Strange - Mode instabilities in luminous stars

If radiation pressure contributes significantly to the total pressure in mo dels for the envelopes of hot and luminous stars, such as massive objects a nd Wolf- Rayet stars, strange - mode instabilities with growth rates in the dynamical range do occur. The properties of these mechanical instabilities are reviewed and an intuitive model of the underlying instability mechanis m is discussed. Adapting spherical geometry the evolution of the instabilit ies has been followed into the nonlinear regime by numerical simulation. Mu ltiple shocks are formed and velocity amplitudes above 10(2) km/sec are rea ched which can imply direct mass loss of the objects.