a second. This rapid variation meant that the energy generation must take place over a relatively small region of roughly , as the speed of light restricts communication between more distant regions. For a size comparison, the diameter of the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
is about . In April–May 1971, Luc Braes and George K. Miley from
Leiden Observatory Leiden Observatory ( nl, Sterrewacht Leiden) is an astronomical institute of Leiden University, in the Netherlands. Established in 1633 to house the quadrant of Rudolph Snellius, it is the oldest operating university observatory in the world, with ...
, and independently Robert M. Hjellming and Campbell Wade at the
National Radio Astronomy Observatory The National Radio Astronomy Observatory (NRAO) is a federally funded research and development center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc. for the purpose of radio a ...
, detected radio emission from Cygnus X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2 +35 1910 = HDE 226868. On the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
, this star lies about half a degree from the 4th-magnitude star Eta Cygni. 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 X-1. Louise Webster and Paul Murdin, at the Royal Greenwich Observatory, and Charles Thomas Bolton, working independently at the University of Toronto's
David Dunlap Observatory The David Dunlap Observatory (DDO) is an astronomical observatory site in Richmond Hill, Ontario, Canada. Established in 1935, it was owned and operated by the University of Toronto until 2008. It was then acquired by the city of Richmond Hill, ...
, announced the discovery of a massive hidden companion to HDE 226868 in 1972. Measurements of the
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters. Based on the high predicted mass of the object, they surmised that it may be a
black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...
, as the largest possible neutron star cannot exceed three times the
mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...
. With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus X-1 was most likely a black hole. More precise measurements of Cygnus X-1 demonstrated variability down to a single millisecond. This interval is consistent with
turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
in a disk of accreted matter surrounding a black hole—the
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole. Cygnus X-1 has since been studied extensively using observations by orbiting and ground-based instruments. The similarities between the emissions of X-ray binaries such as HDE 226868/Cygnus X-1 and
active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated jets. For this reason, Cygnus X-1 is identified among a class of objects called microquasars; an analog of the quasars, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE 226868/Cygnus X-1 may lead to further insights into the mechanics of active galaxies.


Binary system

The
compact object In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects ha ...
and blue supergiant star form a
binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that thi ...
in which they orbit around their
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
every 5.599829 days. From the perspective of Earth, the compact object never goes behind the other star; in other words, the system does not eclipse. However, the inclination of the orbital plane to the
line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...
from Earth remains uncertain, with predictions ranging from 27° to 65°. A 2007 study estimated the inclination as , which would mean that the semi-major axis is about , or 20% of the distance from Earth to the Sun. The
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values bet ...
is thought to be only , meaning a nearly circular orbit. Earth's distance to this system is about . The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is located at roughly 2000 
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...
s from the Sun. This implies that HDE 226868, Cygnus X-1 and this
OB association In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as t ...
may have formed at the same time and location. If so, then the age of the system is about . The motion of HDE 226868 with respect to Cygnus OB3 is , a typical value for random motion within a stellar association. HDE 226868 is about from the center of the association and could have reached that separation in about —which roughly agrees with estimated age of the association. With a galactic latitude of 4° and galactic longitude 71°, this system lies inward along the same
Orion Spur The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...
, in which the Sun is located within the Milky Way, near where the spur approaches the
Sagittarius Arm Sagittarius ( ) may refer to: *Sagittarius (constellation) *Sagittarius (astrology), a sign of the Zodiac Ships *'' SuperStar Sagittarius'', a cruise ship * USS ''Sagittarius'' (AKN-2), a World War II US Navy cargo ship Music *Sagittarius (ba ...
. Cygnus X-1 has been described as belonging to the Sagittarius Arm, 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 , while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object yielded a more precise value of . In all cases, the object is most likely a black hole—a region of space with a 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 The Schwarzschild radius or the gravitational radius is a physical parameter in the Schwarzschild solution to Einstein's field equations that corresponds to the radius defining the event horizon of a Schwarzschild black hole. It is a characteri ...
, which is about for Cygnus X-1. Anything (including
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic parti ...
and photons) that passes through this boundary is unable to escape. New measurements published in 2021 yielded an estimated mass of . Evidence of just such an event horizon may have been detected in 1992 using ultraviolet (UV) observations with the
High Speed Photometer The High Speed Photometer (HSP) is a scientific instrument formerly installed on the Hubble Space Telescope. The HSP was designed to measure the brightness and polarity of rapidly varying celestial objects. It could observe in ultraviolet, visible ...
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 In physics and general relativity, gravitational redshift (known as Einstein shift in older literature) is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well (seem to) lose energy. This loss of energy ...
as the material approaches the horizon. That is, the wavelengths 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. The spin of the compact object is not yet well determined. Past analysis of data from the space-based
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
suggested that Cygnus X-1 was not rotating to any significant degree. However, evidence announced in 2011 suggests that it is rotating extremely rapidly, approximately 790 times per second.


