4 Classification results

As Figs. 27 and 28 show, the spectral classification of O- and early B-type stars over the $\lambda\lambda$ 8500 - 8750 Å region is not trivial. Starting with type B8 the Ca II triplet lines become visible and over the whole B8 - M8 sequence the classification is made easy by a costantly growing number of metallic lines. The intensity of Ca II triplet lines relative to Paschen lines offers for B8 to F8 stars the same diagnostic tool as provided by the Ca II H&K vs. Balmer lines in the optical. Going from G to early M stars, the Paschen lines are not more visible, Ca II triplet lines progressively saturate and the classification criteria therefore shift more and more toward metallic lines like those of Si I, Mg I, Fe I and Ti I. The metallic lines at a given spectral type are as much abundant and strong as those used for classification in the optical. Therefore the $\lambda\lambda$ 8500 - 8750 Å spectra offer the same classification accuracy as spectra in the classical optical region of equivalent $\bigtriangleup\lambda$ = 250 Å (like the $\lambda\lambda$ 3870-4120 Å interval centered on the Ca II H&K lines and the H₈, H$_\epsilon$ and H$_\delta$ Balmer lines).