This book is designed to show how physical principles can be used at the advanced undergraduate level to understand astronomical systems such as planets, stars, galaxies, and the universe as a whole. It emerges from a pair of courses at Rutgers Universitythat attract not just astrophysicsstudentsbut a broadaudienceof physics andengineeringstudents.Theorganizationistherefore“physics-?rst”:westartwith key principles of physics and then examine applications to astronomical systems. At Rutgers, each half of the book constitutes a coherent semester-length course; while there is a little overlap (notably with cosmology in Chaps.11 and 20), the two halves are largely independent and complementary. Part I focuses on gravity, because this is the dominant force in many astronomical systems and it governs many types of motions we observe. The goal of Chaps.2–11 is to develop a progressively richer understanding of gravity and the way astrophysicists use gravitational motion to investigatemass. Part IIcentersononeofthe“bigquestions”wehumansask.Whyare we here? is admittedly beyond the realm of physics, but a related question is within our reach: How did we come to be here? As the Sun was forming, various elements came togetherin the rightcombinationto forma rockyplanet with a tenuousatmosphere. On this planet Earth, the energy from the Sun and the gas in the atmosphere were just right to allow the emergenceof life. The energythat sustains us originates deep insideourstar,thanksto E Dmc2.Theatomsthatcompriseourbodieswereforged inpreviousgenerationsofstars.Literally,wearestardust.ThegoalofChaps.12–20 is to understandthe roles that electromagnetism as well as gas, atomic, and nuclear physics play in this remarkablestory.