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Spiral Galaxy
S- or SB-type Galaxy
A form of galaxy, in which most of the matter that makes up the galaxy is flattened into a disc that rotates around a central core, or nucleus. The forces of gravity at work within the disc create density waves, that in turn generate a spiral structure. Spiral galaxies are classified as 'a', 'b' or 'c', depending on how tight or loose their spiral structures are.
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