Editing Deneb (section)

==Physical characteristics==
[[File:Wide-field view of the Summer Triangle.jpg|thumb|upright=1.2|Wide-field view of the [[Summer Triangle]] and the [[Milky Way]]. Deneb is at the far left centre of the picture, at the end of the darker lane within the Milky Way.<!-- Not sure if you can find Deneb unless you have experience. -->]]
Deneb is a bluish-white star of [[stellar classification|spectral type]] A2Ia, classifying it as a [[blue supergiant]] star.<ref>{{Cite journal |last1=Wagle |first1=Gururaj A. |last2=Ray |first2=Alak |last3=Raghu |first3=Adarsh |date=2020-05-01 |title=Type IIP Supernova Progenitors III: Blue to Red Supergiant Ratio in Low Metallicity Models with Convective Overshoot |journal=The Astrophysical Journal |volume=894 |issue=2 |pages=118 |doi=10.3847/1538-4357/ab8bd5 |doi-access=free |arxiv=2004.14419 |bibcode=2020ApJ...894..118W |issn=0004-637X}}</ref> Since 1943, its [[stellar spectrum|spectrum]] has served as one of the stable references by which other stars are classified.<ref name=baas25_1319/> Its mass is estimated at 19 {{Solar mass|link=y}}. [[Stellar wind]]s causes matter to be lost at an average rate of {{Solar mass|8±3{{e|-7}}}} per year, 100,000 times the Sun's rate of mass loss or equivalent to about one [[Earth mass]] per 500 years.<ref>{{cite journal |bibcode=2002ApJ...570..344A |title=The Spectral Energy Distribution and Mass-Loss Rate of the A-Type Supergiant Deneb |journal=The Astrophysical Journal |volume=570 |issue=1 |pages=344 |last1=Aufdenberg |first1=J. P. |last2=Hauschildt |first2=P. H. |last3=Baron |first3=E. |last4=Nordgren |first4=T. E. |last5=Burnley |first5=A. W. |last6=Howarth |first6=I. D. |last7=Gordon |first7=K. D. |last8=Stansberry |first8=J. A. |year=2002 |doi=10.1086/339740 |arxiv=astro-ph/0201218 |s2cid=13260314}}</ref>

Deneb's distance from the Earth is uncertain. One estimate gives {{convert|802|pc|ly}} assuming Deneb is a member of the [[Cygnus OB7]] association.<ref name=schiller/> Another distance estimate using the [[bolometric magnitude]] implied by its [[effective temperature]] and [[surface gravity]] gives {{convert|762|pc|ly}}.{{Efn|1=From <math>d = 10^{\frac{\mu}{5}+1}</math>, where μ is the [[distance modulus]].}} The original derivation of a [[parallax]] using measurements from the astrometric satellite [[Hipparcos]] gave an uncertain result of {{val|1.01|0.57|ul=mas}}<ref name=aaa323_L49>{{Cite journal |last1=Perryman |first1=M. A. C. |last2=Lindegren |first2=L. |year=1997 |title=The Hipparcos Catalogue |journal=[[Astronomy and Astrophysics]] |volume=323 |pages=L49–L52  |bibcode=1997A&A...323L..49P |last3=Kovalevsky |first3=J. |last4=Hoeg |first4=E. |last5=Bastian |first5=U. |last6=Bernacca |first6=P. L. |last7=Crézé |first7=M. |last8=Donati |first8=F. |last9=Grenon |first9=M. |last10=Grewing |first10=M. |last11=Van Leeuwen |first11=F. |last12=Van Der Marel |first12=H. |last13=Mignard |first13=F. |last14=Murray |first14=C. A. |last15=Le Poole |first15=R. S. |last16=Schrijver |first16=H. |last17=Turon |first17=C. |last18=Arenou |first18=F. |last19=Froeschlé |first19=M. |last20=Petersen |first20=C. S. }}</ref><ref name=GSM>{{Cite book |last=Perryman |first=M. |date=2010 |title=The Making of History's Greatest Star Map |publisher=[[Springer-Verlag]] |doi=10.1007/978-3-642-11602-5 |isbn=978-3-642-11601-8 |series=Astronomers' Universe |url=https://cds.cern.ch/record/1338896 |type=Submitted manuscript |bibcode=2010mhgs.book.....P }}</ref> that was consistent with this distance. However, the 2007 re-analysis gives a much larger parallax resulting in a distance of {{Val|433|60|u=pc}}, or {{val|1410|196|fmt=commas|u=ly}}.<ref name=hipparcos/> 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 known [[Pleiades#Distance|situation with the Pleiades]].<ref name=hipparcos>{{cite journal |arxiv=0708.1752 |bibcode=2007A&A...474..653V |doi=10.1051/0004-6361:20078357 |title=Validation of the new Hipparcos reduction |journal=Astronomy and Astrophysics |volume=474 |issue=2 |pages=653–664 |year=2007 |last1=Van Leeuwen |first1=F. |s2cid=18759600}}</ref>

