Contents Introduction Molecular clouds are generally believed to be the sites where new stars are born. The physics in these clouds is determined by a large number of processes like hydrodynamics, radiation, atomic and molecular processes, magnetic fields and self-gravity.
Gravitational energy is transformed into heat and effectively radiated away by infrared emission.
From the collapsing regions dense cores emerge, which are thought to be the progenitors of new stars. Then the cores start collapsing again. The material is compressed to such high densities that it becomes optically thick for IR radiation. Now the gas can heat and establish a significant amount of pressure which stops the collapse. It is believed that the occurence of highly collimated bipolar outflows is related to this period.