Formation

The largest star in the Cygnus 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 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 226868, while the remainder was most likely expelled by a strong stellar wind. The helium enrichment of HDE 226868's outer atmosphere may be evidence for this mass transfer. Possibly the progenitor may have evolved into a
Wolf–Rayet star Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surfa ...
, which ejects a substantial proportion of its atmosphere using just such a powerful stellar wind. 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).


Accretion disk

The compact object is thought to be orbited by a thin, flat disk of accreting matter known as an
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
. 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—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius, or about 15,000 km. Though highly and erratically variable, Cygnus X-1 is typically the brightest persistent source of hard X-rays—those with energies from about 30 up to several hundred kiloelectronvolts—in the sky. The X-rays are produced as lower-energy photons in the thin inner accretion disk, then given more energy through
Compton scattering Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...
with very high-temperature electrons in a geometrically thicker, but nearly transparent
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
enveloping it, as well as by some further reflection from the surface of the thin disk. An alternative possibility is that the X-rays may be Compton-scattered by the base of a jet instead of a disk corona. The X-ray emission from Cygnus X-1 can vary in a somewhat repetitive pattern called
quasi-periodic oscillations In X-ray astronomy, quasi-periodic oscillation (QPO) is the manner in which the X-ray light from an astronomical object flickers about certain frequencies. In these situations, the X-rays are emitted near the inner edge of an accretion disk in ...
(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 X-1, providing a cross-check with other mass derivations. Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus X-1. The 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 0332+53 was thought to be a possible black hole until pulsations were found. Cygnus X-1 has also never displayed X-ray bursts similar to those seen from neutron stars. Cygnus 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 ), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus X-1 transitions back to the hard state. The spectral transition of Cygnus X-1 can be explained using a two-component advective flow solution, as proposed by Chakrabarti and Titarchuk. 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). The X-ray flux from Cygnus X-1 varies periodically every 5.6 days, especially during 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 226868. There is a roughly 300-day periodicity in the emission, which could be caused by the precession of the accretion disk.


Jets

As accreted matter falls toward the compact object, it loses gravitational potential energy. Part of this released energy is dissipated by jets of particles, aligned
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
to the accretion disk, that flow outward with 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 In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syste ...
. They may be created by magnetic fields within the gas that surrounds the compact object. The Cygnus X-1 jets are inefficient radiators and so release only a small proportion of their energy in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from b ...
. That is, they appear "dark". The estimated angle of the jets to the line of sight is 30°, and they may be precessing. One of the jets is colliding with a relatively dense part of the
interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar ...
(ISM), forming an energized ring that can be detected by its radio emission. This collision appears to be forming a
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...
that has been observed in the optical wavelengths. To produce this nebula, the jet must have an estimated average power of 4–, or . This is more than 1,000 times the power emitted by the Sun. There is no corresponding ring in the opposite direction because that jet is facing a lower-density region of the ISM. In 2006, Cygnus X-1 became the first stellar-mass black hole found to display evidence of gamma-ray emission in the very high-energy band, above . 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 226868.


