Problems in star formation

How do stars form out of gas? Consider a molecular cloud; just out of random flcutuations, there are regions in the clouds where the density is slightly higher. The gravitational force of the denser patch pulls other matter towards it. The clump becomes denser and attracts more matter in a seemingly runaway process. As this clumping is going on, the kinetic energy of the atoms/molecules rushing in the denser parts increases. If higher densitites are to be achieved, this energy has to be dissipated or otherwise the thermal energy due to the infalling matter will prevent any further collapse of the cloud. Both dust and molecules, most notably H₂, play a major role in the collapse of a cloud. As we have seen, dust absorbs UV light and radiates back infrared light that escapes out of the cloud; molecules are also good absorbers of UV light and they radiate in the infrared. Since there is a net outflow of electromagnetic energy out of the core, the core itself, at least in the initial stages of the collapse, can be maintained at a relatively low temperature, thus enabling the infall of more material.

If matter, as it falls into a protostar core, starts to spin around the core itself, it can form a rotating disk. Our solar nebula, the primordial cloud of gas and dust from which the Sun and the planets formed, started as a rotating disk that gradually flattened out due to the centripetal force.

Beta Pictoris
Credits J.L. Beuzit et al. (Grenoble Obs.)