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A '''variable star''' is a [[star]] whose [[luminosity]] varies significantly over time.<ref name=britannica>[https://www.britannica.com/science/star-astronomy#ref52839 Variable star] from britannica.com</ref> Many stars vary a little (the [[Sun]]'s brightness varies periodically by around 0.1% every 11 years in accordance with the [[sunspot cycle]]) but the term "variable star" normally refers to stars whose [[luminosity]] varies by large amounts, often a couple of [[star magnitude|magnitudes]]. These changes in brightness are often periodic though this is not always the case. Variable stars are grouped into two types, intrinsic and extrinsic, depending on whether the star's brightness changes are caused by the star itself or its environment. As of 2018, 52,011 variable stars have been discovered, with examples including [[Algol]] and [[Antares]].<ref name=gcvs>Samus N.N., Kazarovets E.V., Durlevich O.V., Kireeva N.N., Pastukhova E.N., General Catalogue of Variable Stars: Version GCVS 5.1, ''Astronomy Reports'', 2017, vol. 61, No. 1, pp. 80-88 [http://adsabs.harvard.edu/abs/2017ARep...61...80S Online]</ref><ref name=swin>[http://astronomy.swin.edu.au/cosmos/V/Variable+Stars Variable stars] from astronomy.swin.edu.ac</ref>

A '''variable star''' is a [[star]] whose [[luminosity]] varies significantly over time.<ref name=britannica>[https://www.britannica.com/science/star-astronomy#ref52839 Variable star] from britannica.com</ref> Many stars vary a little (the [[Sun]]'s brightness varies periodically by around 0.1% every 11 years in accordance with the [[sunspot cycle]]) but the term "variable star" normally refers to stars whose [[luminosity]] varies by large amounts, often a couple of [[star magnitude|magnitudes]]. These changes in brightness are often periodic though this is not always the case. Variable stars are grouped into two types, intrinsic and extrinsic, depending on whether the star's brightness changes are caused by the star itself or its environment. As of 2018, 52,011 variable stars have been discovered, with examples including [[Algol]] and [[Antares]].<ref name=gcvs>Samus N.N., Kazarovets E.V., Durlevich O.V., Kireeva N.N., Pastukhova E.N., General Catalogue of Variable Stars: Version GCVS 5.1, ''Astronomy Reports'', 2017, vol. 61, No. 1, pp. 80-88 [http://adsabs.harvard.edu/abs/2017ARep...61...80S Online]</ref><ref name=swin>[http://astronomy.swin.edu.au/cosmos/V/Variable+Stars Variable stars] from astronomy.swin.edu.ac</ref>