We have come to the end of a long journey. Before saying a final goodbye to the reader, we wish to present an assortment of mixed fares in this final chapter. The main purpose of the book has been to develop the fundamentals fully. We now give a glimpse of what lies beyond the horizon.

Often it happens that one does not know the details of the physical conditions inside an astrophysical system, but can make rough estimates of different kinds of energies contained in the system (kinetic, potential, magnetic, etc.). To handle such situations, one can suitably integrate the basic equations to obtain an equation connecting different types of total energies of the system. This equation is known as the virial equation. Since this approach is very general, we present a discussion of it in this final chapter, when the reader should be in a position to possess a broad overview of the whole field. While applying hydrodynamics and magnetohydrodynamics to astrophysical systems, often it becomes necessary to incorporate relativistic corrections or to include the effects of radiation pressure. The subjects of relativistic hydrodynamics and radiation hydrodynamics respectively deal with these problems. These are vast fields of study, and we cannot provide proper treatments of these two fields in this elementary book. Just to give an idea of how one proceeds, we discuss the basic equations of these two fields in §17.2 and §17.3. Compared to the usual style of presentation in this book, these two sections would appear like skeletons without flesh and blood.