CHAPTER X 
IONISATION, DIFFUSION, ROTATION 
Ionisation. 
173. The determination of the degree of ionisation of the atoms under 
the conditions of temperature and density found in the stars is important 
in connection with the following applications— 
(а) We derive from it the molecular weight ¡1 which is required for 
nearly all numerical calculations. Accuracy is important since p, is often 
raised to a rather high power in the formulae. We have to find— 
(1) What is the most probable value of ¡x for the stars in general? 
(The standard value adopted by us is 2-1.) 
(2) What is the magnitude of the differential effects (more par 
ticularly as affecting the mass-luminosity relation) caused by differences 
of fx between different stars ? 
(3) What is the change of ¡x between the centre and the outer parts 
of a star? 
(б) A knowledge of the ionisation is required in connection with theories 
of absorption, since each ionisation destroys an absorbing mechanism ; in 
particular, it determines the “guillotine” correction to the opacity on 
Kramers’ theory. 
(c) It determines the energy of ionisation of a star and hence the ratio 
of specific heats y, which is important in the study of the pulsations of 
Cepheids. 
Another subject appropriately treated in connection with ionisation 
is the determination of the deviation of stellar material from the laws of 
a perfect gas. 
The results generally depend appreciably on the chemical constitution 
of a star. The dependence on the chemical element is of two kinds, viz. a 
progressive change from the light to the heavy elements, and exceptional 
phenomena for a small group of consecutive elements which are in a critical 
stage of their K or L ionisation at the temperature and density chosen. 
It may be expected that the latter effects will be largely smoothed out 
in any reasonable mixture of elements. 
In discussing numerical results we have in mind as the most likely 
constitution of the material a predominance of elements in the neighbour 
hood of iron with some admixture of lighter non-metallic elements ; we do 
not think it necessary to allow for more than ten per cent, of elements