Astrophysics (lang-el: Astro - meaning "star", and lang-el: physis – φύσις meaning "nature") is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, and chemical composition) of celestial objects such as galaxies, stars, planets, exoplanets, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at scales much larger than the size of particular gravitationally-bound objects in the universe.

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 involves a substantial amount of physics. The name of a university's department ("astrophysics" or "astronomy") often has to do more with the department's history than with the contents of the programs. Astrophysics can be studied at the bachelors, masters, and Ph.D. levels in aerospace engineering, physics, or astronomy departments at many universities.

History

Although astronomy is as ancient as recorded history itself, it was long separated from the study of physics. In the Aristotelian worldview, the celestial world tended towards perfection—bodies in the sky seemed to be perfect spheres moving in perfectly circular orbits—while the earthly world seemed destined to imperfection; these two realms were not seen as related.

Aristarchus of Samos (c. 310–250 BC) first put forward the notion that the motions of the celestial bodies could be explained by assuming that the Earth and all the other planets in the Solar System orbited the Sun. Unfortunately, in the geocentric world of the time, Aristarchus' heliocentric theory was deemed outlandish and heretical, and for centuries, the apparently common-sense view that the Sun and other planets went round the Earth nearly went unquestioned until the development of Copernican heliocentrism in the 16th century AD. This was due to the dominance of the geocentric model developed by Ptolemy (c. 83-161 AD), an Hellenized astronomer from Roman Egypt, in his Almagest treatise.

The only known supporter of Aristarchus was Seleucus of Seleucia, a Babylonian astronomer who is said to have proved heliocentrism through reasoning in the 2nd century BC. This may have involved the phenomenon of tides, which he correctly theorized to be caused by attraction to the Moon and notes that the height of the tides depends on the Moon's position relative to the Sun. Alternatively, he may have determined the constants of a geometric model for the heliocentric theory and developed methods to compute planetary positions using this model, possibly using early trigonometric methods that were available in his time, much like Copernicus. Some have also interpreted the planetary models developed by Aryabhata (476-550), an Indian astronomer,B. L. van der Waerden (1970), Das heliozentrische System in der griechischen,persischen und indischen Astronomie, Naturforschenden Gesellschaft in Zürich, Zürich: Kommissionsverlag Leeman AG. (cf. Noel Swerdlow (June 1973), "Review: A Lost Monument of Indian Astronomy", Isis 64 (2), p. 239-243.)
B. L. van der Waerden (1987), "The heliocentric system in Greek, Persian, and Indian astronomy", in "From deferent to equant: a volume of studies in the history of science in the ancient and medieval near east in honor of E. S. Kennedy", New York Academy of Sciences 500, p. 525-546. (cf. Dennis Duke (2005), "The Equant in India: The Mathematical Basis of Ancient Indian Planetary Models", Archive for History of Exact Sciences 59, p. 563–576.). and Albumasar (787-886), a Persian astronomer, to be heliocentric models.