At its highest distance estimate, Deneb's [[absolute magnitude]] is estimated as −8.4, placing it among the visually brightest stars known, with an estimated luminosity of nearly {{solar luminosity|200,000|link=y}}.<ref>
{{Cite journal 
 |last1=van de Kamp |first1=P.
 |date=1953
 |title=The Twenty Brightest Stars
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=65 |issue=382
 |pages=30
 |bibcode=1953PASP...65...30V
 |doi=10.1086/126523 
|doi-access=free
 }}</ref><ref>
{{Cite journal
 |last1=Lamers |first1=H. J. G. L. M.
 |last2=Stalio |first2=R.
 |last3=Kondo |first3=Y.
 |date=1978
 |title=A study of mass loss from the mid-ultraviolet spectrum of α Cygni (A2 Ia), β Orionis (B8 Ia), and η Leonis (A0 Ib)
 |journal=[[The Astrophysical Journal]]
 |volume=223 |pages=207
 |bibcode=1978ApJ...223..207L
 |doi=10.1086/156252
}}</ref> By the distance from Hipparcos parallax, Deneb has a luminosity of {{solar luminosity|55,000|link=y}}.<ref name="chesneau2">{{cite journal |last1=Chesneau |first1=O. |last2=Dessart |first2=L. |last3=Mourard |first3=D. |last4=Bério |first4=Ph. |last5=Buil |first5=Ch. |last6=Bonneau |first6=D. |last7=Borges Fernandes |first7=M. |last8=Clausse |first8=J. M. |last9=Delaa |first9=O. |last10=Marcotto |first10=A. |last11=Meilland |first11=A. |last12=Millour |first12=F. |last13=Nardetto |first13=N. |last14=Perraut |first14=K. |last15=Roussel |first15=A. |year=2010 |title=Time, spatial, and spectral resolution of the Hα line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer |url=https://hal.archives-ouvertes.fr/hal-00501515 |journal=Astronomy and Astrophysics |volume=521 |pages=A5 |arxiv=1007.2095 |bibcode=2010A&A...521A...5C |doi=10.1051/0004-6361/201014509 |s2cid=10340205 |last16=Spang |first16=A. |last17=Stee |first17=P. |last18=Tallon-Bosc |first18=I. |last19=McAlister |first19=H. |last20=Ten Brummelaar |first20=T. |last21=Sturmann |first21=J. |last22=Sturmann |first22=L. |last23=Turner |first23=N. |last24=Farrington |first24=C. |last25=Goldfinger |first25=P. J.}}</ref>

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 30 [[List of brightest stars|brightest stars]].<ref>{{cite web
 | title=The 172 Brightest Stars | work=STARS
 | first=James B. | last=Kaler | date=2017
 | url=http://stars.astro.illinois.edu/sow/bright.html | access-date=2021-09-17
}}</ref> Based on its temperature and luminosity, and also on direct measurements of its tiny [[angular diameter]] (a mere 0.002 seconds of arc), Deneb appears to have a diameter about 100{{snd}}200 times [[Solar radius|that of the Sun]];<ref name="chesneau">{{cite journal |last1=Chesneau |first1=O. |last2=Dessart |first2=L. |last3=Mourard |first3=D. |last4=Bério |first4=Ph. |last5=Buil |first5=Ch. |last6=Bonneau |first6=D. |last7=Borges Fernandes |first7=M. |last8=Clausse |first8=J. M. |last9=Delaa |first9=O. |last10=Marcotto |first10=A. |last11=Meilland |first11=A. |last12=Millour |first12=F. |last13=Nardetto |first13=N. |last14=Perraut |first14=K. |last15=Roussel |first15=A. |year=2010 |title=Time, spatial, and spectral resolution of the Hα line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer |url=https://hal.archives-ouvertes.fr/hal-00501515 |journal=Astronomy and Astrophysics |volume=521 |pages=A5 |arxiv=1007.2095 |bibcode=2010A&A...521A...5C |doi=10.1051/0004-6361/201014509 |s2cid=10340205 |last16=Spang |first16=A. |last17=Stee |first17=P. |last18=Tallon-Bosc |first18=I. |last19=McAlister |first19=H. |last20=Ten Brummelaar |first20=T. |last21=Sturmann |first21=J. |last22=Sturmann |first22=L. |last23=Turner |first23=N. |last24=Farrington |first24=C. |last25=Goldfinger |first25=P. J.}}</ref> if placed at the center of the [[Solar System]], Deneb would extend to the orbit of [[Mercury (planet)|Mercury]] or [[Earth's orbit|Earth]]. It is one of the [[List of largest stars|largest white 'A' spectral type stars known]].

