Introduction

The great discoveries in physics and the technological breakthroughs in the twentieth century have completely revolutionized astronomy – the observational study of the physical Universe beyond Earth and its theoretical understanding. These great discoveries included special relativity, general relativity, quantum mechanics, atomic structure, and nuclear structure, together with the elementary particles and their unified interactions. The technological developments of the twentieth century which had the greatest impact on observational astronomy included microelectronics, microdetectors, computers, and space-age technologies. They allowed astronomical observations deep into space with unprecedented resolution and sensitivity. The New Physics, together with these observations, led by the end of the twentieth century to an amazing understanding of an extremely complex Universe that contains more than 1021 stars in more than 100 billion galaxies with enormous variety, diverse environments, and complex evolutions. Nevertheless, astronomy, one of the oldest sciences, is still one of the most rapidly developing. This is because many fundamental questions related to the origin of our physical Universe, to its contents, to its laws, and to the existence of life in it are still unanswered. They may be answered as science progresses, new technologies for high-resolution observations are exploited, and new fundamental theories are developed and tested. In this chapter, we give a brief account of our present knowledge of the physical Universe, our current understanding of it, and our major observational endeavors to widen this knowledge and understanding.

Advances in observational astronomy

Until the invention of the optical telescope for military purposes at the beginning of the seventeenth century, astronomical observations were made with the naked eye. The Universe observable from planet Earth included only five other planets – Mercury, Venus, Mars, Jupiter, and Saturn – orbiting the Sun and a few thousand more distant stars. The invention of the telescope dramatically increased the horizon of the observable Universe, the number of observable stars, and the resolving power of observations.