There are two ways of thinking about stars. On the one hand they are essentially point masses held together by gravity and hurtling through space like the molecules in an infinite container of gas. On the other they are nuclear furnaces in the process of evolu tionary change. The astronomer's point of view depends on the problem he is consilering. Sometimes it is useful to keep both aspects in mind. This is particularly true in the study of clusters of stars.
It seems likely that most stars were formed in clusters, condensing out of large local concentrations of gas and dust. In the ensuing millions or billions of years many of the groups have lost their cohesiveness and have been smeared out into the general stellar pop- ulation. Any cluster that is still recog nizable as such-and there are thousands in our galaxy-embodies, as it were, a double history: the spatial and dynamical evolution of the ensemble of its constit uent stars considered as point masses and the internal and surface evolution of the individual member stars resulting from the nuclear processes taking place within them. As might be expected, each of these processes throws considerable light on the other, and the whole story turns out to be bigger than the sum of its parts.