===Evolutionary state===
Deneb spent much of its early life as an [[O-type main-sequence star]] of about {{solar mass|23}}, but it has now exhausted the [[hydrogen]] in its core and expanded to become a supergiant.<ref name=schiller/><ref name=georgy/> Stars in the mass range of Deneb eventually expand to become the most luminous [[red supergiants]], and within a few million years their cores will collapse producing a [[supernova]] explosion. It is now known that red supergiants up to a certain mass explode as the commonly seen [[type II supernova|type II-P supernova]]e, 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 as [[yellow hypergiant]]s or [[luminous blue variable]]s, or they may become [[Wolf-Rayet star]]s before exploding in a [[Type Ib and Ic supernovae|type 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.<ref name=georgy>{{cite journal |bibcode=2014MNRAS.439L...6G |title=The puzzle of the CNO abundances of α Cygni variables resolved by the Ledoux criterion |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=439 |issue=1 |pages=L6–L10 |last1=Georgy |first1=Cyril |last2=Saio |first2=Hideyuki |last3=Meynet |first3=Georges |year=2014 |doi=10.1093/mnrasl/slt165 |doi-access=free |arxiv=1311.4744 |s2cid=118557550}}</ref>

Stars evolving red-wards for the first time are most likely fusing hydrogen in a shell around a [[helium]] core that has not yet grown hot enough to start fusion to [[carbon]] and [[oxygen]].  Convection has begun [[Stellar evolution#Mature stars#Mid-sized stars#Red-giant-branch phase|dredging]] 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.<ref name=georgy/> 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.<ref name="schiller" />

===Variable star===
[[File:AlphaCygLightCurve.png|thumb|A [[Photometric_system#Photometric_letters|visual band]] [[light curve]] for Deneb, adapted from Yüce and Adelman (2019)<ref name="Yuca2019"/>]]
Deneb is the prototype of the [[Alpha Cygni variable|Alpha Cygni]] (α Cygni) [[variable star]]s,<ref name="Richardson2011" /><ref name="Yuca2019">{{cite journal
|last1=Yüce|first1=K.
|last2=Adelman |first2=S..J.
|title=On the variability of the A0 supergiants 9 Per, HR 1035, 13 Mon, Deneb, and HR 8020 as seen in FCAPT Strömgren photometry
|date=2019
|journal=New Astronomy
|volume=66
|pages=88–99
|doi=10.1016/j.newast.2018.07.002
|bibcode = 2019NewA...66...88Y|s2cid=126285732
}}</ref> whose small irregular amplitudes and rapid pulsations can cause its magnitude to vary anywhere between 1.21 and 1.29.<ref name="gscvquery">{{Cite web |url=http://www.sai.msu.su/gcvs/cgi-bin/search.cgi?search=alf+Cyg|title=GCVS Query forms|website=Sternberg Astronomical Institute|access-date=2019-01-07
}}</ref>  Its variable velocity discovered by Lee in 1910,<ref name=Lee1910/> 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.<ref name="GCVS4">{{cite journal |bibcode=1996yCat.2139....0K |title=VizieR Online Data Catalog: General Catalog of Variable Stars, 4th Ed. (GCVS4) (/gcvs4Kholopov+ 1988) |journal=VizieR On-Line Data Catalog: II/139B. Originally Published in: Moscow: Nauka Publishing House (1985–1988) |volume=2139 |pages=0 |last1=Kholopov |first1=P. N. |last2=Samus' |first2=N. N. |last3=Frolov |first3=M. S. |last4=Goranskij |first4=V. P. |last5=Gorynya |first5=N. A. |last6=Kireeva |first6=N. N. |last7=Kukarkina |first7=N. P. |last8=Kurochkin |first8=N. E. |last9=Medvedeva |first9=G. I. |last10=Perova |first10=N. B.|date=1996}}</ref> The cause of the pulsations of Alpha Cygni variable stars are not fully understood, but their [[irregular variable|irregular nature]] seems to be due to [[Beat (acoustics)|beat]]ing of multiple pulsation periods. Analysis of radial velocities determined 16 different harmonic pulsation modes with periods ranging between 6.9 and 100.8 days.<ref name="Lucy1976"/> A longer period of about 800 days probably also exists.<ref name="Yuca2019" />

===Possible spectroscopic companion===
Deneb has been reported as a possible single line spectroscopic [[Binary star|binary]] with a period of about 850 days, where the spectral lines from the star suggest cyclical radial velocity changes.<ref name="Lucy1976">{{cite journal|bibcode=1976ApJ...206..499L|title=An analysis of the variable radial velocity of alpha Cygni|journal=Astrophysical Journal|volume=206|pages=499|last1=Lucy|first1=L. B.|year=1976|doi=10.1086/154405|doi-access=free}}</ref> Later investigations have found no evidence supporting the existence of a companion.<ref name="Richardson2011">{{cite journal |bibcode=2011AJ....141...17R |title=A Five-year Spectroscopic and Photometric Campaign on the Prototypical α Cygni Variable and A-type Supergiant Star Deneb |journal=The Astronomical Journal |volume=141 |issue=1 |pages=17 |last1=Richardson |first1=N. D. |last2=Morrison |first2=N. D. |last3=Kryukova |first3=E. E. |last4=Adelman |first4=S. J. |year=2011 |doi=10.1088/0004-6256/141/1/17 |arxiv=1009.5994 |s2cid=118300333}}</ref>