Astrophysics is a branch of astronomy that deals mainly with the physics of stars, stellar systems (including galaxies), and interstellar material. The alliance of physics and astronomy, which began with the advent of spectroscopy, made it possible to investigate what celestial objects are and not just where they are. Many branches of physics, including nuclear physics, plasma physics, relativity, and spectroscopy, are used to predict properties of celestial bodies. Astrophysicists also interpret the information obtained from astronomical studies of the electromagnetic spectrum, including light, X-rays, and radio waves.