Stellar Wind Velocity

Stellar winds are continuous outflows of plasma driven from the outer layers of hot, massive stars. The material is accelerated by the intense radiation field, which transfers momentum to the gas through absorption and scattering in spectral lines. This process creates a supersonic flow that can carry away a significant fraction of the star’s mass over its lifetime.

The terminal velocity (v_∞) of a stellar wind is the asymptotic speed reached far from the stellar surface, where the acceleration has effectively ceased. In the simplest radiation‑driven wind theory, v_∞ scales with the surface escape velocity, but it is also reduced by the Eddington factor (Γ), which measures the ratio of radiative to gravitational forces.

A convenient expression that captures these dependencies is derived from the balance of forces at the stellar surface. It relates the wind’s terminal speed to the stellar mass (M), radius (R), and the Eddington factor (Γ = L/L_Edd). The formula shows that higher luminosity (larger Γ) lowers the effective gravity and therefore reduces the terminal velocity, while more massive or compact stars produce faster winds.