Figure: Winding problem

The stars in the disk do not all rotate at the same angular velocity. Stars in the outer edges have a smaller ang. velocity and therefore lag behind stars in the inner parts. If the stars forming the spiral arms would stay in the arm, after a few rotations, any spiral structure would be erased.

The age of spiral galaxies is bigger than 100 rotation times, so one would not expect to find any grand design spiral structure, but only extremely wound up structures. The solution to the apparent paradox lies in the theory of density wave. Stars enter and leave the arms, but do not remain "attached" to it.

Credit: Kippenhahn, 10 Milliarden Sonnen.