Curiously, the turning effect we observe is not coming from stars moving in an orbit around the centre of the galaxy. When we plot the movement of the light coming from the spiral arms (or bar) of a galaxy, given their size and speed of movement it would require the stars to be moving faster than the speed of light. Also if the spiral arms were made up of orbiting stars, the ones closer to the center would move faster than the outer stars causing a winding effect, which we do no observe.

One theory suggests that we are observing a density wave sweep through material, which causes compression and heat, increasing the likelihood of new stars to be born, these stars burn bright and white. As this wave passes through, it leaves behind those stars in its wake, which over time burn less brightly than before.

One analogy that comes to mind an electronic circuit, although electricity travels at the speed of light, the actually charge carriers are moving relatively slowly, know as the drift velocity.