"have played a prominent role in astronomical thought since the discovery by in 1845 of the spiral structure of the ... Through the work of , , , and especially Hubble, by the late 1920s, it became known, and not just speculated, that such spirals were disk-like stellar systems coequal to our own Milky Way system. From the optical study of the kinematics of stars in the solar neighborhood, (1927) and Jan Oort (1927) deduced that the flattening of the galactic disk is due to rotation, and that this rotation occurred differentially, with the stars near the galactic center taking less time to go once around the center than the material farther out."