What visual element observed just ahead of bright blue star-forming regions provides direct proof of the density wave's role?
Darkest dust lanes
Astronomical observation provides direct visual confirmation of the density wave mechanism by examining the spatial relationship between different structures within the arms. Specifically, when examining high-resolution images, the darkest dust lanes—which represent the raw material of cold gas and dust that has not yet been fully compressed or ignited—are consistently located immediately preceding or leading the brightest blue, newly formed star clusters. This spatial correlation visually demonstrates the 'traffic jam' effect: the dust and gas are piled up at the leading edge of the density wave, awaiting the final physical squeeze that will lead to gravitational collapse and the subsequent ignition of the next generation of stars just behind that dust front.
