faintness is due merely to their great distance
from us, since they are much too far off to show any appreciable
parallactic displacement. Mr. Gore, however, thinks there is good
evidence to show that the stars in clusters are really close, and that
the clusters themselves fill a comparatively small space.

One of the finest examples of a cluster is the great globular one, in
the constellation of Hercules, discovered by Halley in 1714. It contains
over 5000 stars, and upon a clear, dark night is visible to the naked
eye as a patch of light. In the telescope, however, it is a wonderful
object. There are also fine clusters in the constellations of Auriga,
Pegasus, and Canes Venatici. In the southern heavens there are some
magnificent examples of globular clusters. This hemisphere seems,
indeed, to be richer in such objects than the northern. For instance,
there is a great one in the constellation of the Centaur, containing
some 6000 stars (see Plate XXI., p. 306).

[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN
CONSTELLATION OF CENTAURUS

From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
of exposure, 1 hour.

(Page 306)]

Certain remarkable groups of stars, of a nature similar to clusters,
though not containing such faint or densely packed stars as those we
have just alluded to, call for a mention in this connection. The best
example of such star groups are the Pleiades and the Hyades (see Plate
XX., p. 296), Coma Berenices, and Praesepe (or the Beehive), the
last-named being in the constellation of Cancer.

Stars which alter in their brightness are called _Variable Stars_, or
"variables." The first star whose variability attracted attention is
that known as Omicron Ceti, namely, the star marked with the Greek
letter [o] (Omicron) in the constellation of Cetus, or the Whale, a
constellation situated not far from Taurus. This star, the variability
of which was discovered by Fabricius in 1596, is also known as Mira, or
the "Wonderful," on account of the extraordinary manner in which its
light varies from time to time. The star known by the name of Algol,[32]
popularly called the "Demon Star"--whose astronomical designation is [b]
(Beta) Persei, or the star second in brightness in the constellation of
Perseus--was discovered by Goodricke, in the year 1783, to be a variable
star. In the following year [b] Lyrae, the star in Lyra next in order of
brightness after Vega, was al