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Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name.
The existence of spiral arms once puzzled scientists, as the stars on the outermost edge of the galaxy would move faster than those near the center as the galaxy rotates. In fact, spiral arms are not a result of star motion, but density waves that induce star formation. That is, the arms appear brighter because there are more young stars (and hence more massive, bright stars that do not survive long), not because stars are moving in a way that produces arms. See discussion in spiral galaxy for further information.
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