Caption: An animation of "stochastic self-propagating star formation (SSPSF) simulated in a simple circular model" of a flocculent spiral galaxy seen face-on.

The mechanism that creates flocculent spiral arms is believed to be stochastic self-propagating star formation (SSPSF). Self-propagating star formation is discussed in IAL 21: Star Formation: The Evolution of Star Formation Regions.

In brief, this is the story of stochastic self-propagating star formation (SSPSF) and flocculent spiral arms:

1. There is a large molecular cloud in which star formation is triggered in one part.

2. Massive OB stars (AKA hot young blue stars) form. Their radiation pressure and stellar winds and the ejecta from their supernova explosions compress nearby regions of the molecular cloud.

3. This compression triggers a new generation of star formation adjacent to the original one, where star formation is tending to turn off because the ultraviolet light (fiducial range 0.01--0.4 μm) from the OB stars is evaporating the interstellar dust and cloud material is being pushed away the same pressure forces that trigger the adjacent star formation.

4. Thus star formation self-propagates through the large molecular cloud.

5. But the molecular cloud is so large that it is differentially revolving around the galaxy center of mass. In most galaxies, the orbital speed (measured in, e.g., km/s) is fairly constant (due their dark matter halo mass distribution), but this means that the angular velocity (measured in, e.g., in degrees per megayear (Myr)) falls off with radius from the galaxy center of mass.

6. Thus, as one goes outward in radius, the molecular cloud material is increasingly trailing.

7. The molecular cloud is getting wound up and is forming spiral arms.

8. But the winding-up problem (see Galaxies file: spiral_arm_winding_problem.html) NEVER arises, because the molecular cloud disperses in a complex way after a few tens of megayears and the orbital periods of the material is of order hundreds of megayears.

9. Because star formation regions are NOT formed by organizing spiral density waves, the star formation regions formed by stochastic self-propagating star formation (SSPSF) ARE messy, and so the spiral arms formed are messy: they are the flocculent spiral arms.