We have been treating stars as though they were isolated objects in the sky, unrelated to anything else in the universe. Actually they are members of groups that ultimately influence the history and evolution of these same stars in various ways. Every star has neighbors all around it in the sky as though it belonged to some vast ensemble of stars that seems to extend indefinitely into space. It is not very easy to get a clear idea of the nature or structure of this ensemble just by looking at the stars with our unaided eyes; modern telescopes and electronic techniques are needed to reveal the structure not only of this ensemble (the Milky Way galaxy) but also of other ensembles like it (the distant galaxies).