, and it attains a maximum at
or
.
The number of carbon compounds which occur lends plausibility to the suggestion that much of the stellar carbon is in combination at temperatures below 5000°.
NITROGEN (7)
The spectrum of neutral nitrogen has not as yet been satisfactorily analyzed into series.[205] It is quite possible that the first ionization that takes place is the ionization of the molecule,[206] which is accompanied by the production of the well known band spectrum. This spectrum has not been observed in the stars; presumably it would appear at lower temperatures than those involved in the coolest spectral classes. It is, however, stated to be a conspicuous feature of the spectrum of the aurora,[207][208] and it is found in the spectrum of the comet head.[209] These occurrences seem to point to very low temperature and pressure at the source. It is possible that much of the nitrogen present in cooler stars is in combination with carbon.[210]
The green Aurora line was thought by Vegard[211] to coincide with a line emitted in the laboratory by solid nitrogen. The conclusion was questioned by McLennan and Shrum,[212] who failed to produce the line under similar conditions, and subsequently found a line, of the same wave-length as the aurora line, in the spectrum of a mixture of oxygen and helium.[213] Various previous attempts to identify the aurora line with a line produced in the laboratory had failed conspicuously.[214]