These proceedings of a workshop held at Isle-aux-Coudres, Quebec, Canada in August 1993, provide an assessment of a fundamental subject in modern stellar astrophysics. Hot, luminous stars play a crucial role in the rapid recycling of matter in dense regions of the Universe. The most luminous objects generate the strongest outflows of material, which in turn have a profound impact on both the interstellar medium and the underlying stars themselves. However, recent observational and theoretical advances have shown that these outflows are highly unstable and inhomogeneous. These characteristics constitute a fundamental property of the outflows that can no longer be disregarded. This text provides an assessment of this important phenomenon from both an observational and a theoretical perspective. It includes up-to- date discussions of the observational diagnostics of time-dependent stellar-wind structure, the theoretical understanding of the origin of such structures, and its consequences for the process of mass loss via stellar winds.

This specialized workshop was conceived during the workshop on "Non isotropic and Variable Outflows from Stars", which was held at the Space Telescope Science Institute in October, 1991. At that meeting, the four of us collectively decided that the time was ripe for an even more focussed discussion of the basic issues in the area of hot-star wind instability and its observable manifestations. Not that the big problems have been solved! Rather, we are currently in a phase of rapid development, both with regard to the models and to the observations. The key issue at this new workshop would be to decide how the time-dependent structures observed in hot-star winds (e. g. , NACs, DACs, blobs, clumps, filaments, shells, puffs, jets, etc. ) relate to radiative and other instabilities. Further questions concern the role of turbulence and the nature of its driver, and the effect of stellar rotation, pulsation, and magnetic fields on time-dependent phenomena in hot-star winds. Of no less importance is the impact of stellar wind variability on the derivation of mass-loss rates, on stellar evolution, and on momentum/energy deposition in the interstellar medium. To attain our goal of maximum confrontation (in the positive sense!) we decided: (1) to limit the workshop to the observers and theoreticians most active in this field in the world; (2) to insist that virtually all participants present a talk, thereby avoiding the distraction of poster sessions; and (3) to allocate approximately half of the allotted time to discussion.

A strange field of speculation is opened by this phenomenon ... here we have a star fitfully variable to an astonishing extent. and whose fluctuations are spread over centuries. apparently in no settled period. and with no regularity of progression. What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so uncertain a source? -- J. F. W. Herschel We can imagine, at least, that the study of Luminous Blue Variable stars began as the official court astrophysicists of Eridu speculated about a new 5000 years ago first-magnitude star which persisted in their southern sky for several years.2 After that the topic languished for a while, but now it has been renewed following recent observations by Blaeu (P Cygni, c.1600) and Herschel (1] Argus, c.1840) ... -- More soberly, it is only within the past few years that we have begun to understand LBV's and to recognize their coherence as a class. Thus only a minority of astronomers are familiar yet with this relatively new topic, whose potential impli cations are of great interest (see below). One purpose of this book, we hope, will be to provide an accessible overview of the subject for interested astronomers and astrophysicists in general."