About five hundred clusters are at present tolerably well known to astronomers, and a large number besides, their character rendered ambiguous by distance, are probably included among both ‘resolvable’ and ‘unresolved’ nebulæ. Such aggregations may be broadly divided into ‘irregular’ and ‘globular’ clusters. Although, as might have been expected, the line of demarcation between the two classes is by no means sharply drawn, each has its own marked peculiarities.

Irregular clusters are framed on no very obvious plan; they are not centrally condensed, they are of all shapes, and their leading stars rarely occupy critical positions. The stars in them are collected together, to a superficial glance, much after the fashion of a flock of birds. Alcyone, it is true, seems of primary dignity among the Pleiades, and the Pleiades may be regarded as typical of irregular clusters; yet the dominance, even here, of a central star may be more apparent than real.

The arrangement of stars in clusters is, nevertheless, far from being unmethodical, even though the method discernible in it be not of the sort that might have been anticipated. It seems, indeed, inconsistent with movements in closed curves, and suggests rather the description of hyperbolic orbits. Obviously, however, its true nature must be greatly obscured to our perception by the annulment, through perspective, of the third dimension of space, whereby independent groupings, flattened down side by side, are rendered scarcely, if at all, distinguishable.