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,
Bivalve shell parts. The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line, which runs along a small distance in from the outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The adductor muscles are what allow the bivalve to close the shell tightly. In some