ABSTRACT
The stars are the most important building blocks of the visible Universe and their nature and evolution is therefore one of the fundamental problems in astronomy. Less is known about the actual birth of stars than the subsequent stages of their evolution, but the accepted picture is of a gravitational collapse in the interstellar medium induced by some local perturbation. Dense regions of the interstellar medium exist which are considered to be areas of star formation, but are impenetrable to visible light and can only be observable with high-frequency radio waves. The lifetimes of the hottest, most massive stars are about one million years, of the Sun about ten thousand million years and of the coolest, least massive stars about a million million years. The Sun will reach its red giant stage 100 million years after it has burnt its hydrogen core, a relatively short time compared to its lifetime in its present state.
