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.
Credits J.L. Beuzit et al. (Grenoble Obs.)
Beta Pictoris, a small young star, is a good example of a Solar - like system in the making. We mentioned that the disk acquires angular momentum (as a skater does while spinning on her axis). However, if the disk is too large or rotates too fast, a lot of energy can be dissipated in the rotation. If the kinetic energy is greater than the gravitational energy then material can be ejected out, as a rider on a merry-go-round will be ejected out if the merry-go-round started to spin fast enough. In reality, it is believed that frictional and magnetic forces in the rotating disk dampen out the rotation of the disk.