Editing Deneb (section)

===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" />