壳层星是在光谱中显示在赤道周围有气体的星周盘环绕的恒星。壳层星快速的自转，给了这种机制一部分的解释，但壳层星依然是神秘的。壳层星的光谱类型从O7.5至F5，但是它们在光谱上的特征是有宽大的吸收谱线，这是由快速的自转和星周盘造成的，也还有其它的光谱特征。壳层星自转的线速度大约是200-250公里/秒，距离足以毁灭恒星的角加速度不远。一般性质的壳层星和光谱很难分辨，因为正常的光谱线会和产生变化的谱线重叠在一起，所以在没有亮度分级和精确的光谱分类时很容易被误解。

壳层星可以细分为4类：

光谱有着长期的变动，早期型壳层星的光谱和Be星或B型恒星的光谱是互相替换的。所有的壳层星都呈现出宽阔的发射谱线取代了吸收的谱线，并且对应于非壳层星的光谱，所以B型壳层星发射出巴耳麦氢谱线，而一般的B型恒星是吸收谱线。早期型的壳层星显示出He I和Fe II的发射线，而晚期型的壳层星显示出Ca II和Ti II的发射线。虽然因为高速自转使得谱线扩散，和造成光度分类上的不确定性，但一般认为壳层星属于主序带的巨大恒星。

A shell star is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They may 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.