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Analysis and Interpretation of Astronomical Spectra 49

Type III Reddish-orange stars with complex band spectra and a few discrete lines. The ab-
sorption bands get darker (more intense) towards the shortwave (blue) side. Such features
show, e.g. Betelgeuse, Antares, and Mira. Not until 1904, it became clear that these ab-
sorption bands are mainly caused by the titanium oxide molecule TiO (today's Class M).

Type IV Very rare reddish stars with absorptions bands getting darker (more intense) to
the longwave (red) side. Angelo Secchi already recognised that it is generated by carbon
(sect. 5.5)!

Type V Finally, stars with "bright lines" – emission lines as we know today.

13.4 The Harvard System

It soon became clear that the classification system of Secchi was too
rudimentary. Based on a large number of spectra and preliminary work
by Henry Draper, Edward Pickering (1846-1919) refined Secchi’s sys-
tem by capital letters from A – Q. The letter A corresponded to the
Secchi class type I for stars with dominant hydrogen lines. Finally this
classification solely survived up to the present time! As director of the
Harvard Observatory, he employed many women, for its time a truly
avant-garde attitude. Three women of his staff took care of the classi-
fication problem until after many detours and “meanders” the system
of Annie J. Cannon (1863 - 1941) became widely accepted around the
end of the World War I.

Its basic structure has survived until today and is essentially based on
the letters O, B, A, F, G, K, M. The well-known and certainly later cre-
ated mnemonic: Oh Be A Fine Girl Kiss Me. With this system, even
today still over 99% of the stars can be classified. This sequence of
letters follows the decreasing atmospheric temperature of the classi-
fied stars, starting from the very hot O-types with several 10,000 K up
to the cool M-types with about 2,400 –3,500 K. This reflects the abso-
lutely ground-breaking recognition that the spectra depend mainly on
the photospheric temperature of the star and secondarily only on other
parameters such as chemical composition, density, rotation speed etc.
This may not be really surprising from today's perspective, since the shares of hydrogen
and helium with 75% and 24%, even about 13.7 billion years after the “Big Bang” still
comprise about 99% of the elements in the universe. This systematic also forms the hori-
zontal axis of the almost simultaneously developed Hertzsprung Russell Diagram (sect. 14).
It was later complemented by the classes:

– R for Cyan (CN) and Carbon Monoxide (CO)
– N for Carbon
– S for very rare stars, whose absorption bands are generated, instead of TiO, by zirconium
oxide (ZrO), yttrium oxide (YO) or lanthanum oxide (LaO). Moreover, the entire class system
was further subdivided with additional decimal numbers from 0–10. Examples: Sun G2,
Pollux K0, Vega A0, Sirius A2, Procyon F5.