Stellar Classification - Secchi Classes

During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:

• Class I: white and blue stars with broad heavy hydrogen lines, such as Vega and Altair. This includes the modern class A and early class F.

  Class I, Orion subtype: a subtype of class I with narrow lines in place of wide bands, such as Rigel and Bellatrix. In modern terms, this corresponds to early B-type stars.

• Class II: yellow stars—hydrogen less strong, but evident metallic lines, such as the Sun, Arcturus and Capella. This includes the modern classes G and K as well as late class F.
• Class III: orange to red stars with complex band spectra, such as Betelgeuse and Antares. This corresponds to the modern class M.

In 1868, he discovered carbon stars, which he put into a distinct group:

• Class IV: red stars with significant carbon bands and lines (carbon stars.)

In 1877, he added a fifth class:

• Class V: emission-line stars, such as γ Cassiopeiae and Sheliak.

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.