At O9 the hydrogen lines show a more pronounced sensitivity to
luminosity than at earlier (hotter) types. In addition, the ratio of Si IV 4089
(which increases in strength as we pass to higher luminosities) to
H (which
becomes narrower and weaker in the more luminous stars), the ratio of Si IV
4116 to the neighboring He I 4121 line, and the ratio of N III 4379 to He I
4387 can be used to judge the luminosity class at O9.
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