we have reached stars of the
fourteenth magnitude, at least 20,000,000 are revealed to us.

If we look into the heavens on a clear moonlight night, we shall further
see that here and there are groups of stars clustered together. These
clusters are termed constellations, and are named after some object
which the arrangement of the stars seemed to suggest. Thus every one is
familiar with that constellation known as the Great Bear, or the
"Plough," so called because of its resemblance to a plough.

The brightest stars of each constellation are named after the letters of
the Greek alphabet, the brightest being called Alpha, the next in
brilliancy Beta, and so on, right through the Greek alphabet. For
example, the seven stars in the Great Bear are known as Alpha, Beta,
Gamma, Delta, Epsilon, Zeta, and Eta.

The constellations are grouped into two divisions, known as the Northern
and Southern constellations respectively.

The visible Northern constellations are 25 in number, and include the
following well-known groups--

Ursa Major The Great Bear.
Ursa Minor The Little Bear.
Draco The Dragon.
Hercules Hercules.
Cygnus The Swan.
Lyra The Lyre.

The visible Southern constellations are 18 in number, and include such
groups as--

Cetus The Whale.
Orion Orion.
Canis Major The Great Dog.
Canis Minor The Little Dog.
Corona Australis The Southern Crown.
Crux Australis The Southern Cross.

_Variable Stars._--Not only are the stars of different magnitudes, but
the brilliancy of some of them changes from time to time. This class of
stars is known as variable stars, and has received the attention of
modern astronomers for many years, in order that the cause of their
variation might, if possible, be ascertained. The periods of variation
differ in length, ranging from a number of days to 60 or 70 years.

One of the most interesting of variable stars is that known as Omicron
Ceti, whose period of change is about 331 days. Its brilliancy varies
from one of the second magnitude to one of the tenth.

Beta Persei is another well-known variable star. This star shines as one
of the second magnitude for 2 days and 13 hours, and then suddenly loses
its light, and in less than 4 hours becomes a star of the fourth
magnitude. Its brilliancy then increases again, and