What physical event triggers the collapse of molecular hydrogen clouds at the wave's leading edge?

Physical squeezing/compression

The initiation of a new star requires immense clouds of molecular hydrogen gas to collapse under their own gravity. In the context of the density wave, this critical trigger happens when a gas cloud, previously drifting peacefully through the lower-density space between the arms, encounters the leading edge of the density wave. The physical phenomenon that occurs here is acute compression. This physical squeezing causes a sharp increase in density within the cloud. Once this increased density forces the cloud to shrink sufficiently so that the force of gravity overcomes the internal pressure opposing collapse, fragmentation occurs, leading directly to the necessary conditions for nuclear fusion to ignite, thereby birthing a star.