Figure: Wave Patterns

Recent study suggests that the wave action works not by literally pushing the stars together but by subtly choreographing their orbits. As a star trundles around the galaxy on an ellipse, the ellipse itself is moving.

This secondary motion is the raw material for the density wave. As shown in panel a, no wave motion occurs when orbits are oriented randomly. Bar wave arises when orbits align as shown in diagram b. Gravity causes the ellipses to move in unison, maintaining their alignment. The wave front is the area of greatest star density, which occurs along the major axis of the ellipses. Spiral wave arises when the ellipses move in unison but are not perfectly aligned; each ellipse is slightly skewed compared with its neighbors.

Credit: http://universe-review.ca/index.htm