A concise but thorough introduction to the observational data and theoretical concepts underlying modern astronomy, Astrophysics in a Nutshell is designed for advanced undergraduate science majors taking a one-semester course. This well-balanced and up-to-date textbook covers the essentials of modern astrophysics--from stars to cosmology--emphasizing the common, familiar physical principles that govern astronomical phenomena, and the interplay between theory and observation. In addition to traditional topics such as stellar remnants, galaxies, and the interstellar medium, Astrophysics in a Nutshell introduces subjects at the forefront of modern research, including black holes, dark matter, gravitational lensing, and dark energy, all updated with some of the latest observational results. To aid physical understanding, mathematical derivations are kept as simple, short, and clear as possible, and order-of-magnitude estimates, dimensional analysis, and scaling arguments are frequently used. These no-nonsense, "back-of-the-envelope" calculations train students to think like physicists. The book is amply illustrated with simple, clear figures and each chapter ends with a set of problems.