ABSTRACT
This chapter presents approximate formulae for the luminosity and lifetime of individual stars, discusses the initial mass function (IMF) which specifies how masses are distributed when stars are bom, and shows averages for star clusters displaying an IMF. It delineates the structure of cloud clumps at the verge of gravitational instability and present Jeans’s criterion for collapse to actually occur. Stars form the most obvious component of the Milky Way. They are, by and large, the ultimate source of luminosity in galaxies. In the post-main-sequence phase, when hydrogen has been exhausted, elements of atomic mass number greater than four are produced by compounding helium nuclei. The energy yield per gram of matter is then one order of magnitude smaller. Nuclear astrophysics is the branch where the synthesis of elements is studied. According to standard cosmological theories, none of the atoms heavier than helium can have been created in the Big Bang in any relevant amount.
