The classification scheme most widely used for globular clusters was developed by Harlow Shapley and Helen B. Sawyer Hogg in the 1920s (Shapley 1930). They introduced Concentration Classes, sometimes referred to as Shapley-Sawyer Concentration Classes, which are named by numbers and typically denoted as Roman numbers, from I-XII, or 1-12, where I (1) denotes the highest concentration or most concentrated, densest structure, and XII the least concentrated.

Below please find this classification scheme illustrated with images of the Messier's Globular Clustersr.

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Class I      
                M75

Class II     
                M2      M80

Class III    
                M54

Class IV     
                M15     M28     M62     M92

Class V      
               M5     M13       M30       M53     M69    M70     M79

Class VI     
                M3

Class VII    
                M10      M22

Class VIII   
                M9      M14      M19

Class IX     
                M4     M12      M72

Class X      
                M56      M68      M107

Class X-XI   
                M71

Class XI     
                M55

Class XII    -

Reference

• Harlow Shapley, 1930. Star Clusters. Harvard Observatory Monographs, No. 2. New York, 1930. Here Ch. II.5, pp. 11-14.

Last Modification: February 19, 2014