Yerkes (or Morgan) Scheme
Created by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy; the shape, real and apparent; and the degree of the central concentration to classify galaxies.
| Spectral Type | Explanation |
|---|---|
| a | Prominent A stars |
| af | Prominent A-F stars |
| f | Prominent F stars |
| fg | Prominent F-G stars |
| g | Prominent G stars |
| gk | Prominent G-K stars |
| k | Prominent K stars |
| Galactic Shape | Explanation |
|---|---|
| B | Barred spiral |
| D | Rotational symmetry without pronounced spiral or elliptical structure |
| E | Elliptical |
| Ep | Elliptical with dust absorption |
| I | Irregular |
| L | Low surface brightness |
| N | small bright nucleus |
| S | Spiral |
| Inclination | Explanation |
|---|---|
| 1 | Galaxy is "Face-on" |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 | |
| 7 | Galaxy is "Edge-on" |
So, for example, the Andromeda Galaxy is classified as kS5.
Read more about this topic: Galaxy Morphological Classification
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