Ordinary Spirals are classified by relative strength of the central bulge & tightness of the spiral arms. Types are Sa which is string bulge and tight with indistinct arms, Sb is intermediate type and Sc is small bulge and loose with well-defined arms. Barred Spirals feature a strong central stellar bar. Its bar rotates as a unit. Spiral arms emerge from the ends of the bar. There is ongoing star formations in the disk. It is supported by relatively rapid rotation.
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