| Spectral Class | Spectral Subclass | Luminosity Class (Supergiants) | Luminosity Class (Giants) | Luminosity Class (Main Sequence) | Luminosity Class (Dwarfs) |
|---|---|---|---|---|---|
| O (hottest) | 0 (hottest) | I (supergiants) | II (bright giants) | V (main sequence stars) | VI (sub-dwarfs) |
| B | 1 | Ia (hypergiants/extremely luminous supergiants | IIIa (luminous regular giants) | Va (extremely luminous main sequence stars) | VII (white dwarfs) |
| A | 2 | 0 (hypergiants/extremely luminous supergiants | III (regular giants) | Vab (luminous main sequence stars) | |
| F | 3 | Iab (normal supergiants) | IIIb (less luminous regular giants) | Vb (normal main sequence stars) | |
| G | 4 | Ib (less luminous supergiants | IV (sub-giants) | Vz (less luminous main sequence stars) | |
| K | 5 | ||||
| M (coolest) | 6 | ||||
| 7 | |||||
| 8 | |||||
| 9 (coolest) |