There have been two leading models for the spiral structure of Galaxies:

  1. Star formation caused by density waves in the galactic disk of the galaxy
  2. The SSPSF model: Star formation caused by shock waves in the interstellar medium

Density wave model: Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotates more slowly than the galaxy’s stars and gas. As gases enters a density wave, it gets squeezed and makes new stars. This idea was developed into density wave theory by Lin and Shu in 1964. They suggested that spiral arms were manifestations of spiral density waves. They assumed that stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated. In other words, the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center.

It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. The following hypothesis exists for star formation caused by density waves:

  • As gas cloud moves into the density wave, the local mass density increases. Since the criteria for cloud collapse (Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
  • As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
  • As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.