The winds of luminous hot stars are driven by absorption in spectral lines and they are called line driven winds.

Hot stars emit the bulk of their radiation in the ultraviolet where the outer atmospheres of these stars have many absorption lines. The opacity in absorption lines is much larger than the opacity in the continuum. The opacity of one strong line, say the C IV resonance line at 1550 Å, can easily be a factor of 106 larger than the opacity for electron scattering.

The large radiation force on ions due to their spectral lines would not be efficient in driving a stellar wind if it were not for the Doppler effect. In a static atmosphere with strong line-absorption, the radiation from the photosphere of the star will be absorbed or scattered in the lower layers of the atmosphere. The outer layers will not receive direct radiation from the photosphere at the wavelength of the line, and so the radiative acceleration in the outer layers of the atmosphere due to the spectral lines is strongly diminished. However, if the outer atmosphere is moving outward, there is a velocity gradient in the atmosphere allowing the atoms in the atmosphere to see the radiation from the photosphere as redshifted. This is because in the frame comoving with the gas the photosphere is receding. As a result the atoms in the outer atmosphere can absorb radiation from the photosphere which is not attenuated by the layers in between the photosphere and the outer atmosphere.