An Etymological Dictionary of Astronomy and Astrophysics
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A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.

→ density; → wave.

One possible explanation for → spiral arms, first put forward by B. Lindblad in about 1925 and developed later by C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material structures, but regions of somewhat enhanced density, created by → density waves. Density waves are perturbations amplified by the self-gravity of the → galactic disk. The perturbation results from natural non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters. Density waves rotate around the → galactic center and periodically compress the disk material upon their passage. If the spiral arms were rigid structures rotating like a pinwheel, the → differential rotation of the galaxy would wind up the arms completely in a relatively short time (with respect to the age of the galaxy), → winding problem. Inside the region defined by the → corotation radius, density waves rotate more slowly than the galaxy's stars and gas; outside that region they rotate faster.
As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.
The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars. Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly "grand design" spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also: → stochastic self-propagating star formation.

→ density; → wave; → theory.