Deneb rivalsRigel, a closer blue supergiant, as the most luminousfirst-magnitude star. However, its distance, and hence luminosity, is poorly known; its luminosity is estimated to be between 55,000 and 196,000 timesthat of the Sun. Distance estimates range from 1,400 to 2,600 light-years; assuming its highest value, it is the farthest star with an apparent magnitude brighter than 2.50.
Deneb is the brighest star in the constellation of Cygnus (top)
α Cygni (Latinised toAlpha Cygni) is the star'sdesignation given byJohann Bayer in 1603. The traditional nameDeneb is derived from theArabic word for "tail", from the phrase ذنب الدجاجةDhanab al-Dajājah, or "tail of the hen".[13] TheIAU Working Group on Star Names has recognised the nameDeneb for this star, and it is entered in their Catalog of Star Names.[14]
Denebadigege was used in theAlfonsine Tables,[15] other variants includeDeneb Adige,Denebedigege andArided. This latter name was derived fromAl Ridhādh, a name for the constellation.Johann Bayer called itArrioph, derived fromAridf andAl Ridf, 'the hindmost' orGallina. German poet and authorPhilippus Caesius termed itOs rosae, orRosemund in German, orUropygium – the parson's nose.[13] The namesArided andAridif have fallen out of use.
An older traditional name isArided/ˈærɪdɛd/, from the Arabicar-ridf 'the one sitting behind the rider' (or just 'the follower'), perhaps referring to the other major stars of Cygnus, which were calledal-fawāris 'the riders'.[16]
Thespectrum of Alpha Cygni has been observed by astronomers since at least 1888, and by 1910 the variableradial velocity had become apparent. This led to the early suggestion byE. B. Frost that this is abinary star system.[21] In 1935, the work ofG. F. Paddock and others had established that this star wasvariable in luminosity with a dominant period of 11.7 days and possibly with other, lower amplitude periods.[22] By 1954, closer examination of the star'scalcium H and K lines showed a stationary core, which indicated the variable velocity was instead being caused by motion of thestar's atmosphere. This variation ranged from +6 to −9 km/s around the star's mean radial velocity.[23] Other, similar supergiants were found to have variable velocities, with this star being a typical member.[22]
Wide-field view of theSummer Triangle and theMilky Way. Deneb is at the far left centre of the picture, at the end of the darker lane within the Milky Way.
Deneb is a bluish-white star ofspectral type A2Ia, classifying it as ablue supergiant star.[26] Since 1943, itsspectrum has served as one of the stable references by which other stars are classified.[5] Its mass is estimated at 19M☉.Stellar wind causes matter to be lost at an average rate of 8±3×10−7M☉ per year, 100,000 times the Sun's rate of mass loss or equivalent to about oneEarth mass per 500 years.[27]
Deneb's distance from the Earth is uncertain. One estimate gives 802 parsecs (2,620 ly) assuming Deneb is a member of theCygnus OB7 association.[7] The original derivation of aparallax using measurements from the astrometric satelliteHipparcos gave an uncertain result of1.01±0.57 mas[28][29] that was consistent with this distance. However, the 2007 re-analysis gives a much larger parallax resulting in a distance of433±60 pc, or1,410±196 ly.[2] The controversy over whether the direct Hipparcos measurements can be ignored in favour of a wide range of indirect stellar models and interstellar distance scales is similar to the better knownsituation with the Pleiades.[2]
At its highest distance estimate, Deneb'sabsolute magnitude is estimated as −8.4, placing it among the visually brightest stars known, with an estimated luminosity of nearly 200,000 L☉.[30][31] By the distance from Hipparcos parallax, Deneb has a luminosity of 55,000 L☉.[10]
Deneb is one of the most luminous first magnitude stars, that is, stars with a brighter apparent magnitude than 1.5. Deneb is also the most distant of the 30brightest stars.[32] Based on its temperature and luminosity, and also on direct measurements of its tinyangular diameter (a mere 0.002 seconds of arc), Deneb appears to have a diameter about 100 – 200 timesthat of the Sun;[10] if placed at the center of theSolar System, Deneb would extend to the orbit ofMercury orEarth. It is one of thelargest white 'A' spectral type stars known.
Deneb spent much of its early life as anO-type main-sequence star of about 23 M☉, but it has now exhausted thehydrogen in its core and expanded to become a supergiant.[7][33] Stars in the mass range of Deneb eventually expand to become the most luminousred supergiants, and within a few million years their cores will collapse producing asupernova explosion. It is now known that red supergiants up to a certain mass explode as the commonly seentype II-P supernovae, but more massive ones lose their outer layers to become hotter again. Depending on their initial masses and the rate of mass loss, they may explode asyellow hypergiants orluminous blue variables, or they may becomeWolf-Rayet stars before exploding in atype Ib or Ic supernova. Identifying whether Deneb is currently evolving towards a red supergiant or is currently evolving bluewards again would place valuable constraints on the classes of stars that explode as red supergiants and those that explode as hotter stars.[33]
Stars evolving red-wards for the first time are most likely fusing hydrogen in a shell around ahelium core that has not yet grown hot enough to start fusion tocarbon andoxygen. Convection has begundredging up fusion products but these do not reach the surface. Post-red supergiant stars are expected to show those fusion products at the surface due to stronger convection during the red supergiant phase and due to loss of the obscuring outer layers of the star. Deneb is thought to be increasing its temperature after a period as a red supergiant, although current models do not exactly reproduce the surface elements showing in its spectrum.[33] On the contrary, it is possible that Deneb has just left the main sequence and is evolving to a red supergiant phase, which is in agreement with estimates of its current mass, while its spectral composition can be explained by Deneb having been a rapidly rotating star during its main sequence phase.[7]
Deneb is the prototype of theAlpha Cygni (α Cygni)variable stars,[35][34] whose small amplitudes and irregular rapid pulsations can cause its magnitude to vary anywhere between 1.21 and 1.29.[36] Its variable velocity discovered by Lee in 1910,[21] but it was not formally placed as a unique class of variable stars until the 1985 4th edition of the General Catalogue of Variable Stars.[37] The cause of the pulsations of Alpha Cygni variable stars are not fully understood, but theirirregular nature seems to be due tobeating of multiple pulsation periods. Analysis of radial velocities determined 16 different harmonic pulsation modes with periods ranging between 6.9 and 100.8 days.[38] A longer period of about 800 days probably also exists.[34]
Deneb has been reported as a possible single line spectroscopicbinary with a period of about 850 days, where the spectral lines from the star suggest cyclical radial velocity changes.[38] Later investigations have found no evidence supporting the existence of a companion.[35]
Names similar to Deneb have been given to at least seven different stars, most notablyDeneb Kaitos, the brightest star in the constellation ofCetus;Deneb Algedi, the brightest star inCapricornus; andDenebola, the second brightest star inLeo. All these stars are referring to the tail of the animals that their respective constellations represent.
In the Chinese love story ofQi Xi, Deneb marks themagpie bridge across theMilky Way, which allows the separated lovers Niu Lang (Altair) and Zhi Nü (Vega) to be reunited on one special night of the year in late summer. In other versions of the story, Deneb is a fairy who acts as chaperone when the lovers meet.
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