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SPECTRAL (STELLAR) CLASSIFICATION SYSTEM
The original version of the current MK system was defined by Morgan, Keenan, and Kellman (1943), and a revised list of standards was published in 1953 by Morgan and Johnson. Additional refinements have been added since then.
MK Spectral Classification System (Standard)
Class |
Color |
Temperature (K) |
Composition |
| O |
Dark Blue |
28,000-50,000 |
ionized atoms, especially helium |
| B |
Blue |
10,000-28,000 |
neutral helium, some hydrogen |
| A |
Light Blue |
7,500-10,000 |
strong hydrogen, some ionized metals |
| F |
White |
6,000-7,500 |
hydrogen and ionized metals; calcium and iron |
| G |
Yellow |
5,000-6,000 |
ionized calcium; both neutral and ionized metals |
| K |
Orange |
3,500-5,000 |
Neutral metals |
| M |
Red |
2,500-3,500 |
Strong molecules; titanium oxide, neutral calcium |
