Spectroscopy of circumstellar shells

Historically, the term circumstellar shells has usually been applied to stars with very large mass loss rates. In this chapter the authors concentrate on the coolest late-type, stars. Although only a small percentage of main-sequence stars are of class O and B, essentially all main-sequence stars with masses greater than about one solar mass are believed, eventually, to experience an episode of rapid mass loss as red giants. This paper concentrates on spectroscopic investigation of CS around cool red giant stars. The authors interpret spectroscopic loosely to include infrared photometry and medium resolution spectroscopy of dust grains as well as high resolution observations of the gas