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Astrophysics is simply the physics of astronomy. Astrophysicists use spectroscopy on light to determine the composition of stars, nebulae, and other matter in the universe. They observe radiation such as microwaves emitted from pulsars, red shifted light from the big bang, and the energy emitted by black holes. They develop and utilize theories of relativity, string cosmology, dark matter, and dark energy. This scientific field utilizes other fields of physics such as thermodynamics and is related to cosmology, which deals with properties of immense bodies.
The field of astrophysics was formed when Isaac Newton coupled Galileo Galilei's hypothesis of the dynamics of planetary orbits, itself based on Nicolaus Copernicus' notion of heliocentrism, with Johannes Kepler's empirical laws of planetary motion, thereby demonstrating that gravity works the same on celestial bodies as it does on Earth. Newton's Law of Universal Gravitation is included in his work Philosophiæ Naturalis Principia Mathematica.
- Astrophysics, at Wikipedia