284 
Rotation and Fission of Stars 
[ch. x 
The Internal Condition of Rotating Stars. 
253. The variation of the rotation of the sun’s atmosphere with latitude 
is of interest as providing direct observational evidence of the variations of 
rotation which theory shews must inevitably occur in every rotating star. 
From the point of view of cosmogony in general, however, variations in the 
speeds of rotation of stellar surfaces are only unimportant by-products of the 
far more extensive variations of rotation in the star’s interior. 
The knowledge of these rotations which we have gained from theory puts 
us in a position to form a fairly definite picture of the internal condition of a 
rotating star. 
In Chapter m we studied the configurations of a non-rotating star in 
some detail. The conditions which were found to obtain in a star devoid of 
'rotation can best be described by imagining the star divided into two distinct 
regions which we may describe as the core and the envelope. 
In fig. 47 (p. 278) the innermost curve gives the distribution of density as we 
pass along the radius of a non-rotating gaseous star of mass about equal to that 
of our sun. This shews that the density falls to about a twentieth of the central 
density at a point only a third of the radius out from the centre of the Star. We 
may describe as the core that part of the star which lies within a sphere whose 
radius is about a third of that of the star, this region containing the main part 
of the star’s mass, and being the only region in which the density is comparable 
with that at the star’s centre. In the outer region, which we describe as the 
envelope, the density is only a small fraction of the central density, and the 
mass of the envelope is only a small fraction of the total mass of the star. 
The curves in fig. 47 are drawn on the assumption that the gas-laws are 
obeyed throughout the star, but we have seen that considerations of stability 
demand deviations from the gas-laws. A star in which the gas-laws were 
obeyed would slowly shrink or contract until it had reached a configuration 
in which the gas-laws were not obeyed in the central regions of the star; 
stability can only be maintained when this central region offers a greater 
resistance to compression than a purely gaseous structure can provide. Thus 
fig. 47 cannot represent an actual stable star; to represent this, substantial 
deviations from the gas-laws must be introduced, with the result (§ 132) that 
both the density and temperature curves become very much flatter in this 
central region than the curves shewn in fig. 47. We may legitimately suppose 
that this modification of the density and temperature curves is confined to the 
core, the envelope continuing to obey the gas-laws. 
Thus we think of the star as consisting of a gaseous atmosphere and of a 
core whose structure may lie almost anywhere between the two extremes 
provided by a structure obeying the gas-laws as represented in fig. 47, and an 
almost incompressible mass of approximately uniform density.