Editing Rigel (section)

=== Variability ===
[[File:RigelLightCurve.png|thumb|right|A [[light curve]] for Rigel, adapted from Moravveji ''et al.'' (2012)<ref name=apj2012_747_108/>]]
Rigel has been known to vary in brightness since at least 1930. The small amplitude of Rigel's brightness variation requires [[Photoelectric sensor|photoelectric]] or [[CCD photometry]] to be reliably detected. This brightness variation has no obvious period. Observations over 18 nights in 1984 showed variations at red, blue, and yellow wavelengths of up to 0.13 magnitudes on timescales of a few hours to several days, but again no clear period. Rigel's [[color index]] varies slightly, but this is not significantly correlated with its brightness variations.<ref name=guinan1985/>

From analysis of ''[[Hipparcos]]'' satellite photometry, Rigel is identified as belonging to the [[Alpha Cygni variable|Alpha Cygni]] class of variable stars,<ref name="waelkens 1998"/> defined as "non-radially pulsating supergiants of the Bep–AepIa spectral types".<ref name=Samus2017/> In those spectral types, the 'e' indicates that it displays emission lines in its spectrum, while the 'p' means it has an unspecified spectral peculiarity. Alpha Cygni type variables are generally considered to be irregular<ref name=AAVSOvartyps/> or have [[quasi-period]]s.<ref name=vangenderen/> Rigel was added to the General Catalogue of Variable Stars in the 74th name-list of variable stars on the basis of the Hipparcos photometry,<ref name=IBVS/> which showed variations with a photographic amplitude of 0.039 magnitudes and a possible period of 2.075 days.<ref name=lefevre/> Rigel was observed with the Canadian [[Microvariability and Oscillations of STars telescope|MOST]] satellite for nearly 28 days in 2009. Milli-magnitude variations were observed, and gradual changes in flux suggest the presence of long-period pulsation modes.<ref name=apj2012_747_108/>
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