The Physics and Chemistry

To appreciate the following story, we need to be familiar with the setting, the cast, their observational appearance and their physical interactions. Here, each member of the cast – gas, dust, cosmic rays, magnetic fields and radiation – is introduced and their potential behaviour is studied. Stars form in clouds. Like atmospheric clouds, they are largely molecular and opaque. However, star-forming clouds are made more complex by a quite stunning range of physical, chemical and dynamical processes. The processes work in synthesis to provide triggers, regulators and blockers during the collapse stages. Energy is transferred from large scales down to small scales. Simultaneously, feedback occurs from small scales back onto large scales. As we shall see, the progress of a parcel of gas is far from systematic – at each stage in the development there is a chance that it will be rejected. It will, therefore, prove rewarding to first acquire a broad overview.