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 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