Star formation in a galactic cluster
A model has been developed for the collapse of an interstellar cloud with turbulence. The differential equations which describe the evolution of the cloud include ionic and dust cooling and also the dissipation of energy due to the collision of turbulent elements moving at supersonic speeds. Under some conditions the collision of two elements can give rise to a star and the rate of star formation and the mass of the stars formed changes as the cloud collapses. The pattern found is that the stars first produced have masses of about 1.4 M . and the masses get less as star formation continues. Stars produced by this mechanism have little associated angular momentum. Some of the stars which happen to move in high density regions of the cloud may increase their mass greatly by accretion; these stars will be the more massive stars and they will also rotate most rapidly, a theoretical prediction which agrees with observation. On the basis of the model the proportion of stars which would have planetary systems is estimated. This shows that there should be of order 10 6 planetary systems per galaxy.