The Trumpler classification system, introduced almost 100 years ago, is still the most commonly used means of identifyng open clusters today.

Robert J. Trumpler was a Swiss-born American astronomer who wrote an excellent 1930 paper entitled "Preliminary Results on the Distances, Dimensions and Space Distribution of Open Star Clusters".  The paper is the first documentation of the cluster classification system that he created.  In addition, the paper identified 37 previously unrecognized star clusters, which he named  An1 through An37.   The abbreviation "An" stand for "Anonymous Clusters".  In time, the abbreviation evolved from "An" to "Tr".   

Most of Dr. Trumpler's significant scientific work regarded either positional astronomy or the study of star clusters and the Milky Way.  He enjoyed teaching and was highly regarded by both the faculty and students at the Berkeley Department of Astronomy.

The Trumpler system uses three primary features to describe a cluster.  The definitions below are taken word for word from the paper:

Degree of Concentration (ranging from I to IV):

I: Detached clusters with strong central concentration
II: Detached clusters with little central concentration
III: Detached clusters with no noticeable concentration, in which the stars are more or less thinly but nearly uniformly scattered.
IV: Clusters not well detached but passing gradually into the environs, appearing like a star field condensation.

Range of Brightness (ranging from 1 to 3):

1:  Most cluster stars nearly of the same apparent brightness
2: Medium range in brightness of stars
3: Clusters composed of bright and faint stars; generally a few very bright and some moderately bright stars standing out from a host of fainter ones.

Number of Stars (either p, m or r):

P:  Poor clusters with less than 50 stars
m:  Moderately rich clusters with 50 to 100 stars
r:  Rich clusters with more than 100 stars.

The open star cluster classification consists of the combination of the 3 characteristic symbols above.  For example, I-3-m designates a detached cluster with strong central concentration, composed of 50 to 100 bright and faint stars.  Since there are 4 codes for Concentration, 3 for Brightness Range, and 3 for Richness, that makes for 4x3x3 = 36 different Trumpler classes.

The attached 3 posters show a comparison of the cluster classifications using images from each category.  I struggled with how to best show this comparison.  I have developed one poster each for rich, medium and poor clusters.  Each poster has 9 examples, using the first 3 degrees of concentration along with the 3 brightness range degrees.  

I haven’t included the "IV" degree of concentration in these posters.  I did originally, but I found that those clusters are not distinct from the background, did not add much to the presentation, and seemed to distract from the rest of the display.

For each image, the field of view is a 0.5 degree square.  North is up.

After studying these objects carefully, I still struggle with understanding the classification logic for certain objects.  Left to my own devices, I probably would have categorized a few of the objects differently.  I wonder if seeing these clusters in an image instead of an eyepiece affects the interpretation of the categories?

I will be including the Trumpler classifications in my next edition of my free Compendium, planned for this fall.  There are surprisingly few sources of free information for these classifications.

My collection of 140 open star clusters is seen here.