Gravitational torques and star formation

Any irregularities that occur in the structure of a collapsing cloud will produce non-radial gravitational forces and, if such irregularities are sheared by differential rotation into trailing spiral features of large amplitude, the associated gravitational torques will transfer angular momentum outward on an orbital time-scale. Even if no redistribution of angular momentum occurs during the collapse and a flattened disc-like configuration is formed, such a disc will be a fragile structure at best, and with realistic initial conditions it will be highly unstable to the development of trailing spiral density enhancements. Thus it seems inescapable that in a realistic collapsing cloud there will sooner or later occur rapid outward transfer of angular momentum by gravitational torques. The only stable outcome of the collapse will then be a system in which most of the mass is in a central star or binary system, and less than half remains in a disc around the star; less than one-quarter can remain in a disc with the size of our Solar System.