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.