Density wave theory [1] posits that the "arms" of a spiral galaxy are more like stellar gravitational traffic jams than persistent objects-- stars pass through

Density wave theory [1] posits that the "arms" of a spiral galaxy are more like stellar gravitational traffic jams than persistent objects-- stars pass through and clump up in arms, but the stars in each arm change through the eons. If you plot mass extinction events against when the solar system transits through the arms, eleven extinction events in the last 500 million years occurred while going passing through one of these high density regions. [2] [1]: https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory [2]: https://arxiv.org/abs/1309.4838 https://arxiv.org/abs/1309.4838