There are several types of star with masses greater than 1.5 that are known to be oscillating. Examples are Scuti stars, which populate the low-luminosity extension of the Cepheid instability strip, with periods ranging from 20 min. to a few hours. There are also the slowly pulsating B stars (periods are between 1 and 3 days), oscillating Be stars, with periods between 0.5 and 2 days, and Cephei stars, whose periods range from 2 to 8 hr. All these stars are multiperiodic, and most, if not all, B-type variables owe their pulsations to the kappa mechanism operating in the metal-opacity bump. It is firmly believed that in any given star the modes that have so far been observed represent only a small fraction of the modes that are actually excited. The reason is that the amplitude-limiting mechanism of the modes observed involves a draining of energy into other modes of oscillation of yet lower amplitude. Our confidence in this conclusion stems from the fact that in the case of Cepheids, where only one or two modes are involved, the theory predicts amplitudes large enough to be observed, and it has thus been possible to confirm its validity. When applied to Scuti stars, for example, the same theory shows that there must be a large number of modes excited to low amplitude. STARS will be able to observe this rich source of diagnostic oscillations, from which detailed inferences concerning the internal structure of those stars will be possible. Amongst the unstable modes, there are several with large amplitudes near the edge of the energy-generating core. Such modes are excellent probes for investigating the nature of the region of convective overshoot. The stars are also potentially susceptible to material redistribution by rotationally induced circulation currents. Measurements of the internal angular velocity from rotational splitting of oscillation frequencies will be of paramount importance for assessing the role of this phenomenon in influencing the course of the stars' evolution.
Oscillations of Wolf-Rayet stars have recently been discovered by Blacha, Schaller and Maeder (1992). This holds considerable promise for an asteroseismic investigation of the evolution of very massive stars.