While the stars in general appear to travel independently of one
another, except when they are combined in binary or trinary systems,
there are notable exceptions to this rule. In some quarters of the sky
we behold veritable migrations of entire groups of stars whose members
are too widely separated to show any indications of revolution about a
common center of gravity. This leads us back again to the wonderful
group of the Pleiades. All of the principle stars composing that group
are traveling in virtually parallel lines. Whatever force set them
going evidently acted upon all alike. This might be explained by the
assumption that when the original projective force acted upon them
they were more closely united than they are at present, and that in
drifting apart they have not lost the impulse of the primal motion. Or
it may be supposed that they are carried along by some current in
space, although it would be exceedingly difficult, in the present
state of our knowledge, to explain the nature of such a current. Yet
the theory of a current has been proposed. As to an attractive center
around which they might revolve, none has been found. Another instance
of similar ``star-drift'' is furnished by five of the seven stars
constituting the figure of the ``Great Dipper.'' In this case the
stars concerned are separated very widely, the two extreme ones by not
less than fifteen degrees, so that the idea of a common motion would
never have been suggested by their aspect in the sky; and the case
becomes the more remarkable from the fact that among and between them
there are other stars, some of the same magnitude, which do not share
their motion, but are traveling in other directions. Still other
examples of the same phenomenon are found in other parts of the sky.
Of course, in the case of compact star-clusters, it is assumed that
all the members share a like motion of translation through space, and
the same is probably true of dense star-swarms and star-clouds.