ABSTRACT
The stars show a tremendous range in luminosity, though much less in mass. A star begins its career by condensing out of the gas and dust in the interstellar medium–that is to say, the gas and dust lying between existing stars. There are numerous cases in which many stars of similar type, and presumably of similar age, are concentrated in a limited area. These are known as stellar associations. Star formation in a given area may well be triggered off by a nearby supernova explosion–the death of a very massive star, which literally blows itself to pieces, or the destruction of the white dwarf component of a binary pair. Once nuclear reactions begin the star settles down to a period of stability; a star with a mass similar to that of the Sun will spend about 10 000 million years on the Main Sequence, so that the Sun is about half-way through this stage in its evolution.
