By F0, the hydrogen lines have lost most of their sensitivity to
luminosity. Note, however, that they can still be used to distinguish the
supergiant classes from lower luminosities. Near F0, the luminosity class is
estimated from the strength of lines due to ionized iron and
titanium. Excellent
luminosity-sensitive features include the Fe II, Ti II double blend at
4172-8,
and similar blends at 4395-4400,
4417 and
4444.
The strength of these
blends are usually estimated with respect to other less luminosity sensitive
features, such as Ca I 4227, Fe I 4271 and Mg II 4481.
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