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
Helium and metals; weak Hydrogen |
Helium, ionized metals, stronger Hydrogen |
Hydrogen lines dominant, singly-ionized metals |
weaker, neutral and singly-ionized metals |
||Singly-ionized Calcium most prominent, Hydrogen weaker,
neutral metals |
metals, molecular lines begin to appear |
Oxide molecular lines dominant, neutral metals |
and neutral metal lines |
Oxide, neutral metal
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.