n days to complete its orbit, and here is a mighty body
hastening round its sun in three! To do this in the time the large dark
planet must be very near to Algol; indeed, astronomers have calculated
that the surfaces of the two bodies are not more than about two million
miles apart, and this is a trifle when we consider that we ourselves are
more than forty-six times as far as that from the sun. At this distance
Algol, as observed from the planet, will fill half the sky, and the heat
he gives out must be something stupendous. Also the effects of
gravitation must be queer indeed, acting on two such huge bodies so
close together. If any beings live in such a strange world, the pull
which draws them to their mighty sun must be very nearly equal to the
pull which holds them to their own globe; the two together may
counteract each other, but the effect must be strange!

From irregularities in the movements of Algol it has been judged that
there may be also in the same system another dark body, but of it
nothing has been definitely ascertained.

But all variable stars need not necessarily be due to the light being
intercepted by a dark body. There are cases where two bright stars in
revolving round each other produce the same effect; for when seen side
by side the two stars give twice as much light as when one is hidden
behind the other, and as they are seen alternately side by side and in
line, they seem to alter regularly in lustre.

CHAPTER XVI

STAR CLUSTERS AND NEBULAE

Could you point out any star cluster in the sky? You could if you would
only think for a minute, for one has been mentioned already. This is the
cluster known as the Pleiades, and it is so peculiar and so different
from anything else, that many people recognize the group and know where
to look for it even before they know the Great Bear, the favourite
constellation in the northern sky, itself. The Pleiades is a real star
cluster, and the chief stars in it are at such enormous distances from
one another that they can be seen separately by the eye unaided, whereas
in most clusters the stars appear to be so close together that without a
telescope they make a mere blur of brightness. For a long time it was
supposed that the stars composing the Pleiades could not really be
connected because of the great distances between them; for, as you know,
even a hair's-breadth apparently between stars signifies in reality many
millions of miles.

Light tra