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The application of physical theory to celestial systems such as stars, planets, galaxies, supernovae, and black holes. Astrophysics proper is concerned with explaining phenomena more so than making observations, the latter falling under the purview of astronomy.

Astrophysics is the application of physical theories to celestial systems, in order both to make predictions about the systems and to test the theories themselves. Such systems include but are not limited to

Often astrophysics pushes theories developed with terrestrial laboratory data into regimes that are difficult if not impossible to create on Earth, providing a way of testing how far such theories can be extended. Conversely, it uses principles learned in more familiar settings to describe and predict how exotic systems will interact and evolve, providing the only window into the behavior of phenomena located too far away to influence in controlled experiments.

Because of the physical vastness of the observable universe, together with the compositional diversity of its contents, astrophysics draws on and spurs development in many other fields of physics, from to , from to and , from to .

In contrast to the more observationally-oriented field of , is concerned more with applying and testing theories. It is less about what the contents of the universe are and more about explaining the phenomena we do observe, providing physical mechanisms for how celestial systems work as we observe. When such investigation involves the "big questions" regarding the properties, origin, and development of the universe as a whole, the pursuit becomes known as .

At its core, astrophysics contains the ideal that the same physics seen on Earth applies to the rest of the universe, and so it is the link that extends our understanding of nature to the most distant and exotic of phenomena.