Planetary nebulae nuclei and white dwarfs exhibit global oscillations at several stages of their evolution, see Fig. 1.1. These oscillations are believed to arise from partial ionization of abundant elements in the stellar envelope as the stars cool, e.g. C and O at K, He at K and H at K. In general, white dwarfs are fainter than , but the oscillations are also much larger than for main-sequence stars, typically between 1% and 30% of the integrated optical flux, and this can easily be observed by STARS.
Several areas of science can be addressed by such observations. Winget et al. (1991) have demonstrated that with several days of near continuous optical coverage it is possible to measure the mass of an isolated white dwarf, its rotation rate (and hence angular momentum), to search for weak magnetic fields and, from the rate of change of pulsation periods, to study the effect of contraction and cooling of the star. Such an independent calibration of cooling rates is very important for determining the temperature - age relation for white dwarfs and, from the white-dwarf luminosity function, the age of the galactic disk (e.g. Iben and Laughlin 1989). From this we will be able to study the effects of compositional stratification by measuring the relative UV and optical amplitudes as function of pulse phase, providing a key test of ideas about the formation of white dwarfs.
The Whole-Earth Telescope has been very successful in the early studies of these stars. However, STARS should be able to detect a much richer spectrum of low-amplitude normal modes than has been possible from the ground, and consequently we anticipate a substantial refinement in the conclusions that so far have been drawn.