Fragmentation of Collapsing Clouds

The preceding discussion of the Jeans collapse criterion presents us with an immediate problem. The Jeans critical density favors the collapse of massive clouds, but most stars that we observe have masses of the order of 1 solar mass or less, and almost none have masses larger than about 100 solar masses. The solution to this dilemma is thought to lie in fragmentation of collapsing clouds, as illustrated in the following figure.

As large clouds, which have lower critical densities, begin to collapse, they get more dense. As they do, subregions in the original cloud that did not have enough density to collapse will eventually exceed the Jeans density. If there are sufficient fluctuations in the cloud, such regions may begin to collapse independently. These regions, in turn, may fragment as they collapse and grow more dense and some of their subregions exceed the critical density.

By this process, the original large cloud fragments into a hierarchy of smaller collapsing clouds. Although we cannot claim to understand all the details of such a sequence, it is clear schematically that this might lead to clusters of stars in the range of 1 solar mass, as observed.