How did the universe originate and how did it reach its present state? What formed the earth and how did life arise? These are among the central questions that have concerned every human culture and they obviously involve events buried in the distant past. How can we progress in understanding so as to choose among the various theories for the origin of the universe, or possibly formulate a better theory? This process of exploration is what we call astronomy, and the final purpose of astronomy is thus nothing less than to account for the origin and physical nature of the universe around us.