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5. LUMINOSITY EFFECTS AT O9

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 Hdelta (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.

Figure 4

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