m us.

CHAPTER XXIII

THE STARS--_continued_

The stars appear to us to be scattered about the sky without any orderly
arrangement. Further, they are of varying degrees of brightness; some
being extremely brilliant, whilst others can but barely be seen. The
brightness of a star may arise from either of two causes. On the one
hand, the body may be really very bright in itself; on the other hand,
it may be situated comparatively near to us. Sometimes, indeed, both
these circumstances may come into play together.

Since variation in brightness is the most noticeable characteristic of
the stars, men have agreed to class them in divisions called
"magnitudes." This term, it must be distinctly understood, is employed
in such classification without any reference whatever to actual size,
being merely taken to designate roughly the amount of light which we
receive from a star. The twenty brightest stars in the sky are usually
classed in the first magnitude. In descending the scale, each magnitude
will be noticed to contain, broadly speaking, three times as many stars
as the one immediately above it. Thus the second magnitude contains 65,
the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The
last of these magnitudes is about the limit of the stars which we are
able to see with the naked eye. Adding, therefore, the above numbers
together, we find that, without the aid of the telescope, we cannot see
more than about 5000 stars in the entire sky--northern and southern
hemispheres included. Quite a small telescope will, however, allow us to
see down to the ninth magnitude, so that the total number of stars
visible to us with such very moderate instrumental means will be well
over 100,000.

It must not, however, be supposed that the stars included within each
magnitude are all of exactly the same brightness. In fact, it would be
difficult to say if there exist in the whole sky two stars which send us
precisely the same amount of light. In arranging the magnitudes, all
that was done was to make certain broad divisions, and to class within
them such stars as were much on a par with regard to brightness. It may
here be noted that a standard star of the first magnitude gives us about
one hundred times as much light as a star of the sixth magnitude, and
about one million times as much as one of the sixteenth magnitude--which
is near the limit of what we can see with the very best telescope.

Though the first