The Origin of the Galactic Rotation and of the Connection Between Physical Properties of the Stars and Their Motions

The origin of the rotation of the galaxy and of the spiral nebulae in general is traced back to a time when the nebulae were recently formed from a common system of pri- mordial gas. At this stage the diameters of the nebulae were of the same order as their mutual separations, the system as a whole had gravitational instability, and the mutual attractions and relative motions of the extended nebulae produced large angular momenta, but only very small angular motions. During the process of contraction in any nebula, the linear and angular velocities increase, leaving the total angular mo- mentum constant. The connection between mass and velocity dispersion is accounted for by the forma- tion of massive bodies in the denser regions and less massive bodies in the less dense regions of the galaxy. The velocity dispersion also depends on the place in the system where the objects were formed. The physical basis for the decrease in group motion with increasing velocity dispersion is found in the accumulated effect of viscous forces of different, specified types in the gaseous nebula, which includes a great number of small condensations. In the dense central part of the galaxy, viscous forces produce in the end rotation as a solid; farther out in the galactic plane they produce another type of stable, circular motion, in which centrifugal forces exactly balance the gravitational forces. The great preponderance of circular, "planetary" motions may be explained in this way. Stars formed in regions where the viscosity was insufficient to produce a stable state of motion, or before this state had been reached, move in effipses with higher eccentricity, and have for this reason greater velocity dispersion and smaller group motion than stars formed in the galactic plane. The quadratic relationship found to exist between group motion and velocity dispersion is to be expected, at least as a first approximation. The formation of planetary systems like that of the sun may possibly have been similar to that of the galaxy. If formed in this way, planetary systems should be rather com- mon in the galaxy