Comparison of Globular Star Clusters

The Shapley–Sawyer Concentration Class

The Shapley–Sawyer Concentration Class is a classification system on a scale of one to twelve using Roman numerals for globular clusters according to their concentration. The most highly concentrated clusters are classified as Class I, with successively diminishing concentrations ranging to Class XII, such as Pal 10.

The image above shows globular clusters with a high concentration class (IV to VI) in the first row, clusters with a medium concentration (VII to IX) in the second row, and globular clusters with a very low concentration (XI to XII) in the bottom row.

Interstellar Reddening

Extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening.

The object's color excess E(B-V), which is given in magnitudes, is a measure of the amount of reddening due to interstellar dust.

In the image above, the value E(B-V) is given for each globular cluster in the lower right corner. E(B-V) increases continuously from the upper left to the lower right. This is the reason for the increasing redness of the globular clusters.

Image Details

The following links give detailed information about the globular cluster images.