The branch of science which deals with the study of position, composition, motion and other related facts of the heavenly bodies, is called astronomy. It is an observational science and not experimental science like other sciences, because we cannot control the conditions of the experiments or events which are occurring in the heavenly bodies i.e., we can not repeat these experiments in our laboratory since the conditions like temperature, pressure etc. in these heavenly bodies are too extreme and they cannot be artificially created in the laboratory. In other branches of science like physics, we can control over the experimental conditions in the laboratory.
The main source of information about the heavenly bodies is the study of the electromagnetic radiations emitted from them. There are two distinct regions of radiations (i.e., visible radiations of wavelength 4x10-7m to 8x10-7m and radio waves of wavelength 8mm to 17m) reaching the earth from the heavenly bodies. These are called the two ‘windows of astronomy'. It means the visible lights and radio waves coming from the heavenly bodies are the two windows to study about the sun, the stars etc. i.e. about the universe.
Let us begin our study of universe with the various theories put forward regarding the evolution of universe