379 
350 , 35i] The Origin of the Galactic System 
combined with the rotational velocity of the original nebula, provides a 
satisfactory explanation of the observed star-streaming in the galactic plane, 
this now being interpreted in terms of the ellipsoidal velocity distribution of 
Schwarzschild, and not in terms of the two intermingled star-streams of 
Kapteyn. On this view the velocities of the stars in any small region may 
be resolved into: 
(1) a uniform motion of rotation about the axis of the galaxy, 
(2) an individual motion, superposed on to the foregoing, with the law 
of distribution given by Schwarzschild’s ellipsoidal law. 
The combined motion gives a velocity distribution of the type included 
in formula (339'1), and so represents a possible state of steady motion. 
The foregoing view of the origin of the galactic system accords with the 
fact that the planes of the orbits of binary stars shew a preference for 
parallelism to the plane of the galaxy. In the past different investigators have 
reached different conclusions on this question*, but a recent investigation 
by Kreikenf provides strong evidence that there is a real tendency to 
parallelism. 
It is clear from photographs of spiral nebulae that, when condensation 
of their outer parts first sets in, these do not immediately break into a 
uniformly distributed cloud of stars. Most nebulae in the early stages of 
development, and some in late stages, exhibit condensations which are far 
too large to be single stars, and are probably clusters each containing a great 
number of stars. The theory of gravitational instability makes it easy to 
understand how these large clusters come to exist. For it shews that all 
displacements of a gaseous medium which have a wave-length above a certain 
critical length are unstable, but that those of greatest wave-length are most 
unstable. The result must be that the condensations form on the largest 
possible scale first, and then gradually smaller condensations form inside 
these until the shortest wave-length is reached which gives rise to instability, 
the final condensations being of stellar mass. Nebulae such as M 51 (Plate XV) 
and M 81 (Plate XII) shew this process going on. 
These larger condensations may very possibly be precisely those groups 
of stars whose relics appear as moving clusters or open clusters in the galaxy, 
while the few which succeed in escaping altogether from the main mass, 
before the process of disintegration is far advanced, may be the observed 
globular clusters. This conception reconciles the strong general impression 
which the galaxy produces of being a chaos of moving clusters, with the 
equally strong impression it produces of being the last stage in the develop 
ment of a spiral nebula. 
* Aitken, The Binary Stars, p. 218. 
f E. A. Kreiken, AI.N. lxxxvii. (1927), p. 101. 
24-2