This type embraces mainly stars of relatively small apparent brightness. The brightest is γ Velorum with m = 2.22. We shall find that the absolute magnitude of these stars nearly coincides with that of the stars of type B.
The type is grouped into five subdivisions represented by the letters Oa, Ob, Oc, Od and Oe. These subdivisions are conditioned by the varying intensities of the bright bands named above. The due sequence of these sub-types is for the present an open question.
Among interesting stars of this type is ζ Puppis (Od), in the spectrum of which Pickering discovered a previously unknown series of helium lines. They were at first attributed (by Rydberg) to hydrogen and were called “additional lines of hydrogen”.
Type B (Orion type, Helium stars). All lines are here dark. Besides the hydrogen series we here find the He-lines (396, 403, 412, 414, 447, 471, 493 μμ).
To this type belong all the bright stars (β, γ, δ, ε, ζ, η and others) in Orion with the exception of Betelgeuze. Further, Spica and many other bright stars.
On [plate III] ε Orionis is taken as representative of this type.
Type A (Sirius type) is characterized by the great intensity of the hydrogen lines (compare [plate III]). The helium lines have vanished. Other lines visible but faintly.
The greater part of the stars visible to the naked eye are found here. There are 1251 stars brighter than the 6th magnitude which belong to this type. Sirius, Vega, Castor, Altair, Deneb and others are all A-stars.
Type F (Calcium type). The hydrogen lines still rather prominent but not so broad as in the preceding type. The two calcium lines H and K (396.9, 393.4 μμ) strongly pronounced.
Among the stars of this type are found a great many bright stars (compare the third chapter), such as Polaris, Canopus, Procyon.