Alpha Cygni variable

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Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time other parts expand. They are supergiant stars of spectral types B or A. Variations in brightness on the order of 0.1 magnitudes are associated with the pulsations, which often seem irregular, due to beating of multiple pulsation periods. The pulsations typically have periods of several days to several weeks.

The prototype of these stars, Deneb (α Cygni), exhibits fluctuations in brightness between magnitudes +1.21 and +1.29. Small amplitude rapid variations have been known in many early supergiant stars, but they were not formally grouped into a class until the 4th edition of the General Catalogue of Variable Stars was published in 1985. It used the acronym ACYG for Alpha Cygni variable stars.[1] Many luminous blue variables (LBVs) show Alpha Cygni-type variability during their quiescent (hot) phases, but the LBV classification is generally used in these cases.

A large number (32) were discovered by Christoffel Waelkens and colleagues analysing Hipparcos data in a 1998 study.[2]


The pulsations of Alpha Cygni Variable stars are not fully understood. They are not confined to a narrow range of temperatures and luminosities in the way that most pulsating stars are. Instead, most luminous A and B supergiants, and possibly also O and F stars, show some type of unpredictable small-scale pulsations. Nonadiabatic strange mode radial pulsations are predicted but only for the most luminous supergiants. Pulsations have also been modelled for less luminous supergiants by assuming they are low mass post-red supergiant stars, but most Alpha Cygni variables do not appear to have passed through the red supergiant stage.[3][4]

The pulsations are likely induced by kappa mechanism, caused by iron opacity variations, with strange modes producing the observed short periods for both radial and non-radial pulsations. Non-adiabatic g-modes may produce longer period variations, but these have not been observed in Alpha Cygni variables.[5]



Designation (name) Constellation Discovery Maximum Apparent magnitude (mV)[6] Minimum Apparent magnitude (mV)[6] Period (days) Spectral class Luminosity Comment
CE Cam (HD 21389) Camelopardalis Percy & Welsh (1983)[7] 4m.54 A0Iab 63,000[8]  
CS Cam Camelopardalis Rufener (1982)[9] 4m.259 B9Ia 75,900[10]  
η CMa (Aludra) Canis Major Kazarovets et al. (1999)[11] 2m.38 2m.48 4.70433 B5Ia 105,000[8]  
ο2 CMa Canis Major Waelkens et al. (1998)[2] 2m.98 3m.04 24.44 B3Ia 219,000[12]  
κ Cas Cassiopeia Percy & Welsh (1983)[7] 4m.12 4m.21 2.64690 B1Ia 331,000[12]  
6 Cas Cassiopeia Abt (1957)[13] 5m.34 5m.45 30 B2 Ia+ 200,000[14] Hypergiant
ο2 Cen Centaurus 5m.12 5m.22 46.3 A2Ia 136,000[15]  
ν Cep Cepheus Percy & Welsh (1983)[7] 4m.25 4m.35 A2 Iab 254,000[8]  
DL Cru Crux Waelkens et al. (1998)[2] 6m.24 6m.28 2.8778 B1.5Ia 242,000[16]  
Deneb[4] Cygnus Lee (1910)[17] 1m.21 1m.29 A2 Ia 196,000[18] Prototype
σ Cyg Cygnus Abt (1957)[13] 4m.19 4m.26 120.2 B9 Iab 39,000[19]  
55 Cyg Cygnus Hill et al. (1976)[20] 4m.81 4m.87 B2.5 Ia ~400,000[21]  
3 Gem Gemini Waelkens et al. (1998)[2] 5m.75 13.70 B3 Ia 200,000[22]  
ρ Leo Leo Olsen (1974)[23] 3m.83 3m.9 3.4271 B1Iab 295,000[12]  
β Ori (Rigel)[4] Orion Waelkens et al. (1998)[2] 0m.17 0m.22 B8Ia 279,000[24]


Brightest member
ε Ori (Alnilam) Orion Cousins (1960)[26] 1m.64 1m.74 B0.5Iabea 275,000[12]


χ2 Ori Orion Waelkens et al. (1998)[2] 4m.68 4m.72 2.8682 B2Ia 446,000[12]  
9 Per Perseus Abt (1957)[13] 5m.15 5m.25 A2 Ia 141,000[28]  


Designation (name) Galaxy Discovery Maximum Apparent magnitude (mV) Minimum Apparent magnitude (mV) Period (days) Spectral class Luminosity Comment
LHA 115-S 18[29] SMC 13m.3 Complex B[e]sg   Possible LBV
HDE 268835[29] LMC 10m.60 10m.68 >100 B8p    
HD 37974[29] LMC 10m.92 11m.00 400 B0.5Ia+    


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  • Samus N.N., Durlevich O.V., et al. Combined General Catalog of Variable Stars (GCVS4.2, 2004 Ed.)