Globular clusters' classification uses the system of H. Shapley and H.B. Sawyer. This classification uses Roman numeral from I (highest stellar density cluster) to XII (lowest stellar density cluster):
Class I, II, III: Visible high stellar density at their core. With a halo around decreasing in luminosity as a function of the distance from the core.
Class IV, V, VI: The core stellar density is still visible, but is more spread out and not as dense.
M75 is a globular cluster of class I in Sagittarius.
Class VII, VIII, IX: The cluster stellar density is more homogeneous and less contrasted.
M62 is a globular cluster of class IV
Class X, XI, and XII: The cluster surface luminosity is completely homogeneous with no increase in stellar density visible at the core.
M22 is a globular cluster of class VII in Sagittarius
M55 is a globular cluster of class XI in Sagittarius
The smaller the number of stars, the higher the core's stellar density.
Other deep sky objects: