The term
star-cluster is sometimes applied, though improperly, to assemblages
which are rather groups, such, for instance, as the Pleiades. In their
most characteristic aspect star-clusters are of a globular shape --
globes of suns! A famous example of a globular star-cluster, but one
not included in the Milky Way, is the ``Great Cluster in Hercules.''
This is barely visible to the naked eye, but a small telescope shows
its character, and in a large one it presents a marvelous spectacle.
Photographs of such clusters are, perhaps, less effective than those
of star-clouds, because the central condensation of stars in them is
so great that their light becomes blended in an indistinguishable
blur. The beautiful effect of the incessant play of infinitesimal rays
over the apparently compact surface of the cluster, as if it were a
globe of the finest frosted silver shining in an electric beam, is
also lost in a photograph. Still, even to the eye looking directly at
the cluster through a powerful telescope, the central part of the
wonderful congregation seems almost a solid mass in which the stars
are packed like the ice crystals in a snowball.
The same question rises to the lips of every observer: How can they
possibly have been brought into such a situation? The marvel does not
grow less when we know that, instead of being closely compacted, the
stars of the cluster are probably separated by millions of miles; for
we know that their distances apart are slight as compared with their
remoteness from the Earth.
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