Cloud Formation, Evolution and Destruction

We now begin to trace the journey towards a star. How long does this take? The answer is surprisingly short: a good many clouds already contain new stars and these stars tend to be young. The typical cloud cannot spend long, if any time at all, in a dormant state. The first challenge is to accumulate gas into a molecular cloud. One dilemma is to decide what occurred first: the production of the cloud or the molecules? We all agree that a star-forming cloud must become gravitationally bound. Here, we shall investigate what this implies and what bound states are possible. To do this, we need to bring in concepts from hydrostatics and hydrodynamics. Changes in pressure and gradients in pressure take on great significance. Where will the environment’s pressure confine a cloud? And where will a cloud lose pressure, deflating like a balloon? Or, where will the cloud’s own gravity provide the confinement? Theory is vital in this chapter since the changes in clouds cannot be observed on human time scales: the evolution is too slow and must be hypothesised. Many scenarios begin from a static equilibrium. This provides the initial conditions to which each protostars can be traced back. Yet the observed motions are impressively fast and we find that classical theory cannot keep up. We are led towards something more violent.