The Extension to the Supergiants of the Cyanogen Criterion of Stellar Luminosity.

The behavior of CN in stars of accurately known spectral types and absolute magnitudes has been studied by combining intensity estimates of the bands at X 3882 and X 4215. When plotted against spectral type, the observed band intensities show progressively earlier maxima in going from dwarfs to supergiants. The usual equations of dissociative equilibrium can be extended successfully to the atmos- pheres of the supergiants to account for the observations. The absolute maximum in CN intensity is reached near type Ki by stars with visual absolute mag- nitudes in the neighborhood of -2. For types G4-Ki the luminosity effects in the bands are strong enough to allow good spectroscopic absolute magnitudes to be determined from their intensities. G-type stars of high luminosity are easily detected, even with very low dispersion, by picking out spectra with intense CN bands; and in this way discovery surveys can be extended to faint apparent magnitudes