Shell star variables. Pleione and Gamma 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. Shell star A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broad absorption lines are due to rapid rotation of the photosphere,
S-type star shells are seen around a number of carbon stars, but not S-type stars. Infrared excesses indicate that there is dust around most intrinsic S stars, but the outflow has not been sufficient and longlasting enough to form a visible detached shell. The shells are thought to form during a superwind phase very late in the AGB evolution. BD Camelopardalis is a naked-eye example of an extrinsic S star. It is slow irregular variable in a symbiotic binary system with a hotter companion which may also be variable. The mira variable Chi Cygni is an intrinsic S star. When near maximum