Editing Cygnus X-1 (section)

===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/>