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—

(a) We derive from it the molecular weight μ which is required for nearly all numerical calculations. Accuracy is important since μ is often raised to a rather high power in the formulae. We have to find—

(1) What is the most probable value of μ for the stars in general? (The standard value adopted by us is 2·1.)

(2) What is the magnitude of the differential effects (more particularly as affecting the mass-luminosity relation) caused by differences of μ between different stars?

(3) What is the change of μ between the centre and the outer parts of a star?

(b) 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 γ, 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.