The first classification was introduced by E.C Pickering (Harvard
1890). A slightly modified version of this classification is still the principal system
used. This Harvard system is a temperature sequence based on spectral
characteristics. The sequence is OBAFGKM with decimal subdivisions, e.g. G5 is half way between
G0 and K0.
| Star | Principal spectral characteristics. |
| O: | Lines of ionized helium. Often neutral helium and weak hydrogen lines are also visible. |
| B: | Neutral helium lines, hydrogen lines strengthening in B6 through B9 |
| A: | Very strong hydrogen lines in A0 weakening towards A9. Ionized calcium increasing from A0 to A9. |
| F: | Ionized calcium increasing in strength, hydrogen weakening. Other elements beginning to strengthen. |
| G: | Ionized calcium very strong, hydrogen weaker. Metal lines especially Iron become prominent. |
| K: | Strong metallic line, molecular bands of CH and CN. |
| M: | Red stars strong absorption bands of titanium oxide and many metallic lines. |
In 1943 a secondary system was introduced by Morgan, Keenan, and Kellman to classify stars by luminosity as well as temperature. They also gave numerical definitions of the Harvard spectral types and designated bright stars to be the standards.
Their MK luminosity system is:
| O: | Extremely luminous 'super' super giants extremely rare in the Galaxy. |
| Ia: | Luminous super giants |
| Ib: | Dimmer super giants |
| IIa: | Bright giants |
| III: | Normal giants |
| IV: | Subgiants |
| V: | Dwarfs (main sequence stars) |
| VI: | Subdwarfs. |
For example, Betelgeux is a M2 Iab star, part way between Ia and Ib. The MK system is only applicable to stars of normal chemical composition, this includes 95% of all stars. The original Harvard system also included classes R&N dominated by carbon lines and S dominated by zirconium oxide. In the modern system R and N types are merged as one carbon star class using spectral types such as C2,4 in which temperature and carbon band strength are indicated by the two numbers. S stars use a similar system. For stars with minor spectral abnormalities, a modified MK type is assigned, e.g. K0III-CN3 would be a giant K star with anomalously strong cyanide bands. (by R. Shaw.)
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