Tentative evidence, based on photoelectric observations of the Ca+ emission core, is presented for the existence of circumstellar envelopes in several G and K type “giants.” A significant emission asymmetry in α Tau may imply a chromospheric rather than a circumstellar source.

A physical mechanism that can result in the generation of extended expanding atmospheres is discussed. The process involves the unloading of stellar material following the arrival of a shock wave at the edge of the star. The basic principles are developed from a discussion of a simplified case that has been studied in the laboratory; they are then applied to the atmosphere of a star. A radiation-hydrodynamics computation of a model cepheid is then used to obtain quantitative atmospheric profiles. The computed continuum and spectral lines during the unloading process are then examined. A discussion of the possibility that the unloading process occurs in stars other than cepheids suggests the existence of a shock visibility factor associated with ionization or dissociation in the region behind the shock front and leads to a possible alternate interpretation of the variable star instability strips in the H-R diagram.