Starbirth. Illustration of the structure of a galactic spiral arm and its starbirth regions. The arms of a spiral galaxy are thought to be density waves, formed as stars bunch up as they orbit the galactic centre and move in and out of the arms. The gas and dust, strewn throughout a galaxy, are concentrated along the spiral arms in dust lanes. This reflects the origin of the dust from heavier elements produced by the supernovae of previous stars. It is also thought that the density waves of the spiral arms may trigger starbirth as they move through the slower moving dust lanes, leading to the clusters of young stars and other starbirth regions.