erior bulk, our sun is much more powerful as
a light-giver. One hundred of the companions of Sirius would not give as
much light as our sun! This is a result of very considerable
significance. It teaches us that besides the great bodies in the
universe which attract attention by their brilliancy, there are also
other bodies of stupendous mass which have but little
brilliancy--probably some of them possess none at all. This suggests a
greatly enhanced conception of the majestic scale of the universe. It
also invites us to the belief that the universe which we behold bears
but a small ratio to the far larger part which is invisible in the
sombre shades of night. In the wide extent of the material universe we
have here or there a star or a mass of gaseous matter sufficiently
heated to be luminous, and thus to become visible from the earth; but
our observation of these luminous points can tell us little of the
remaining contents of the universe.

The most celebrated of all the variable stars is that known as Algol,
whose position in the constellation of Perseus is shown in Fig. 83. This
star is conveniently placed for observation, being visible every night
in our latitude, and its interesting changes can be observed without any
telescopic aid. Everyone who desires to become acquainted with the great
truths of astronomy should be able to recognise this star, and should
have also followed it during one of its periods of change. Algol is
usually a star of the second magnitude; but in a period between two and
three days, or, more accurately, in an interval of 2 days 20 hours 48
minutes and 55 seconds, its brilliancy goes through a most remarkable
cycle of variations. The series commences with a gradual decline of the
star's brightness, which in the course of four and a half hours falls
from the second magnitude down to the fourth. At this lowest stage of
brightness Algol remains for about twenty minutes, and then begins to
increase, until in three and a half hours it regains the second
magnitude, at which it continues for about 2 days 12 hours, when the
same series commences anew. It seems that the period required by Algol
to go through its changes is itself subject to a slow but certain
variation. We shall see in a following chapter how it has been proved
that the variability of Algol is due to the occasional interposition of
a dark companion which cuts off a part of the lustre of the star. All
the circumstances can thus be ac