Unstable Luminosity and Temperature Combinations
The localization of particular kinds of variable stars in regions of the HR diagram exhibited
in the left frame implies that there are certain combinations of surface temperature and luminosity that
make a star susceptible
to pulsations. In general, we may expect that this is due to an alteration in the
internal energy
flow (usually by opacity changes) that upsets the hydrostatic equilibrium in certain parts of the star.