This chapter describes the properties of stars that can be derived from observations. Stars may be the most important component of the Universe, and to illustrate this point we may note that life on Earth is sustained by light and heat from the nearest star, our Sun. The stare themselves make up the great galaxies, and are the principal source of galactic light, except in certain disturbed galaxies. On a clear moonless night you can see the faint band of light across the sky which is known as the Milky Wat/ (figure 2.1). Galileo’s telescope showed that the Milky Way is’ composed of millions of individual stars, and now we kn,ow that the entire Milky Way, our own Galaxy, contains about 1011 stars. Because the stars are a great distance from Earth it is not easy to observe all their properties. Although the energy output and temperature can both be found fairly readily, and the spectrum of the emitted light obtained, it is difficult or impossible to measure the mass and radius of the average star. Furthermore, no direct data whatever can be measured for the interiors of stars, with the possible excep¬tion of the Sun.

From careful analysis of the observed properties of stars, scientists have deduced how stars change with time. This triumph of astrophysics was largely possible because the behaviour of stars is controlled by relatively simple laws of physics. This contrasts with the evolution of galaxies, where a much more complicated situation, that has so far defied convincing explanation, exists. An understanding of the observational properties of stars provides an essential prelude to an astrophysical explanation of the life-styles of different stars

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