Astrophysics: Decoding the Cosmos is an accessible introduction to the key principles and theories underlying astrophysics. This text takes a close look at the radiation and particles that we receive from astronomical objects, providing a thorough understanding of what this tells us, drawing the information together using examples to illustrate the process of astrophysics. Astrophysics; Contents; Preface; Acknowledgments; Introduction; Appendix: dimensions, units and equations; PART I THE SIGNAL OBSERVED; 1 Defining the signal; 2 Measuring the signal; PART II MATTER AND RADIATION ESSENTIALS; 3 Matter essentials; 4 Radiation essentials; PART III THE SIGNAL PERTURBED; 5 The interaction of light with matter; 6 The signal transferred; 7 The interaction of light with space; PART IV THE SIGNAL EMITTED; 8 Continuum emission; 9 Line emission; PART V THE SIGNAL DECODED; 10 Forensic astronomy; Appendix A: Mathematical and geometrical relations