The primary luminosity discriminant at G0 is the strength of the Sr II 4077 line. Of use as well are the blends of Ti II and Fe II which were used in the F-type stars to distinguish luminosity types. For instance, Ti II, Fe II 4172-8, and Ti II, Fe II 4444 (in ratio with Mg II 4481) continue to be useful. The 0,0 and 1,1 bands of the violet system of CN ( 3883 and 3871) - which can be detected in these spectra as a depression of the continuum near to these wavelengths - display a peculiar behavior in stars near to G0. The strength of these bands increases as we pass from V to IV, but then weaken again by class III, only to strengthen again in the supergiant stars. This appears to be a characteristic of spectra near G0 III, in the Hertzsprung Gap.
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