Photospheric line profiles in the optical spectra of Be stars show rapid variations which have been identified as the effect of nonradial pulsation. The most common patterns are low order sectorial modes which divide the stellar surface along nodes of constant longitude. Since these pulsations are strongest in the equatorial regions of the star, it has been suggested that they are the source of the extra force necessary to eject material from the star into the circumstellar disk.
If nonradial pulsation is important as a cause of the Be phenomenon, it is necessary to understand the driving mechanism. Until fairly recently there was no theoretical justification for a pulsational instability in the early-type main sequence B stars. New calculations, however, have shown that iron lines may contribute enough absorption to support pulsation by the familiar opacity mechanism.
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