Fig. 4. Spectra of α Cygni and α Tauri.
Fig. 5. Spectrum of α Orionis.
Fig. 6. Spectrum of γ Cassiopeiæ
Vogel proposed a modification of Secchi’s classification, by subdividing each of his three types into two or three others, and including the Wolf-Rayet stars under the second type. His definitions are as follows:
Type I is distinguished by the intensity of the light in the more refrangible end of the spectrum, the blue and violet. The type may be divided into three subdivisions, designated a, b and c:
In Ia the metallic lines are very faint, while the hydrogen lines are distinguished by their breadth and strength.
In Ib the hydrogen lines are wanting.