The open clusters are classified according to a scheme developed by of R.J. Trumpler. There are 3 indicators depending on the concentration and detachment from the stellar background, range in brightness and number of stars:
Concentration and detachment from the surrounding star field
Class I: The cluster is strongly detached from the stellar background with a strong core stellar density.
Class II: The cluster is detached from the stellar background with a light core stellar density.
Class III: The cluster is detached from the stellar background without a denser core.
Class IV: The cluster is weakly detached from the stellar background, the area having a higher stellar density but no visible core.
Range in brightness
Class 1: All the stars present about the same brightness.
Class 2: The stars present a regular range of brightness.
Class 3: Beside some very bright stars, many weaker stars with a wide magnitude range.
Number of stars
p: The cluster is poor in stars (less than 50 stars)
m: The cluster has a medium number of stars (from 50 to 100 stars)
r: The cluster is rich in stars (more than 100)
The letter 'n' at the end of the classification indicates a nebula linked to the cluster.
M11 - the Wild Duck Cluster is a very rich open cluster located near the Milky Way's center. Of class (I 2 r)
M26 - open Clusters of class II 3 m
NGC 2818 - open Cluster of class III 1 m n and NGC 2818A - Planetary nebula IIIb of class III 1 m n
M73 - open Cluster of class IV 1 p in Aquarius
Other deep sky objects: