What happens to individual stars as they pass through the density wave front?

They speed up after emerging from the compressed region, moving past the wave front into the dimmer inter-arm space.

The key difference between the pattern speed of the density wave and the orbital speed of the stars and gas dictates the dynamics of the spiral arm. The density wave rotates relatively slowly, acting like a temporary bottleneck. When a star or gas cloud enters the arm, it encounters the compression, slows down briefly, and undergoes star formation. However, because individual stars orbit the galactic center much faster than the wave pattern itself rotates, once the star passes through the compressed zone, it accelerates relative to the wave and moves into the less dense, inter-arm region. This continuous cycle of entering, forming stars, and leaving ensures that the stellar populations within the visible arm are constantly being refreshed, even as the overall pattern remains stable.