endeavour to frame some valid hypothesis as to the relation of
our solar system to other systems.

Thus far our view has been confined to the career of a single
star,--our sun,--with the tiny, easily-cooling balls which it has
cast off in the course of its development. Thus far, too, our
inferences have been very secure, for we have been dealing with a
circumscribed group of phenomena, the beginning and end of which
have been brought pretty well within the compass of our
imagination. It is quite another thing to deal with the actual or
probable career of the stars in general, inasmuch as we do not
even know how many stars there are, which form parts of a common
system, or what. are their precise dynamic relations to one
another. Nevertheless we have knowledge of a few facts which may
support some cautious inferences. All the stars which we can see
are undoubtedly bound together by relations of gravitation. No
doubt our sun attracts all the other stars within our ken, and is
reciprocally attracted by them. The stars, too, lie mostly in or
around one great plane, as is the case with the members of the
solar system. Moreover, the stars are shown by the spectroscope
to consist of chemical elements identical with those which are
found in the solar system. Such facts as these make it probable
that the career of other stars, when adequately inquired into,
would be found to be like that of our own sun. Observation daily
enhances this probability, for our study of the sidereal universe
is continually showing us stars in all stages of development. We
find irregular nebulae, for example; we find spiral and
spheroidal nebulae; we find stars which have got beyond the
nebulous stage, but are still at a whiter heat than our sun; and
we also find many stars which yield the same sort of spectrum as