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 has puzzled scientists. One would expect that the stars on the outermost edge of the galaxy would move faster than those near the center as the galaxy rotates, but this is not the case. The best explanation so far is that the arms are caused by a density wave , much like traffic on a highway. As a slow moving car passes through, traffic gets congested around that car, but as the car moves on, the area of high density traffic moves along. In the same way, the stars clump together, then move apart, giving the arms. See discussion in spiral galaxy for further information.