An Etymological Dictionary of Astronomy and Astrophysics

The problem encountered in the explanation of the → spiral arms of galaxies if the material making up a spiral arm is static, that is
remains in the arm. Since galaxies exhibit → differential rotation, stars near the center take less time to orbit the center than those farther from the center. The arm would, after a few → galactic rotations, become increasingly curved and wind around the → galaxy ever tighter until it ultimately disappears. This is inconsistent with observations.