By F5, the hydrogen lines have lost all sensitivity to luminosity, and we must now rely solely on lines and blends of ionized species. The luminosity sensitive features are essentially the same as at F0, except that at F5 and later types, we can use the Sr II 4077 line with some confidence. Also note that at F5 (and later types), the Ca II K-line shows a slight positive sensitivity to luminosity, in the sense that it becomes slightly broader in the more luminous stars.
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