Astrophysics is the physics of stars, and more broadly the physics of the universe. It allows us to understand the structure and evolution of planetary systems, stars, galaxies, interstellar gas, and the cosmos as a whole. In this concise introduction, leading astrophysicist James Binney describes the rapid expansion of the field of astrophysics over the past century, thanks to the vast amounts of data collected from telescopes that explored all regions of the electromagnetic spectrum and the rapid increase in computing power, factors that have allowed increasingly effective mathematical models to be developed. The author explains how the application of the fundamental principles of physics—regarding energy, mass, and momentum—along with the two pillars of relativity and quantum mechanics have led to a substantial understanding of phenomena ranging from rapidly rotating pulsars to the collision of giant spiral galaxies. This is an accessible and scientifically accurate introduction to astrophysics at a simple mathematical level, for readers who want to engage meaningfully with its fundamental concepts.