As emission predominates more and more, the spectrum resembles those of the normal members of the sequence less and less. If a star has an absorption spectrum it can always be assigned a place in the sequence, and this method of arrangement appears to be logical. But it is clear that the sequence so formed is no longer physically homogeneous. The stars that have no absorption lines, although some of them have obvious affinities with stars that have absorption spectra, have [Pg 166] moreover no place in a sequence formed on the basis of absorption intensities. It is, of course, possible to devise a self-consistent scheme for the arrangement of a limited number of the stars, and such a scheme is, for many purposes, both desirable and convenient. It is, however, exceedingly hard to know where the division should be drawn between “absorption” and “emission” stars. Perhaps the most satisfactory plan is to treat all stars as a sequence, with special comment for the large number of them that require it. THE CLASS STARS (a) The Balmer Lines.—The spectra of the stars are dominated by the Balmer series of hydrogen, which, with the exception of the and lines in the cooler stars, are stronger at than any other line seen in the stellar sequence. The maximum of the Balmer series has been stated[452] to occur at , and this value was used by Fowler and Milne[453] in calibrating their temperature scale based upon ionization theory. It is in accordance with theory that the subordinate lines of hydrogen, with ionization potential 13.54 volts, and excitation potential 10.15 volts, should have their maximum at about 10,000°.