density-wave theoryA theory that attempts to explain how the structure of a spiral galaxy is sustained by the flow of material in and out of the regions of the spiral arms under the influence of gravity. It was proposed in 1964 by C.C. Lin and F.H. Shu and based on work by B. Lindblad. The spiral structure is not permanent: it is regarded as a wave pattern in which the spiral arms are regions of low gravitational potential where matter concentrates. The spiral arms therefore comprise the loci of wave crests of a density wave. The wave pattern is not tied to the matter but moves through it at an angular velocity that can differ appreciably from that of the matter; the relative motion is on average about 30 km s–1. It is this motion that somehow maintains the overall appearance of the galaxy.
Stars move into the traveling gravitational troughs of the spiral arms and can remain there for a considerable time until their orbits take them out again. Interstellar gas also comes under the influence of the density wave. It has been suggested that when the gas encounters the moving wave it undergoes a sudden compression, which in turn could be a possible trigger for interstellar cloud collapse and star formation. Young stars should therefore be found preferentially in spiral arms, which is indeed the case.
Although the density-wave theory explains the spiral shape of galaxies and the presence of young stars in spiral arms, it does not show how the density wave originated. In addition it predicts the dissipation of the spiral arms after only a few galactic rotations, i.e. after several hundred million years, which is much shorter than the lifetime of galaxies. The theory is widely believed to apply to galaxies suffering gravitational perturbations from a passing neighbor or from the central bar of a barred spiral galaxy: these galaxies have sharply defined arms. The patchier arms of many isolated galaxies may not be due to density waves but to self-propagating star formation.