562 The Genesis of Double Stars 



follow that the analogous figure for compressible fluid is also un- 

 stable, as will be pointed out more fully hereafter. 



Professor Jeans has discussed in a paper of great ability the 

 difficult problems oifered by the conditions of equilibrium and of 

 stability of a spherical nebula 1 . In a later paper 2 , in contrasting 

 the conditions which must govern the fission of a star into two parts 

 when the star is gaseous and compressible with the corresponding 

 conditions in the case of incompressible liquid, he points out that for 

 a gaseous star "the agency which effects the separation will no 

 longer be rotation alone ; gravitation also will tend towards separa- 

 tion. ...From numerical results obtained in the various papers of my 

 own, ...I have been led to the conclusion that a gravitational 

 instability of the kind described must be regarded as the primary 

 agent at work in the actual evolution of the universe, Laplace's 

 rotation playing only the secondary part of separating the primary 

 and satellite after the birth of the satellite." 



It is desirable to add a word in explanation of the expression 

 "gravitational instability" in this passage. It means that when 

 the concentration of a gaseous nebula (without rotation) has pro- 

 ceeded to a certain stage, the arrangement in spherical layers of 

 equal density becomes unstable, and a form of bifurcation has been 

 reached. For further concentration concentric spherical layers 

 become unstable, and the new stable form involves a concentration 

 about two centres. The first sign of this change is that the spherical 

 layers cease to be quite concentric and then the layers of equal 

 density begin to assume a somewhat pear-shaped form analogous 

 to that which we found to occur under rotation for an incompressible 

 liquid. Accordingly it appears that while a sphere of liquid is stable 

 a sphere of gas may become unstable. Thus the conditions of stability 

 are different in these two simple cases, and it is likely that while 

 certain forms of rotating liquid are unstable the analogous forms for 

 gas may be stable. This furnishes a reason why it is worth while to 

 consider the unstable forms of rotating liquid. 



There can I think be little doubt but that Jeans is right in 

 looking to gravitational instability as the primary cause of fission, 

 but when we consider that a binary system, with a mass larger than 

 the sun's, is found to rotate in a few hours, there seems reason to look 

 to rotation as a contributory cause scarcely less important than the 

 primary one. 



With the present extent of our knowledge it is only possible to 

 reconstruct the processes of the evolution of stars by means of 



1 Phil. Trans. E.S. Vol. cxcix. A (1902), p. 1. See also A. Roberts, S. African Astoc. 

 Adv. Sci. Vol. i. (11)03), p. 6. 



2 Astrophysical Journ. Vol. xxii. (1905), p. 97. 



