he Stars are so great that 1000 years would make hardly any difference
in the appearance of the heavens.

Changes, however, there certainly would be. Even in the short time
during which we have any observations, some are already on record. One
of the most interesting is the fading of the 7th Pleiad, due, according
to Ovid, to grief at the taking of Troy. Again, the "fiery Dogstar," as
it used to be, is now, and has been for centuries, a clear white.

The star known as Nova Cygni--the "new star in the Constellation of the
Swan"--was first observed on the 24th November 1876 by Dr. Schmidt of
Athens, who had examined that part of the heavens only four days before,
and is sure that no such star was visible then. At its brightest it was
a brilliant star of the third magnitude, but this only lasted for a few
days; in a week it had ceased to be a conspicuous object, and in a
fortnight became invisible without a telescope. Its sudden splendour was
probably due to a collision between two bodies, and was probably little,
if at all, less than that of the Sun itself. It is still a mystery how
so great a conflagration can have diminished so rapidly.

But though we speak of some stars as specially variable, they are no
doubt all undergoing slow change. There was a time when they were not,
and one will come when they will cease to shine. Each, indeed, has a
life-history of its own. Some, doubtless, represent now what others
once were, and what many will some day become.

For, in addition to the luminous heavenly bodies, we cannot doubt that
there are countless others invisible to us, some from their greater
distance or smaller size, but others, doubtless, from their feebler
light; indeed, we know that there are many dark bodies which now emit no
light, or comparatively little. Thus in the case of Procyon the
existence of an invisible body is proved by the movement of the visible
star. Again, I may refer to the curious phenomena presented by Algol, a
bright star in the head of Medusa. The star shines without change for
two days and thirteen hours; then in three hours and a half dwindles
from a star of the second to one of the fourth magnitude; and then, in
another three and a half hours, reassumes its original brilliancy. These
changes led astronomers to infer the presence of an opaque body, which
intercepts at regular intervals a part of the light emitted by Algol;
and Vogel has now shown by the aid of the spectroscope that Algol do