The outer atmospheres of luminous cool giant stars and early-type stars can be driven outward by the strong radiation fields from the stellar photospheres. In the case of the cool stars, radiative driving occurs because of absorption of photons by dust grains that can form in the outer atmospheres. The grains can absorb radiation over a broad range of wavelengths, so the outflows of the cool stars are said to be ‘continuum driven’ winds. In the case of hot early-type stars the winds are driven by the scattering of radiation by line opacity, so their outflows are called ‘line driven’ winds.

The essential difference between continuum driven and line driven winds is the role of the Doppler shift between a parcel of outflowing matter and the photosphere. In the acceleration of a stellar wind to terminal velocity, the stellar light incident on the parcel of the wind is increasingly redshifted up to the final value of Δλ = λv∞/c. For a cool star with a continuum driven wind, this redshift corresponds to a few Å, which is such a narrow band that within it neither the continuum opacity nor the incident radiation field changes significantly. So the Doppler shifting is not important in continuum driven winds. In the case of line driven winds both the line opacity and the radiation field in the lines change significantly over the Doppler shifts associated with the winds.