Astronomy is the study of matter in outer space. It is usually considered a branch of physics. But because it encompasses (literally) an astronomical number of subjects— everything from the study of a star’s surface to the end of the universe—it is often considered a field of its own. Astrophysics is a branch of astronomy dealing with the physics of celestial bodies and the universe as a whole. It deals with problems that range from the structure, distribution, evolution, and interaction of stars and galaxies to the orbital mechanics of a near-Earth asteroid.