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Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties ( luminosity, density, temperature, chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at its largest scale; conversely, since the energies involved in cosmology, especially the Big Bang, are the largest known, the observations of the cosmos also serve as the laboratory for physics at its smallest scales as well.

In practice, virtually all modern astronomical research involves a substantial amount of physics. The name of a school's doctoral program ("Astrophysics" or "Astronomy") in many places like the United States often has to do more with the department's history than with the contents of the programs.

1 History

Although astronomy is as old as recorded history, it was long separated from the study of physics. In the Aristotelian worldview, the celestial pertained to perfection—bodies in the sky being perfect spheres moving in perfectly circular orbits—while the earthly pertained to imperfection; these two realms were seen as unrelated.

For centuries, the apparently common-sense view that the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt and other planetA planet (from the Greek , planetes or "wanderers") is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3s went round the EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla went unquestioned, until Nicolaus CopernicusNicolaus (or Nicholas Copernicus (original name Kopernik or Koppernigk Polish Mikolaj Kopernik German Nikolaus Kopernikus February 19, 1473 May 24, 1543) was an astronomer, mathematician and an economist who developed a heliocentric ( Sun-centred) theory suggested in the 16th century15th century 16th century 17th century more centuries) As a means of recording the passage of time, the 16th century was that century which lasted from 1501 to 1600. Events Beginning of the " Little Ice Age" a cooling period that resulted in lower crop yi that the EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla and all the other planets in the Solar System orbited the Sun. Galileo GalileiGalileo Galilei ( Pisa, February 15, 1564 Arcetri, January 8, 1642), was a Tuscan astronomer, philosopher, and physicist who is closely associated with the scientific revolution. His great achievements include perfecting the telescope, a variety of astron made quantitative measurements central to physics, but in astronomy his observation didn't have astrophysical significance.

The availability of accurate observational data led to research into theoretical explanations for the observed behavior. At first, only ad-hoc rules were discovered, such as Kepler's laws of planetary motionJohannes Kepler's primary contribution to astronomy/ astrophysics were the three laws of planetary motion . Kepler derived these laws, in part, by studying the observations of Brahe. Isaac Newton would later design his laws of motion and universal gravita, discovered at the start of the 17th century. Later that century, Isaac Newton, bridged the gap between Kepler's laws and Galileo's dynamics, discovering that the same laws that rule the dynamics of objects on earth rules the motion of planets and the moon. Celestial mechanics, the application of Newtonian gravity and Newton's laws to explain Kepler's laws of planetary motion, was the first unification of astronomy and physics.

After Isaac Newton published his Principia, maritime navigation was transformed. Starting around 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. The needs of navigation provided a drive for progressively more accurate astronomical observations and instruments, providing a background for ever more available data for scientists.

At the end of the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases, specific lines corresponding to unique chemical elements. In this way it was proved that the chemical elements found in the Sun (chiefly hydrogen) were also found on Earth. Indeed, the element helium was first discovered in the spectrum of the sun and only later on earth, hence its name. During the 20th century, spectrometry (the study of these spectral lines) advanced, particularly as a result of the advent of quantum physics that was necessary to understand the astronomical and experimental observations.