Nebulium and Hydrogen in New Stars

Abstract

IN a recent paper (NATURE, Jan. 28, p. 136) S. R. Pike pointed out that the condition of coexistence of O⁺⁺ and H lines in the spectra of new stars leads to a density of the order 10^-7 gm./c.c., which is directly opposed to the current ideas that forbidden transitions can occur only at very low densities, as determined, for example, by C. T. Elvey (NATURE, Jan. 7, p. 12) using the ‘expanding shell’ theory of novæ. Since Mr. Pike's arguments are equally applicable to the spectra of planetaries, it seems of importance to analyse the subject further and to find the probable way out of the difficulty. I think that the real cause of the discrepancy mentioned is involved in the use of Saha's formula, which holds only in thermodynamic equilibrium, and of course cannot be applied to the gaseous shells as produced by the expansion of novæ or those of planetary nebulæ, because in these cases the exciting radiation is extremely diluted; a more general ionisation formula is to be used.