n some cases indeed the
relation may only be apparent, one being really far in front of the
other. In very many cases, however, the association is real, and they
revolve round one another. In some cases the period may extend to
thousands of years; for the distance which separates them is enormous,
and, even when with a powerful telescope it is indicated only by a
narrow dark line, amounts to hundreds of millions of miles. The Pole
Star itself is double. Andromeda is triple, with perhaps a fourth dark
and therefore invisible companion. These dark bodies have a special
interest, since it is impossible not to ask ourselves whether some at
any rate of them may not be inhabited. In [Greek: epsilon] Lyrae there
are two, each again being itself double. [Greek: xi] Cancri, and
probably also [Greek: theta] Orionis, consist of six stars, and from
such a group we pass on to Star Clusters in which the number is very
considerable. The cluster in Hercules consists of from 1000 to 4000. A
stellar swarm in the Southern Cross contains several hundred stars of
various colours, red, green, greenish blue, and blue closely thronged
together, so that they have been compared to a "superb piece of fancy
jewellery."[73]

The cluster in the Sword Handle of Perseus contains innumerable stars,
many doubtless as brilliant as our Sun. We ourselves probably form a
part of such a cluster. The Milky Way itself, as we know, entirely
surrounds us; it is evident, therefore, that the Sun, and of course we
ourselves, actually lie in it. It is, therefore, a Star Cluster, one of
countless numbers, and containing our Sun as a single unit.

It has as yet been found impossible to determine even approximately the
distance of these Star Clusters.

NEBULAE

From Stars we pass insensibly to Nebulae, which are so far away that
their distance is at present quite immeasurable. All that we can do is
to fix a minimum, and this is so great that it is useless to express it
in miles. Astronomers, therefore, take the velocity of light as a unit.
It travels at the rate of 180,000 miles a second, and even at this
enormous velocity it must have taken hundreds of years to reach us, so
that we see them not as they now are but as they were hundreds of years
ago.

It is no wonder, therefore, that in many of these clusters it is
impossible to distinguish the separate stars of which they are composed.
As, however, our telescopes are improved, more and more clusters are
being