Shown is the radial velocity v(r) of a line driven wind from an O star, as function of time
The rather arbitrary initial conditions try to relax to the so-called beta velocity law of Castor et al. (1975) and Pauldrach et al. (1986)
However, a new, radiation-hydrodynamical instability occurs, first described by Lucy and Solomon (1970), and extensively studied by Owocki and Rybicki (1984, 1985, 1986), Owocki et al. (1988), Owocki (1991), Owocki and Puls (1996, 1999), Feldmeier (1995) and Feldmeier et al. (1997)
The instability creates strong shock fronts in the wind
These are reverse shocks, which stop fast, thin gas and accrete it onto very narrow, very dense shells
Occasionally, weak forward shocks occur at the outer shell edge, when the shell ploughs through the outer wind gas
