Astrophysics is the branch of astronomy that deals with the physics of the universe, especially with “the nature of the heavenly bodies, rather than their positions or motions in space”. Among the objects studied are galaxies, stars, planets, 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.
In practice, modern astronomical research often involves a substantial amount of work in the realm(s) of theoretical and/or observational physics. Highly elusive areas of study for astrophysicists, which are of immense interest to the public, include their attempts to determine: the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe. Topics also studied by theoretical astrophysicists include: solar system formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.
Astrophysics can be studied at the bachelors, masters, and Ph.D. levels in physics or astronomy departments at many universities.