As far as I understand it, a nebula initially tends to have a small amount of rotation (on average) and then as it collapses under its own gravity this rotation increases significantly due to conservation of angular momentum (assuming that there is no external torque,right?!). What I'm unsure about, is why do nebulae tend to end up as rotating discs (after starting from an initial, potentially arbitrary shape)? Is it somewhat analogous to the reason why the Earth is an oblate sphere, i.e. due to the effects of rotation, because the top and bottom parts of the Earth have a smaller tangential velocity and are therefore more heavily influenced by the force of the Earth's own gravity, whereas near the equator the tangential velocity is much higher (since it is proportial to the radius) causing the Earth to bulge outwards?!