A main-sequence star with a spectral type in the range B–F whose spectrum contains prominent narrow absorption lines originating in surrounding gas. Shell stars are usually Be stars, in which case both emission and absorption arise in an equatorial disk of ejected gas which we happen to view edge-on from Earth. They are fast rotators and show small-amplitude changes in brightness. Many have variable spectra. Pleione, also known as BU Tauri, a member of the Pleiades cluster, is a well-known example of a shell star, with a range of mag. 4.8–5.5 and spectral type B8. Shell stars that show irregular variability due to changes in, or even disappearance of, the shell are called Gamma Cassiopeiae variables.