A significant fraction of the stars in the universe resides in the rotationally supported disks of galaxies. Disks are mostly thin and flat, but the disk is often warped away from its principal plane in the outer parts. Disk galaxies usually manifest spiral patterns, and rather more than half host bars. Most, but not all, disk galaxies have a central bulge, perhaps also a thick stellar disk, and generally a small fraction of the stars resides in a quasi-spherical stellar halo, while the central attraction at large distances from the center is dominated by a dark halo. The material in most disks overwhelmingly orbits in a single sense, although a small fraction of galaxies have been found to host substantial counter-rotating components.
This chapter is primarily concerned with the dynamics of rotationally supported disks of stars. Stellar disks are believed to have formed over time from gas that had previously settled into centrifugal balance in the gravitational well of the galaxy, and the process of star formation continues to the present day in most disk galaxies. While stars are the dominant dynamical component today, the small gas fraction (usually 10% by mass) can still play an important dynamical role in some contexts.
Disk dynamics is a rich topic for two principal reasons: (a) the organized orbital motion facilitates gravitationally-driven collective behavior and (b) outward transfer of angular momentum extracts energy from the potential well. Space limitations preclude a detailed development and this review will mostly be confined to a summary of the principal results and open issues. The derivations of the principal formulae can be found in the excellent textbook by Binney & Tremaine (2008, hereafter BT08). Furthermore, no attempt is made to cite every paper that relates to a topic.
Closely related topics are described in other chapters in this volume: the structure of the disks of the Milky Way (Freeman), the formation and structure of bulges (Kormendy), the atomic and molecular gaseous components of the Milky Way (Dickey & Bania respectively), and our current understanding of the processes that lead to the formation of galaxies (Steinmetz). The distributions of light and mass within galaxies are described in volume 6 (Gallagher).