Astrophysics is a branch of astronomy that analyzes the properties and interactions of cosmological objects based on known physical law. The term is slightly misleading, as anyone who goes into astronomy must also be proficient in physics. It can be said that this field is very similar to the fields of astronomy and cosmology.
The two main divisions in this field are observational and theoretical astrophysics. There is no such thing as experimental astrophysics because the scales and objects being observed are far too large or far away to experiment on with modern technology. Because light takes time to travel to us on Earth, the most distant regions of the universe are actually windows into the ancient universe, when the universe was far denser and more energetic. Because this field sometimes deals with theories of the early, compact universe, it can overlap strongly with particle physics, which provides predictions of how matter would behave in the ancient universe.
Astrophysicists are known for studying such phenomena as black holes, galaxies, superclusters, neutron stars, quasars, the Big Bang, dark matter and energy, cosmic strings, stellar evolution, the cosmic microwave background radiation, and many others. The cosmos is a good arena for studying pure physics because on such large scales, the particular type of element making up objects becomes less significant, and more general variables, such as mass and velocity, take primacy. Sometimes, this field is called "the study of the very large and the very small."
Many important insights to mankind's understanding of the universe have been contributed by astrophysicists. They have predicted the likely age of the universe, the size of the observable universe, how long the Sun will last before it exhausts its nuclear fuel, the commonness of black holes and other exotic celestial bodies, what the universe looked like billions of years ago, the average temperature of interstellar or intergalactic space, the shapes of galaxies, and the way that matter is distributed across the observable universe. Astrophysics always continues to evolve and produce new insights into the structure of the universe.