WHAT ARE STARS?: Definition of stars, their structure and where they are to be found. Formation of stars. The Milky Way - made up of approximately 15,000 million stars. The Sun, our best-known star. The Solar System: Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The Earth receives just enough energy for life to exist on it. Different kinds of stars and their evolution.