Why is it surprising to find spiral arms in spiral galaxies?
A) Spiral arms require new stars, and previous new stars should have already used up the interstellar medium; no new stars should be forming now.
B) The variations in speed of the stars across galaxies are not high enough to twist the shape of the arms into a spiral.
C) There is no known mechanism to generate spiral arms.
D) The motions of the stars and differential rotation of the galaxy should have wound their arms tightly and made them blend and disappear.

Question

Quizplus · Accepted Answer

The presence of spiral arms in galaxies is surprising because, given the differential rotation of galaxies (where parts closer to the center rotate faster than those further out), one would expect the spiral arms to wind up tightly over time, eventually blending together and disappearing. This has not been observed, indicating that our understanding of the dynamics of spiral galaxies and the mechanisms that maintain their spiral structure is incomplete.

Why Is It Surprising to Find Spiral Arms in Spiral