The Classification of Open Clusters
In 1930, R.H. Trumpler introduced a system for classifying open clusters. The Trumpler system is composed of three elements that describe the concentration, range in brightness and richness of a cluster.
Concentration
1 – Detached; strong concentration toward center
2 – Detached; weak concentration toward center
3 – Detached; no concentration toward center
4 – Not well detached from surrounding star field
Range in brightness among component stars
1 – Small range in brightness
2 – Moderate range in brightness
3 – Large range in brightness
Richness
P – Poor: less than 50 stars
M – Moderately rich: 50 to 100 stars
R – Rich: more than 100 stars
An "n" appended to the Trumpler class indicates that nebulosity is involved.
Thus NGC457, classified as 2 3 r, is detached from the field, only weakly concentrated, exhibits a wide range of stellar magnitudes and contains more than 100 stars.
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