One way to generate spiral structure is through star formation triggered by a spiral density wave. A spiral density wave is traveling through the galaxy disk, causing the collapse of gas clouds which form new stars. In this case, the spiral structure appears prominently (especially in blue optical images) because new stars emit most strongly in the blue. If you look at many spiral galaxies in redder colors (or even the near-infrared), the spiral pattern become less prominent, because you are now looking at the older stars which are more uniformly distributed.
Another way to generate spiral structure is through gravitational interactions between galaxies (collisions or mergers). Tidal forces can create strong spiral structure that is seen in both young and old stars because it is a physical arrangement of stars. An example of that is Arp 240 (shown below [1]). While the galaxies may have had spiral arms before their collision (from the previously described density waves), the spiral structure in the image below has undoubtedly been amplified.
Another way to generate spiral structure is through gravitational interactions between galaxies (collisions or mergers). Tidal forces can create strong spiral structure that is seen in both young and old stars because it is a physical arrangement of stars. An example of that is Arp 240 (shown below [1]). While the galaxies may have had spiral arms before their collision (from the previously described density waves), the spiral structure in the image below has undoubtedly been amplified.
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