The densities of these clouds are still very low, comparable to and even less dense than the vacuum which is achieved in physics laboratories. From observations with a high spatial resolution, it is found that the density in the clouds is not distributed in a homogeneous way. The cores of such clumps contain gas of a density ten times higher than the average density of the clumps. Star formation is a process which owes its origin to the self-gravitation of the molecular cloud clumps. If no mechanisms of support against the collapse of the clumps were present, the star formation process would be much more effective and happen on much shorter time scales than it really is and does.
Magnetic fields provide support to the clouds against their self-gravitation.
When the ionization fraction of the molecules in the cloud is very low, magnetic support becomes synonymous with ambipolar diffusion. The charged particles interact with the magnetic fields, and the neutrals can be supported against their self gravity through the frictional drag they experience as they slip relative to the ions. Due to ion-neutral collisions, magnetic energy is eventually lost, decreasing the strength of the magnetic field, and hence decreasing the effectiveness of the magnetic support. Another mechanism which has been proposed to explain cloud support, is rotation. However, at the start of the cloud collapse, the rotation of the cloud cores is coupled to the rotation of the entire cloud, because of magnetic braking.