If there's no friction in space, why do the arms of spiral galaxies sweep backwards?
• The short answer is: differential rotation. Stars in a spiral galaxy, which includes a flat disc with a central bulge, orbit about its centre. Those further from the centre have longer orbits and therefore longer orbital periods, even though their velocities are similar to or greater than those of stars closer in. This results in more distant stars lagging behind.
But this raises further questions. Why do most galaxies have spiral arms in the first place? And why, given the much greater age of…
