Cliromospheric activity is found in ten types of stars, both single and binary. Variability at many time scales is reviewed, from minutes to centuries. The emphasis is on what is not understood, incorrecty understood, and understood but not appreciated. Observational results in hand include photometry, spectroscopy, spectrophotometry, speckle interferometry, magnetic flelds, and orbltal period variations. Certain physical mechanisms responsible for variability are discussed, making the distinction between the well-established, tlie suspected, and the posslble. Specific topics
include flares, rotation, differential rotation, synchronization, circularization, starspots, spot models, migratlon periods, spot cycles, magnetic activity cycles, sector structure, Maunder minima,
pulsation, light curve solutions, convective envelope changes, mass transfer, mass loss, and orbital period variations. Specific directions for future research are identified.
Results are presented from a study of variability in the stellar wind of G8 Cygni [07.5 III((f))], based on 33 lUE high resolution spectra taken over ~ 7 days. Significant changes are evident in the Si IV resonance line profiles on timescales as short as ~ 1 hour, cliaracterised by broad absorption enhancements at relatively low velocity (0.5 to 0.7 of the terminal velocity, υ∞) which evolve into high velocity discrete absorption components (at > 0.8 over a period of ~ 1 day. The recurrence timescale for the appearance of the broad features is ~ 15-20 hours. Three well monitored consecutive sequenccs of progressive opacity enhancements are described. Their properties are parameterised and incorporated into a self-consistent phenomenological model where the opacity enhancements and ‘underlying’ wind sure radiatively coupled.
The model results show that the absorption enhancements do not propagate from the photosphere in any simple way, and are not due to spherically symmetric phenomena. We conclude that the discrete absorption variability in G8 Cyg is not due to ‘shells’ or ‘puffs’ of matter, but instcaid arise from material passing through perturbations in the flow, which may be illustrated in terms of spirally wound-up streams.