Frank Shu

"The rich variety of stars live in our Galaxy mostly singly or in pairs ... Sometimes, however, hundreds or thousands of stars can be found in loose groups called s. The is a daily young open closer, and the which surrounds the famous "seven sisters" of this cluster attests to the fact that these stars must only recently have been born out of the surrounding gas and dust. The oldest stars in our Galaxy are found in tighter groups called s. Rich globular clusters may contain more than a million members."

"The revealed the to have an unexpected richness of structure. Many of the observed features have now been identified as collective effects arising from the of the ring material. These effects include , the main topics of this review chapter. Both kinds of waves were first discussed in the astronomical literature in connection with the dynamics and structure of , and our discussion contrasts the similarities and differences between the and s. After developing the theory of free and forced waves of both types, we discuss how the observed waves can be used as diagnostics to obtain crucial parameters that characterize the physical state of the rings."

"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."