In simplest terms, the spectral class of a star is a measurement of its color and temperature. Hot stars are more massive and bluish, and cooler stars are less massive stars and reddish. The spectrum is divided into seven categories specified by letter, which in descending order are: O, B, A, F, G, K, and M. Furthermore, each class may be further divided into ten subclasses (or "gradations") specified by numbers, from descending from 0 to 9 inclusive.
- Example: A star of class B0 would be hotter than a class B7 star, which in turn would be hotter than a A3-class star.
White dwarf stars, however, use a completely different scale. A white dwarf's spectral class always starts with the letter D (for "degenerate mass"). Following that comes one of the following letters: A, B, C, O, Z, or Q, which indicate certain elements detected in the star's spectral lines.
| Spectral Class | Temperature | Color |
|---|---|---|
| O5 | 40,000 K | blue |
| B0 | 27,000 K | blue-white |
| B5 | 16,000 K | |
| A0 | 10,000 K | white |
| A5 | 8,200 K | |
| F0 | 7,200 K | yellow-white |
| F5 | 6,700 K | |
| G0 | 6,000 K | yellow |
| G2 | 5,800 K | (Sol) |
| G5 | 5,500 K | |
| K0 | 5,100 K | orange |
| K5 | 4,300 K | |
| M0 | 3,700 K | red |
| M5 | 3,000 K |
There are also several rarer spectral classes that have been recorded.
| L: | Infared |
| T: | Substellar, T-Tauri |
| W: | Wolf-Rayet |
| C: | Carbon giants (combines R- and N- types) |
| S: | Red giant (ZiO) |
| Q: | Nova |
See also: Luminosity class, Hertzsprung-Russell diagram, Main sequence