The scope of this book is not typical of the majority of texts in cosmic plasma physics. The consecutive consideration of physical principles, starting from the most general ones, and simplifying assumptions allows students interested in astronomy to find the answers to two key questions: (1) what approximation is the simplest, but still sufficient for the description of a phenomenon in cosmic plasmas, and (2) how to build an adequate model of this phenomenon. Astronomers of the future will need such tools. Astronomy of the future will become an increasingly more precise science, similar to physics or plasma physics. This is what the book aims to teach.