Density Wave Theory
One popular theory says that the spiral structure is a wave that moves through the disk causing the stars and gas to clump up along the wave---a density wave.
Density Wave Theory
Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy.
Diamond Ring Effect ...
Density Wave Theory - A theory to account for the spiral arms of spiral galaxies. According to the density wave theory, spiral arms are the crests of waves moving through a galaxy like water waves move through water ...
Density Wave Theory
(a) One possible explanation for spiral arms. According to this theory, the spiral arms represent regions of somewhat enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas.
This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.
A resonance hypothesized by Lindblad in the 1920s in his attempt to explain the existence of spiral arms (see Density Wave Theory).
The mostly accepted theory of the spiral arms is the density wave theory. It says that the stars indeed rotate at different speed, but the density of the interstellar medium form a wave.
NOT simple rotation ---
windup too tight
self-propagating star formation
density wave theory : slow pattern speed
density wave triggered by bar or passing neighbor ...
According to spiral density wave theory, this is precisely what has happened in the Milky Way. Some disturbance in the past produced the wave, which has been moving through the Galactic disk ever since.
This is sort of what is going on in the spiral arms. The concept that explains this is known as the spiral density wave theory, but I like to call it the naked jogger model. Let me explain it first to show you what I mean.
 See also: Light, Wave, Density, Orbit, Rotation
  
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