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
- Our own Solar System, as well as systems of planets around other stars;
- Stars themselves, including the nuclear fusion and energy transport necessary to describe;
- Extreme, compact objects such as white dwarfs and neutron stars and the very energetic explosions (e.g. supernovae) that may accompany their deaths or births;
- Black holes, which are described well by general relativity;
- Collections of many bodies interacting via gravity, such as the Asteroid Belt, globular clusters, or entire galaxies;
- The matter and radiation thinly spread between stars and galaxies;
- Cosmological structures and remnants of the early universe (such as the CMB); as well as
- The less well understood substances (or collective effects identified for convenience as substances) of dark matter and dark energy, especially insofar as they interact with other, more readily observable objects.
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 newtonian-mechanics to general-relativity, from electromagnetism to quantum-mechanics and nuclear-physics, from statistical-mechanics to magnetohydrodynamics.
In contrast to the more observationally-oriented field of astronomy, astrophysics 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 cosmology.
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.