Radiation driving in spectral lines not only boosts the
radiation force, it also solves the problem of the star
potentially blowing itself apart:
As the line-driven material starts to move off the surface of
the star, it is Doppler-shifted, making a previously narrow line
broader, and increasing its ability to absorb light.
of optically thick (opaque) lines
allows a hot star wind to bootstrap itself into existence!
And causes the radiation force to deviate from strictly 1/
behavior: the radiation force on lines can be less than gravity
inside the star but more than gravity above the starís surface.