In this textbook we study dynamical phenomena in astrophysical objects from viewpoints of fluid dynamics and related fields. In this chapter, as the first step for the study, we consider temperature-density diagram, and discuss what astrophysical objects can be regarded as fluid dynamical systems on this diagram. To clarify the conditions under which astrophysical systems can be regarded as fluid dynamical ones, we examine Debye length, mean free path, gyration radius and so on. We see that the domain on the diagram where the fluid dynamical treatments are possible is rather broad, and many astrophysical objects are diverged on the domain, from low to high temperature and from low to high density. Since the domain where hydrodynamical treatments are possible is wide, in some situations magnetic fields, radiation fields and general relativity are important to determine the fluid dynamical behaviors.