Approximate colors of stars in each
spectral class.
| Spectral Class | Approximate Temperature (K) |
Hydrogen Balmer Lines |
Other Spectral Features |
| O | 40,000 | Weak | Ionized helium |
| B | 20,000 | Medium | Neutral helium |
| A | 10,000 | Strong | Ionized calcium weak |
| F | 7,500 | Medium | Ionized calcium weak |
| G | 5,500 | Weak | Ionized calcium medium |
| K | 4,500 | Very weak | Ionized calcium strong |
| M | 3,000 | Very weak | Titanium oxide strong |
Approximate colors of stars in each
spectral class.
The first widely used classification system was devised by astronomers at Harvard during the 1890s and 1900s. The spectra were first classified in groups labeled A through Q, but some groups were later dropped, merged with others, or reordered. The final classification includes the seven spectral classes, or types, still used today: O, B, A, F, G, K, M.
This sequence of spectral types, called the spectral sequence, is important because it is a temperature sequence. The O stars are the hottest, and the temperature continues to decrease down to the M stars, the coolest of all.
updated January 08, 2000 by Matt Oltersdorf