A double star is one that divides into two with the help of a more or less powerful telescope. The effect is a strange, and might have appeared beforehand a most unlikely one. Yet it is of quite ordinary occurrence. Double stars are no freak of nature, but part of her settled plan; or rather, they enter systematically into the design of the Mind which is in and above nature.

The first recognised specimen of the class was ζ Ursæ Majoris, the middle ‘horse’ of the Plough, called by the Arabs ‘Mizar,’ which Riccioli found at Bologna, in 1650, to consist of a 2½ and a 4 magnitude star within fourteen seconds of arc of each other. Both are radiantly white, and they make a glorious object even in a very small telescope. The accident of a bright comet passing, on February 8, 1665, close to γ Arietis (‘Mesarthim’) led to the discovery of its duplex nature by Robert Hooke in the course of his observations on the comet. The components, each of the fourth magnitude, and eight seconds apart, are perfectly alike both in light and colour. Meanwhile Huygens had seen θ Orionis—perceived to be quadruple in 1684—as triple in 1656; a Crucis, in the southern hemisphere, was divided by some Jesuit missionaries sent by Louis XIV to Siam in 1685, and α Centauri by Richaud at Pondicherry in 1689; making in all five double stars detected during the seventeenth century.

Sidereal science has a great future before it. The prospects of its advance are incalculable; the possibilities of its development virtually infinite. No other branch of knowledge attracts efforts for its promotion, at once so wide-spread, so varied, and so enthusiastic; and in no other is anticipation so continually outrun by the brilliant significance of the results achieved.

For the due appreciation, however, of these results, some preliminary knowledge is required, and is possessed by few. To bring it within the reach of many is the object aimed at in the publication of the present volume. Astronomy is essentially a popular science. The general public has an indefeasible right of access to its lofty halls, which it is the more important to keep cleared of unnecessary technical impediments, since the natural tendency of all sciences is to become specialised as they advance. But literary treatment is the foe of specialisation, and helps to secure, accordingly, the topics it is applied to, against being secluded from the interest and understanding of ordinarily educated men and women. Now, in the whole astonishing history of the human intellect, there is no more astonishing chapter than that concerned with the sidereal researches of the last quarter of a century. Nor can the resources of thought be more effectually widened, or its principles be more surely ennobled through the vision of a Higher Wisdom, than by rendering it, so far as possible, intelligible to all.

The question whether nebulæ are external galaxies hardly any longer needs discussion. It has been answered by the progress of discovery. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way. A practical certainty has been attained that the entire contents, stellar and nebular, of the sphere belong to one mighty aggregation, and stand in ordered mutual relations within the limits of one all-embracing scheme—all-embracing, that is to say, so far as our capacities of knowledge extend. With the infinite possibilities beyond, science has no concern.

The chief reasons justifying the assertion that the status of the nebulæ is intra-galactic, are of three kinds. They depend, first, upon the nature of the bodies themselves; secondly, upon the stellar associations of many of them; thirdly, upon their systematic arrangement as compared with the systematic arrangement of the stars.

The detection of gaseous nebulæ not only directly demonstrated the non-stellar nature of a large number of these objects, but afforded a rational presumption that the others, however composed, were on a commensurate scale of size, and situated at commensurable distances. It may indeed turn out that gaseous and non-gaseous nebulæ form an unbroken series, rather than two distinct classes separated by an impassable barrier.

The study of the stars inevitably leads us to consider the advancing movement in the midst of them of the sun and its attendant train of planets. There can be no reasonable doubt—and the thought is an astounding one—that we are engaged on a voyage through space, without starting-point or goal that we can know of, but which may prove not wholly uneventful. Its progress may possibly bring about, as millenniums go by, changes powerfully influential upon human destinies; nay, an incident in its course may, at any time, by the inscrutable decree of Providence, terminate the terrestrial existence of our race, and consign the records of its civilisation, in dust and cinders, to the arid bosom of a dead planet. A curious sense of helplessness, tempered, however, by a higher trust, is produced as we thus vividly realise, perhaps for the first time, how completely we are at the mercy of unknown forces—how irresistibly our little ‘lodge in the vast wilderness’ of the universe is swept onward over an annual stretch of perhaps five hundred millions of miles, under the mysterious sway of bodies reduced by their almost infinite distances to evanescent dimensions.

But, as things are constituted, the translation of the sun's household is a necessity, albeit one of startling import to ourselves. The stellar system is maintained by the balance of forces, and motion is the correlative of force.

About two hundred and fifty stars have been formally registered as variable, and many more are open to the like suspicion. Gore's ‘Revised Catalogue’ includes 243 entries, besides 39 provisional additions; Chandler's nearly contemporaneous list enumerates 225 objects. Of these 160 are reckoned as ‘periodical,’ the rest as ‘irregular’ or ‘temporary.’ Periodical stars are further divided into those with ‘long,’ and those with ‘short’ periods. Nor is the distinction by any means arbitrary. The stars seem to separate of themselves into two principal groups, undergoing fluctuations in cycles of respectively less than fifty, and between two and four hundred days. The paucity of stars with periods of intermediate lengths is shown graphically in fig. 11, where the height of the curve represents the numbers of stars subject to changes proportionate in duration to the horizontal distance from left to right.

Variations requiring several months for their completion differ both in degree and kind from those run through in a few days. They are of much greater amplitude, ranging over five to eight instead of, at the most, two magnitudes; they are accomplished with less punctuality; and they are frequently attended by symptoms of atmospheric ignition entirely foreign to quicker vicissitudes. Most important of all, they affect bodies of peculiar constitution. Nearly all long-period variables are red stars with banded spectra; those of short period are white or yellowish in colour, and display Sirian or solar spectra.

The stars, speaking broadly, are suns. But what is a sun? We can only reply by taking function into consideration. A sun is a great radiating machine, and the obvious criterion for admission to the order is fitness for this office. Qualification to be a centre of light and heat is the dominant characteristic of each of its true members. Now the solar emissive activity is concentrated in a shining shell of clouds known as the ‘photosphere,’ which the entire energies of the organism (so to speak) seem directed to maintain and renew. And with reason, since its efficiency as a radiator depends upon the perpetuation of the condensing process by which this brilliant surface is produced.

The possession of a photosphere must then be regarded as an essential feature of the suns of space. But such a structure can only be formed in an incandescent atmosphere, the action of which modifies, more or less powerfully, the light emitted from it. The spectroscope can then alone decide whether a given sidereal object be, in the proper sense, a sun. For it is not so much the quantity as the quality of its radiations that determines the point. They must be such as can be supposed to emanate from condensed and vividly glowing matter bathed in cooler, though still ignited, vapours. That is to say, they must be primarily unbroken from end to end of the rainbow-tinted riband formed by prismatic dispersion, while showing the secondary effects of absorptive encroachments.