Spiral arms are not rigid spokes of stars. They are density waves, much like a traffic jam: individual stars drift in and out while the pattern turns more slowly and lasts far longer. Pitch angle winds the arms tight or opens them out, all of it on a disk about 1,000 light-years thick across a 100,000 light-year span. Without dark matter the rotation curve falls back to a Keplerian decline and the outskirts fly apart.
Spiral arms are not rigid spokes of stars. They are density waves, much like a traffic jam: individual stars drift in and out while the pattern turns more slowly and lasts far longer. Pitch angle winds the arms tight or opens them out, all of it on a disk about 1,000 light-years thick across a 100,000 light-year span. Without dark matter the rotation curve falls back to a Keplerian decline and the outskirts fly apart.
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