A shell star, also termed Gamma Cassiopeiae variable (GCAS), is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter. The shell stars are fast rotators, giving a partial explanation on the mechanism, but shell stars are still considered enigmatic. Shell stars belong to spectral types O7.5 to F5, but their spectra are characterized by enormously widened absorption lines due to fast rotation and the disc that contributes also to other spectral peculiarities. Rotation velocities are about 200–250 km/s, not far from the point when the rotational acceleration would disrupt the star. The spectra and general qualities of shell stars are hard to interpret, since normal spectral features are overlaid with variable emission, so luminosity class and exact spectral class might easily be misinterpreted.
Shell stars are subdivided in four categories:
- early Be stars of spectral types 07.5 to B2.5
- middle Be stars of types B3 to B6.5,
- late Be stars of types B7 to B9.5, and
- A-F shell stars ranging from A0 to F5.
The spectrum is variable in the long term, and the early type shell stars may switch from and to exhibiting a Be type spectrum or an ordinary B spectrum. All shell stars exhibit broadened emission instead of absorption lines in spectrum for the counterparting non-shell star type, so that B shell stars show emission of the Balmer Hydrogen lines where a normal B star show absorption. The earlier types usually exhibit He I and usually Fe II emission, the later shell stars exhibits Ca II and Ti II emission. It is believed that the shell stars belong to somewhere in the range of main sequence up to giant star size, although the exact luminosity class is unclear by the general emission broadening caused by the rotation.
The prototype shell star is Gamma Cassiopeiae, while the brightest example is Achernar. A certain peculiar shell star is HR 2309 – a sharp-lined shell star, indicating a small and well defined ring structure. Other examples include: