1. LIII. Ionization in the solar chromosphere
2. Ionisation in Stellar Atmospheres
3. Values can be found in R. H. Fowler,Statistical Mechanics(Cambridge University Press, Cambridge, England, 1936), 2nd ed., p. 569.
4. There are exceptions, for example, R. v. d. R. Woolley and D. W. N. Stibbs,The Outer Layers of a Star(Oxford Clarendon Press, London, 1953), pp. 188–189,