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°.
The position of the maximum can be placed elsewhere by the use of special stars in estimating the line-intensities. The intensity of the hydrogen lines is in fact unusually difficult to determine, as they differ from star to star in width, wings, and probably also in central intensity. Using a series of individual stars, Menzel[454] placed the maximum at
, with the note that “on the average the lines seem to be strongest in Classes
to
, but the mean intensity is often greatly exceeded in certain