HDE 226868

HDE 226868 is a supergiant star with a
spectral class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting th ...
of O9.7 Iab, which is on the borderline between class-O and class-B stars. It has an estimated surface temperature of 31,000  K and mass approximately 20–40 times the
mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...
. Based on a stellar evolutionary model, at the estimated distance of 2,000 parsecs, this star may have a radius equal to about 15–17 times the solar radius and has approximately 300,000–400,000 times the
luminosity of the Sun The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal ...
. For comparison, the compact object is estimated to be orbiting HDE 226868 at a distance of about 40 solar radii, or twice the radius of this star. The surface of HDE 226868 is being 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 magnitudes during each 5.6-day binary orbit, with the minimum magnitude occurring when the system is aligned with the line of sight. The "ellipsoidal" pattern of light variation results from the
limb darkening Limb darkening is an optical effect seen in stars (including the Sun), where the central part of the disk appears brighter than the edge, or ''limb''. Its understanding offered early solar astronomers an opportunity to construct models with such ...
and
gravity darkening Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where the poles of a star are brighter than the equator, due to rapid rotation and oblate shape. When a star is oblate, it has a larger radius at its equa ...
of the star's surface. When the spectrum of HDE 226868 is compared to the similar star Epsilon Orionis, the former shows an overabundance of helium and an underabundance of carbon in its atmosphere. The ultraviolet and
hydrogen-alpha H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28  nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha ...
spectral lines of HDE 226868 show profiles similar to the star P 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 km/s. Like other stars of its spectral type, HDE 226868 is thought to be shedding mass in a stellar wind at an estimated rate of solar masses per year. This is the equivalent of losing a mass equal to the Sun's every 400,000 years. 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. 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, the radio emission, and the X-rays themselves all vary. The
Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, ...
of HDE 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 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. The gas and dust between the Sun and HDE 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 (''AV'') is 3.3  magnitudes. Without the intervening matter, HDE 226868 would be a fifth-magnitude star, and thus visible to the unaided eye.


Stephen Hawking and Kip Thorne

Cygnus X-1 was the subject of a bet between physicists Stephen Hawking and
Kip Thorne Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Richard P. Fe ...
, 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: According to the updated tenth-anniversary edition of ''A Brief History of Time'', Hawking has conceded the bet due to subsequent observational data in favor of black holes. In his own book ''
Black Holes and Time Warps ''Black Holes & Time Warps: Einstein's Outrageous Legacy'' is a 1994 popular science book by physicist Kip Thorne. It provides an illustrated overview of the history and development of black hole theory, from its roots in Newtonian mechanics unti ...
'', Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eight ...
, finding the framed bet, and signing it. 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". This date was confirmed by Kip Thorne on the January 10, 2018 episode of ''Nova'' on
PBS The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virginia. PBS is a publicly funded nonprofit organization and the most prominent provider of educati ...
.


In popular culture

Cygnus X-1 is the subject of a two-part song series by
Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source o ...
progressive rock band Rush. The first part, "Book I: The Voyage", is the last song on the 1977 album ''
A Farewell to Kings ''A Farewell to Kings'' is the fifth studio album by Canadian rock band Rush, released in September 1977 by Anthem Records. After touring their previous album '' 2112'' (1976), which saw the group reach a new critical and commercial peak, they ...
''. The second part, "Book II: Hemispheres", is the first song on the following 1978 album, '' Hemispheres''. The lyrics describe an explorer aboard the spaceship ''
Rocinante Rocinante () is Don Quixote's horse in the two-part 1605/1615 novel ''Don Quixote'' by Miguel de Cervantes. In many ways, Rocinante is not only Don Quixote's horse, but also his double; like Don Quixote, he is awkward, past his prime, and ...
'', 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. In the Disney Sci-Fi 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.


See also

*
X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...
* List of nearest black holes * Stellar black hole


References


External links

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Cygnus X-1 at Constellation Guide

NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk
Michael Parker, 29 May 2015
NuSTAR's First View of High-Energy X-ray Universe
NASA/JPL-Caltech June 28, 2012 {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System 098298 O-type supergiants Cygnus (constellation) 226868 Stellar black holes X-ray binaries Cygni, V1357 Durchmusterung objects