[114] H. B. 805, 1924. [115] Am. Ast. Soc. Rep., 190, 1923. [116] Eddington, Ap. J., 48, 205, 1918. [117] M. C. Johnson, M. N. R. A. S., 85, 56, 1924. [118] Ibid. [119] Adams and Joy, Pub. A. S. P., 36, 142, 1924. [120] Chapter V, p. 64. [121] H. A., 28, 175, 1901. [122] Pub. A. S. P. 36, 225, 1924. [123] Br. A. Rep., 1924. [124] A. J. Cannon, H. B. 762, 1924. [Pg 55] CHAPTER V ELEMENTS AND COMPOUNDS IN STELLAR ATMOSPHERES THE identification of stellar lines and bands with those observed in the laboratory has furnished a rich source of data for astrophysics. About 25 per cent of the observed solar lines are assigned to elements in Rowland’s Table of Solar Spectrum Wave-lengths. The majority of the solar lines which are still unidentified are faint. Notwithstanding practical difficulties of identification caused by blending, and the consequent uncertainty of wave-length, most of the observed lines, at least in the cooler stars, have been satisfactorily accounted for. There remain some important strong lines and bands of unknown origin, which have been usefully summarized by Baxandall.[125] The present chapter contains a summary of the stellar occurrence and astrophysical behavior of the chief spectrum lines which are of known origin and series relations. A few other lines, such as those of C++, N++, and O+ are included, as their series relations will probably be forthcoming in the near future. The observed chemical elements are arranged in order of atomic number. At the conclusion of the chapter the elements which have not been detected in stellar spectra are enumerated. The series notation employed follows the system advocated by Russell and Saunders,[126] which appears to meet, more fully than any other, the practical needs of modern spectroscopy. HYDROGEN (1) Hydrogen is represented in stellar spectra by the Balmer series (); the ultimate lines are those of the Lyman series () in the far ultra-violet, and cannot therefore be [Pg 56] traced in the stellar spectrum. The occurrence of the secondary spectrum of hydrogen, ascribed to the hydrogen molecule H₂, has been suspected,[127] but not definitely established. Only one of the lines has been recorded, and this should almost certainly be attributed[128] to N++. The familiar Balmer series appear as emission lines in the Wolf-Rayet stars, but normally they are absorption lines in all succeeding classes. The intensity of the hydrogen lines is at a maximum[129] in the neighborhood of Class . They vary greatly in width, however, within a given spectral class,[130] and it is difficult to find a method of photometry applicable to the comparison of lines of very different widths. The maximum of the Balmer lines has been placed by Menzel[131] at . The writer is inclined to believe that no significant maximum can in fact be derived for the Balmer lines; beyond , however, their intensity falls off rapidly.