1. The O Stars
No luminosity classification has been attempted for stars earlier than O9. The spectral type has been determined from the ratio He I 4471: He II 4541. The types determined from this ratio appear to be consistent with the appearance of other spectral features in a sequence of effective excitation. The types obtained in this manner are in very close agreement with those determined by H.H. Plaskett. (1)
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If the spectral types of the O stars are determined from the single
ratio of the absorption lines He I 4471: He II 4541, results
accurate to a tenth of a class between O5 and O9 can be obtained on
plates of the dispersion used (125 A per mm at H
). This single ratio
appears to be the most useful criterion of spectral type for O5-O9
stars on spectra similar to those used. The classification of the
Wolf-Rayet stars as a group will not be discussed; the number of stars
in this class is very small, and individual description of each
spectrum seems to be necessary.
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The standard O stars are listed in Table 1. Notes concerning spectral features for some of the stars are given; in the case of those of class O9, luminosity differences are also noted.
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