I review driving mechanisms for stellar winds, using first the
example of the coronal, pressure-driven solar wind, but then
focussing mainly on radiation-pressure driven winds from hot,
For the latter, I review the central role of line-opacity as a
coupling between matter and radiation, emphasizing how the Doppler
shift of an accelerating wind outflow exposes the strong line opacity to
a substantial continuum flux, and thus allows the line force to
sustain the outward acceleration against gravity.
Through the CAK formalism that assumes a power-law distribution of
line-opacity, I derive the mass loss rate and wind velocity law, and
discuss how these are altered by various refinements like a
finite-disk correction, ionization variations in opacity, and a
non-zero sound speed.
I also discuss how multiline scattering in Wolf-Rayet (WR) winds can allow
them to exceed the single scattering limit, for which the wind and radiative
momenta are equal.
Through a time-dependent perturbation analysis, I show how the
line-driving leads to a fast, inward "Abbott-wave" mode for
long wavelength perturbations, and a strong
Line-Deshadowing-Instability at short wavelengths, summarizing also
1D and 2D numerical simulations of the nonlinear evolution of this
I next discuss how rapid stellar rotation alters the latitudinal variation of
mass loss and flow speed, and how this depends on treatment of
gravity darkening, nonradial line forces, and "bi-stability" shifts
Finally, I conclude with a discussion of the large mass loss epochs of
Luminous Blue Variable (LBV) stars, and how these might be modeled via
super-Eddington, continuum driving moderated by the "porosity"
associated with extensive spatial structure.