Magnetic braking

Summary

Magnetic braking is the loss of angular momentum of a rotating star through its magnetized wind. The magnetic field forces the outflowing gas to corotate with the star out to a large distance, the Alfvén radius. So each unit of lost mass carries away much more angular momentum than it had at the surface. It explains why stars like the Sun slow their rotation as they age, and it drives the shrinking of the orbits of close binary stars.