The main branches of physics are astrophysics, classical mechanics, electromagnetism, optics, quantum mechanics, relativity and thermodynamics. Physics is the branch of science concerning the study of the nature and properties of matter and energy, and the relationship between them.
The physics branch of astrophysics concerns the application of physics to stars and celestial matter, while classical mechanics studies bodies acted on by forces and bodies in motion. Electromagnetism combines the study of electricity and magnetism as a singular branch of physics, and optics focuses on the behavior of light. Quantum mechanics covers atomic and subatomic systems. Relativity entails the description of phenomena taking place at a reference point in motion to an observer, while thermodynamics studies the relationships between heat and other forms of energy.