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Introduction to Astrophysics/Galaxies:Shapes and Evolution
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The three main types of galaxies are elliptical (E), irregular (I), and spiral (S).
Galaxies are typically found in clusters. These clusters are categorized as regular or irregular. Regular clusters contain mainly E galaxies, and are large, dense, and spherical. These clusters have formed from many collisions, causing the S galaxies to become E galaxies. Irregular clusters are thin and have no symmetry. They contain mostly S galaxies.