[yunruse]

This is the Lin-Shu density wave theory [0]; since the 1960s it has been proposed by some that both this and the mechanism described in the Stochastic Self-propagating Star Formation model [1] work together to maintain the distinctive shape.

[0] https://en.wikipedia.org/wiki/Density_wave_theory [1] https://en.wikipedia.org/wiki/SSPSF_model