Accretion discs in astrophysics

Steady accretion disks are considered, and attention is given to the role of viscosity, radiation mechanisms, time dependence, instability, variability, and the inner boundary in the physics of accretion disks. The major uncertainties in accretion disk theory are the dissipation process (viscosity) and the emission process. It is noted that disk modeling is in some sense more an art than a science, and since a steady disk can be constructed for almost any combination of viscosity and radiation processes, the possibilities are almost endless. Conversely, it is likely that more can be learned about the properties of accretion disks in astrophysics by observation and subsequent modeling than by pure theorizing.