Astrophysics is a science that employs the methods and principles of
physics and chemistry in the study of astronomical objects and phenomena. As
one of the founders of the discipline, James Keeler, said, astrophysics "seeks
to ascertain the nature of the heavenly bodies, rather than their positions or
motions in space— what they are, rather than where they are", which is
studied in celestial mechanics.
Among the subjects studied are the Sun (solar physics), other stars, galaxies,
extrasolar planets, the interstellar medium, and the cosmic microwave
background. Emissions from these objects are examined across all parts of the
electromagnetic spectrum, and the properties examined include luminosity,
density, temperature, and chemical composition.