The Harvard
Spectral Sequence

By late in the last century it was realized that the spectra of stars (in particular, their patterns of absorption lines) had systematic features that could be classified into what came to be known as the Harvard Spectral Sequence. The sequence was denoted by a series of letters O, B, A, ... with the spectral characteristics listed in the following table.

Harvard Spectral Sequence
Spectral Class Characteristics
O Ionized Helium and metals; weak Hydrogen
B Neutral Helium, ionized metals, stronger Hydrogen
A Balmer Hydrogen lines dominant, singly-ionized metals
F Hydrogen weaker, neutral and singly-ionized metals
G Singly-ionized Calcium most prominent, Hydrogen weaker, neutral metals
K Neutral metals, molecular lines begin to appear
M Titanium Oxide molecular lines dominant, neutral metals
R, N CH, CN, and neutral metal lines
S Zirconium Oxide, neutral metal lines

For even finer gradation in the spectral sequence, each category in this classification can be subdivided into 10 subclasses using numbers from 0 to 9. Thus, for example, the classes O and B can be subdivided into the finer classes O0, O1, O2, O3, ... O9, B0, B1, B2, ... B9.

Here are some postscript files of example spectra for each of these spectral classes.