Skumanich rotation law ($\Omega \propto t^{-1/2}$)

If the moment of inertia I changes slowly and wind-driven magnetic braking of a solar-type star has torque $-K{\Omega }^3$, then ${\Omega }^{-2}(t)={\Omega }^{-2}(t_0)+2K(t-t_0)/I$. At sufficiently late times within this unsaturated approximation, rotation scales as $t^{-1/2}$. This empirical main-sequence trend is not a universal law for saturated rapid rotators, changing stellar structure or every magnetic geometry.