How and Why do stars form?
What causes the clouds to become unstable and condense into smaller masses? Sir James Jeans (1877-1946) realised that there must be a critical value for a mass of gas such that above a certain limit, gravity would overcome the thermal motion of the particles which would otherwise disperse the cloud. Above this mass, the gas cloud is unstable and will collapse. This is the Jeans Instability.

Stages in Star Formation

1.) Dense cloud regions become unstable (possibly triggered by a shock wave from a supernova) collapsing on themselves.

2.) Additional gas is accreted and the cloud fragments into dense cloud cores.

3.) The core continues to collapse, photons generated at the hot center become trapped. This forms a Protostar.

4.) The protostar contracts, while still accreting mass, possibly from a disk, forming a pre-main-sequence star, then main-sequence star.