AGamma Cassiopeiae variable (γ Cassiopeiae variable) is a type ofvariable star, named for its prototypeγ Cassiopeiae.

TheGeneral Catalogue of Variable Stars (GCVS) categorises γ Cassiopeiae stars aseruptive variables and describes them as rapidly-rotating B class giants or subgiants, although many of them aremain sequence stars. It distinguishes them from those Be stars that only show smaller amplitude brightness variations. The GCVS uses the codeGCAS to denote γ Cassiopeiae variables.^[2]

γ Cassiopeiae variables show irregular changes in brightness on a timescale of decades. These typically have amplitudes of the order of a magnitude. For example, γ Cassiopeiae is usually about magnitude 2.5 and has varied between magnitudes 1.6 and 3.0. The variations are associated with changes in the spectrum between normal absorption spectra andBe star spectra, often also includingshell star characteristics.^[2]

Pleione and γ Cassiopeiae itself are both variable stars that have intermittent shell episodes where strong shell features appear in the spectrum and the brightness increases or decreases significantly. At other times the shell is not detectable in the spectrum, and even the emission lines may disappear.^[3]

γ Cassiopeiae variables are understood to be hot stars which have equatorialdecretion disks which periodically disappear and reform, or possibly just change dramatically in scale. They are probably all very rapid rotators and most can be classified asBe stars. They are often alsoshell stars at least part of the time, where the disk is seen edge-on and produces very narrow absorption lines in addition to the broaderphotospheric lines and possible emission lines. Regardless of whether they are shell stars by the most narrow definition, the periods when they produce strong disks and increase in brightness are known as shell events.^[3]

Gamma Cassiopeiae variables are inherently luminous and relatively common, so there are many naked-eye examples.

• Bohlender, D. (2016). "Searching for and Monitoring Ae and a Shell Stars at the DAO".Bright Emissaries: Be Stars as Messengers of Star-Disk Physics.506: 275.Bibcode:2016ASPC..506..275B.
• Slettebak, A. (1982)."Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars".Astrophysical Journal Supplement Series.50:55–83.Bibcode:1982ApJS...50...55S.doi:10.1086/190820.

• Gamma Cassiopeiae variable stars

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