How does differential rotation affect the shape of the Milky Way’s spiral arms?

Differential rotation—the phenomenon where a galaxy's rotation speed varies at different distances from its center—is a significant factor in shaping spiral arms. Any large-scale disturbance within a galactic disk, such as a sequence of stellar formation events initially appearing in a near-linear pattern, will eventually be drawn out into a spiral shape due to this differential rotation.1

While the overall spiral pattern of the Milky Way is primarily attributed to a dynamical effect known as a density-wave pattern, differential rotation acts as a mechanism that can naturally produce a spiral structure from various disturbances. In this context, the rotation of the disk stars around the galactic centre occurs at different rates depending on the distance from the nucleus, a motion first interpreted by the astronomer Jan H. Oort. Consequently, this shearing effect stretches localized features into the characteristic spiral arms observed in many galaxies.12