Galaxy Morphological Classification - Yerkes (or Morgan) Scheme

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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