What phenomenon explains the spiral arms as effective nurseries?

Density Wave Theory

The primary physical phenomenon responsible for making the spiral arms such effective sites for star nurseries is the Density Wave Theory. This theory posits that the spiral pattern observed is not a fixed collection of the same stars and gas perpetually moving together. Rather, it is a wave of slightly higher density that propagates through the galactic disk, similar to a traffic jam moving along a highway. This wave moves independently of the local material. As massive, cold molecular clouds drift into this high-density leading edge of the wave, they are physically squeezed. This compression is the necessary trigger that increases density sufficiently to initiate the gravitational collapse required to form stars, making the arms dynamic sites of stellar birth rather than static features.