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traversed by Uranus and Neptune' amid the depths of sidereal space are even more remarkably drawn out, regarding them in their helicoidal character.
As the Sun travels through space the planets sweep onward with him.* But has he, besides his planetary dependants, any companions on his voyage? Do any of his brother suns travel along with him? As yet we have no means of knowing, for a strange difficulty arises. If the Sun has companions, these must, of course, be relatively near to him. It does not follow that they will appear brighter than other stars, because if they are no larger than the Sun, we know that other orbs (as Sirius and Arcturus) must largely exceed them in real size, and so their relative proximity may not be rendered apparent. But this is not all. The stars which astronomers select as most likely to afford measurable indications of proximity are those whose apparent motions on the heavens are exceptionally large. Now the companions of our Sun on his voyage through the sidereal system doubtless travel on a nearly parallel course; and therefore, setting aside their orbital motions around each other, or around the common centime of gravity of the family, they must appear, as viewed by us, to be almost at rest. They would of course indicate in a more marked manner than any other stars the effect of the Earth's annual
* It is worthy of notice that the Sun's northern hemisphere travels forwards. Is it not conceivable that in this peculiarity we may find some explanation of the greater heat which has been said to be emitted from the northern solar hemisphere? See also p. 210, footnote.
motion (or, technically, they would have a large annual parallactic displacement); but then astronomers would not be led to look for such effects, since we know as a matter of fact that the stars hitherto examined for signs of annual parallax are those which, either
Observed proper motions in Ursa Major and neighbourhood.
through exceptional brilliancy or through exceptionally large proper motion, seem likely to be near to us.
We find signs in the heavens leading us to regard the existence of such ' companions of the Sun' as at least not wholly improbable. Here, for example, is a picture (fig. 90) borrowed from my ' Other Worlds,'
in which the chief stars of the constellation Ursa Major are depicted, and some few others belonging to Draco and Bootes. To each star is attached a small arrow indicating the direction of its motion, and the amount of such motion in 36,000 years. We see here decided signs of star-drift We can scarcely doubt that the five principal stars of Ursa Major included within the dotted line are travelling together though space; while the four stars above, belonging to Draco,
Observed proper motion of Stars in head of Aries.
seem similarly to be companion suns. The remaining stars may also not improbably form a single family.
The group of stars shown in fig. 91 seem to form a system within which probably there are orbital motions of considerable magnitude.
It is worthy of notice that in the two instances here referred to there are evidences of association apart from the observed proper motions. For in the second the stars forming the system seem segregated in a somewhat marked manner from neighbouring stargroups. The stars in Ursa Major, again, have been noted by Fr. Secchi as having very similar spectra—in other words, as resembling each other very closely in structure and condition. In this circumstance we have a peculiarity which may one day enable us to select with some confidence those stars which are our Sun's special companions, which voyage along with him through the sidereal system, and share with him perchance in a reign over some special domain of space.
And here I have to draw this work to a conclusion. Too limited by far has been the space I have allotted to my subject, and yet, for many reasons, it would have been unwise to have exceeded this space. I have followed so closely the course I marked down in the beginning that I see no occasion to change one word in the introductory pages. Yet at many stages where I had promised myself a pause for survey and reflection I have been compelled to pass on without stay. Many inviting paths opening out on one hand and the other have been perforce left unexplored. As I have proceeded I have become more and more sensible of the vastness of the subject I have undertaken to discuss. Not a book, but a library of books, is needed to deal adequately even with only what is at present known about the Sun; not a few students of science, but all the astronomers and physicists now living, might devote their powers to the study of solar phenomena, and yet find that the army of labourers needed to be largely recruited. Only in the coming time, when the students of science will be the majority instead of a minute minority of mankind, will this subject, and those countless other kindred subjects which await investigation, be adequately dealt with. Let us hope that time is not far off.