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 Clusters.

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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