B(e) star
A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically much weaker. Other observational characteristics include optical linear polarization and often infrared radiation that is much stronger than in ordinary B-class stars, called infrared excess. As the Be nature is transient, Be stars might exhibit a normal B-type spectrum at times, and hitherto normal B stars may become Be stars.
While most Be stars are on the main sequence, the identifier actually refers to a heterogeneous group of objects including pre main sequence stars, supergiant stars, protoplanetary nebulae, and others [1]. They may be subclassed into B[e] supergiants, Herbig Be stars, compact planetary nebula B[e], symbiotic B[e], and a catch-all "unclear" category.
The first star recognized as Be star was Gamma Cassiopeiae, observed 1866 by Angelo Secchi, the first star ever observed with emission lines. With the understanding of the processes of emission line formation in the early 20th century it became clear that these lines must come from the circumstellar environment, not from the star itself. Nowadays, all the observational characteristics are explained with a gaseous disk that is formed of material ejected from the star. The infrared excess and the polarization result from the scattering of stellar light in the disk, while the line emission is formed by re-processing stellar ultraviolet light in the gaseous disc.
Be stars are generally acknowledged to rotate rapidly, which has been impressively confirmed by interferometric measurements of the rotational distortion of Achernar. However, rotation alone is probably not sufficient to form the disk, but an additional ejection mechanism is required, such as a magnetic field or nonradial stellar pulsation. The transient nature of the Be phenomenon is most likely connected to the nature of that secondary process, but the details are currently still being discussed.
Be stars are typically variable and can either be classified as Gamma Cassiopeiae variable due to the transient nature of the disk and the scattering processes, or as Lambda Eridani variable on account of their pulsational nature.
References
Porter J., Rivinius Th.: Classical Be stars, 2003 PASP 115, 1153