The idea that the nebulæ were far more remote from us than the stars long held sway, even after it had been given up by its chief supporter. When Sir William Herschel, by means of his then unapproached telescopic power, resolved the Milky Way more or less completely into stars, and showed that numerous objects which had been classed as nebulæ were really clusters of stars, it was natural to suppose that those which still retained their cloudy appearance under the highest telescopic powers were also clusters or systems of stars, which only needed still higher powers to show their true nature. This idea was supported by the fact that several nebulæ were found to be more or less ring-shaped, thus corresponding on a smaller scale to the form of the Milky Way; so that when Herschel discovered thousands of telescopic nebulæ, he was accustomed to speak of them as so many distinct universes scattered through the immeasurable depths of space.
Now, although any real conception of the immensity of the one stellar universe, of which the Milky Way with its associated stars is the fundamental feature, is, as I have shown, almost unattainable, the idea of an unlimited number of other universes, almost infinitely remote from our own and yet distinctly visible in the heavens, so seized upon the imagination that it became almost a commonplace of popular astronomy and was not easily given up even by astronomers themselves. And this was in a large part due to the fact that Sir William Herschel's voluminous writings, being almost all in the Philosophical Transactions of the Royal Society, were very little read, and that he only indicated his change of view by a few brief sentences which might easily be overlooked. The late Mr. Proctor appears to have been the first astronomer to make a thorough study of the whole of Herschel's papers, and he tells us that he read them all over five times before he was able thoroughly to grasp the writer's views at different periods.
But the first person to point out the real teaching of the facts as to the distribution of the nebulæ was not an astronomer, but our greatest philosophical student of science in general, Herbert Spencer. In a remarkable essay on 'The Nebular Hypothesis' in the Westminster Review of July, 1858, he maintained that the nebulæ really formed a part of our own Galaxy and of our own stellar universe. A single passage from his paper will indicate his line of argument, which, it may be added, had already been partially set forth by Sir John Herschel in his Outlines of Astronomy.
'If there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our own sidereal system. If there were but two nebulæ, and both were so placed, the coincidence would be excessively strange. What, then, shall we say on finding that there are thousands of nebulæ so placed? Shall we believe that in thousands of cases these far-removed galaxies happen to agree in their visible positions with the thin places in our own galaxy? Such a belief is impossible.'
He then applies the same argument to the distribution of the nebulæ as a whole:—'In that zone of celestial space where stars are excessively abundant, nebulæ are rare, while in the two opposite celestial spaces that are farthest removed from this zone, nebulæ are abundant. Scarcely any nebulæ lie near the galactic circle (or plane of the Milky Way); and the great mass of them lie round the galactic poles. Can this also be mere coincidence?' And he concludes, from the whole mass of the evidence, that 'the proofs of a physical connection become overwhelming.'
Nothing could be more clear or more forcible; but Spencer not being an astronomer, and writing in a comparatively little read periodical, the astronomical world hardly noticed him; and it was from ten to fifteen years later, when Mr. R.A. Proctor, by his laborious charts and his various papers read before the Royal and Royal Astronomical Societies from 1869 to 1875, compelled the attention of the scientific world, and thus did more perhaps than any other man to establish firmly the grand and far-reaching principle of the essential unity of the stellar universe, which is now accepted by almost every astronomical writer of eminence in the civilised world.
The Evolution of the Stellar Universe
Amid the enormous mass of observations and of suggestive speculation upon this great and most interesting problem, it is difficult to select what is most important and most trustworthy. But the attempt must be made, because, unless my readers have some knowledge of the most important facts bearing upon it (besides those already set forth), and also learn something of the difficulties that meet the inquirer into causes at every step of his way, and of the various ideas and suggestions which have been put forth to account for the facts and to overcome the difficulties, they will not be in a position to estimate, however imperfectly, the grandeur, the marvel, and the mystery of the vast and highly complex universe in which we live and of which we are an important, perhaps the most important, if not the only permanent outcome.
The Sun a Typical Star
It being now a recognised fact that the stars are suns, some knowledge of our own sun is an essential preliminary to an inquiry into their nature, and into the probable changes they have undergone.