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 offered by the conditions of equilibrium and of 
stability of a spherical nebula’, In a later paper’, 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,...1 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 
“sravitational 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. R.S. Vol. cxcrx. A (1902), p. 1. See also A. Roberts, S. African Assoc. 
Adv, Sci. Vol. 1. (1903), p. 6. 
2 Astrophysical Journ. Vol. xx. (1905), p. 97. 
