The homogeneity of the spectra in a given class is striking, and the fact that large numbers of stars display exactly similar spectra has a significance—considered in another chapter[504]—to which the classification problem cannot do more than call attention. The similarity of the spectra becomes the more striking when it is remembered that the range of conditions embraced within any one class is very wide; the ratio in mean density may be as great as 10[505] between stars of the same class but of differing absolute magnitude.[506] [Pg 194] The close spectral similarity between, giants and dwarfs, in spite of the great differences in physical conditions, should not, however, be misinterpreted. The observed facts are in exact accordance with what might have been anticipated. In the first place, thermal ionization is governed by the surface gravity, and only indirectly by the mean density.[507] TABLE XXX Class Characteristic Lines O H He Si+++ C++ N++ He+ H He Si+++ He+ O+ H He Si+++ O+* H He* H He H He Si+ Ca+ H He Si+ Ca+ Mg+ H He Si+ Ca+ Mg+ H* Si+* Ca+ Mg+ H Ca+ Mg+* Ca Fe H Ca+ Mg+ Ca Fe H Ca+ Ca Fe Ti H Ca+ Ca Fe Ti H Ca+ Ca Fe Ti G bd. H Ca+ Ca Fe Ti G bd. H Ca+ Ca Fe Ti G bd. H Ca+* Ca Fe* Ti* G bd.* H Ca+ Ca* Fe Ti TiO₂ H Ca+ Ca Fe Ti TiO₂ H Ca+ Ca Fe Ti TiO₂ The division into “arc,” “spark,” and “superspark” is clearly shown by the table. Maxima of the lines which are used as criteria of class are marked with an asterisk. [Pg 195] It is shown in Chapter III that the range in surface gravity is far smaller than the range in mean density. Secondly, the basis of the classification has been shown to be the degree of thermal ionization.[508] Granted that the value of the partial electron pressure is low enough, in dwarfs as well as in giants, for thermal ionization to predominate over ionization by collision, a mass of gas will pass through the same succession of ionization-stages with changing temperature, whatever the surface gravity. Any given stage of ionization will, however, be reached at a lower temperature, the lower the pressure, since, as pointed out in Chapter X,[509] lowered pressure tends to increase the degree of ionization, and will help to produce a given degree of ionization at a lower temperature.