History
Who believed the celestial region was made up of Aether?
Astrophysics
Although astronomy is as ancient as recorded history itself, it was long separated from the study of terrestrial physics. In the Aristotelian worldview, bodies in the sky appeared to be unchanging spheres whose only motion was uniform motion in a circle, while the earthly world was the realm which underwent growth and decay and in which natural motion was in a straight line and ended when the moving object reached its goal. Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either Fire as maintained by Plato, or Aether as maintained by Aristotle. In the 17th century, natural philosophers such as Galileo, Descartes, and Newton began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same natural laws. 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). Laboratory experiments with hot gases showed that the same lines could be observed in the spectra of known gases, specific lines corresponding to unique chemical elements. In this way it was proved that the chemical elements found in the Sun and stars (chiefly hydrogen) were also found on Earth. Indeed, the element helium was first discovered in the spectrum of the Sun and only later found on Earth, hence its name. During the 20th century, spectroscopy (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. See also:  Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure  Timeline of white dwarfs, neutron stars, and supernovae  Timeline of black hole physics  Timeline of gravitational physics and relativity