A variable star that exhibits sudden changes in brightness, in the form of outbursts, flares, or fades. The variations are caused by activity in the star’s chromosphere or corona, and may be accompanied by mass loss through the ejection of shells or as an enhanced stellar wind. See Flare Star; FU Orionis Star; Gamma Cassiopeiae Star; Irregular Variable; Nebular Variable; R Coronae Borealis Star; RS Canum Venaticorum Star; S Doradus Star; T Tauri Star; UV Ceti Star.