Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space". Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.
The greatest astronomers of the first half of the 20th century were the astrophysicists. For example, Arthur Eddington, Cecilia Payne, Hans Bethe, and Subrahmanyan Chandrasekhar elucidated the physical nature of stars using the new quantum theories of atomic, nuclear and particle physics. In recent decades, about half of the prizes of the American Astronomical Society are awarded for work in astrophysics and half in astronomy.
Gutti Jogesh Babu; Eric D. Feigelson (1 August 1996). Astrostatistics. CRC Press. p. 4. ISBN 978-0-412-98391-7.