ABSTRACT
Clusters and nebulae are among the most striking of stellar objects. Interstellar space is never completely empty, but the density is very low indeed. Denser clumps of material accumulate in the giant molecular clouds, and star formation follows; many stars may be produced from the same cloud, resulting in a stellar cluster. Open clusters are not permanent structures; they are disrupted by passing ‘field’ stars, and will eventually disperse. The most famous of all open clusters is that of the Pleiades, in Taurus. Globular clusters are quite different from open clusters. They are vast symmetrical systems, containing from 10 000 to at least a million stars, and so strongly condensed that near their centres the individual stars may be only light-months apart. The globular clusters are members of what is termed the galactic halo, which is roughly spherical and extends out to several hundreds of thousands of light-years.
