Part 12 (1/2)
VI. THE NEBULAR HYPOTHESIS.
Inquiring into the pedigree of an idea is not a bad means of roughly estimating its value. To have come of respectable ancestry, is _prima facie_ evidence of worth in a belief as in a person; while to be descended from a discreditable stock is, in the one case as in the other, an unfavorable index. The a.n.a.logy is not a mere fancy. Beliefs, together with those who hold them, are modified little by little in successive generations; and as the modifications which successive generations of the holders undergo, do not destroy the original type, but only disguise and refine it, so the accompanying alterations of belief, however much they purify, leave behind the essence of the original belief.
Considered genealogically, the received theory respecting the creation of the Solar System is unmistakeably of low origin. You may clearly trace it back to primitive mythologies. Its remotest ancestor is the doctrine that the celestial bodies are personages who originally lived on the Earth--a doctrine still held by some of the negroes Livingstone visited. Science having divested the sun and planets of their divine personalities, this old idea was succeeded by the idea which even Kepler entertained, that the planets are guided in their courses by presiding spirits: no longer themselves G.o.ds, they are still severally kept in their orbits by G.o.ds. And when gravitation came to dispense with these celestial steersmen, there was begotten a belief, less gross than its parent, but partaking of the same essential nature, that the planets were originally launched into their orbits from the Creator's hand. Evidently, though much refined, the anthropomorphism of the current hypothesis is inherited from the aboriginal anthropomorphism, which described G.o.ds as a stronger order of men.
There is an antagonist hypothesis which does not propose to honour the Unknown Power manifested in the Universe, by such t.i.tles as ”The Master-Builder,” or ”The Great Artificer;” but which regards this Unknown Power as probably working after a method quite different from that of human mechanics. And the genealogy of this hypothesis is as high as that of the other is low. It is begotten by that ever-enlarging and ever-strengthening belief in the presence of Law, which acc.u.mulated experiences have gradually produced in the human mind. From generation to generation Science has been proving uniformities of relation among phenomena which were before thought either fortuitous or supernatural in their origin--has been showing an established order and a constant causation where ignorance had a.s.sumed irregularity and arbitrariness. Each further discovery of Law has increased the presumption that Law is everywhere conformed to. And hence, among other beliefs, has arisen the belief that the Solar System originated, not by _manufacture_ but by _evolution_. Besides its abstract parentage in those grand general conceptions which positive Science has generated, this hypothesis has a concrete parentage of the highest character. Based as it is on the law of universal gravitation, it may claim for its remote progenitor the great thinker who established that law. The man who gave it its general shape, by promulgating the doctrine that stars result from the aggregation of diffused matter, was the most diligent, careful, and original astronomical observer of modern times. And the world has not seen a more learned mathematician than the man who, setting out with this conception of diffused matter concentrating towards its centre of gravity, pointed out the way in which there would arise, in the course of its concentration, a balanced group of sun, planets, and satellites, like that of which the Earth is a member.
Thus, even were there but little direct evidence a.s.signable for the Nebular Hypothesis, the probability of its truth would still be strong. Its own high derivation and the low derivation of the antagonist hypothesis, would together form a weighty reason for accepting it--at any rate, provisionally. But the direct evidence a.s.signable for the Nebular Hypothesis is by no means little. It is far greater in quant.i.ty, and more varied in kind, than is commonly supposed. Much has been said here and there on this or that cla.s.s of evidences; but nowhere, as far as we know, have all the evidences, even of one cla.s.s, been fully stated; and still less has there been an adequate statement of the several groups of evidences in their _ensemble_. We propose here to do something towards supplying the deficiency: believing that, joined with the _a priori_ reasons given above, the array of _a posteriori_ reasons will leave little doubt in the mind of any candid inquirer.
And first, let us address ourselves to those recent discoveries in stellar astronomy, which have been supposed to conflict with this celebrated speculation.
When Sir William Herschel, directing his great reflector to various nebulous spots, found them resolvable into cl.u.s.ters of stars, he inferred, and for a time maintained, that all nebulous spots are cl.u.s.ters of stars exceedingly remote from us. But after years of conscientious investigation, he concluded that ”there were nebulosities which are not of a starry nature;” and on this conclusion was based his hypothesis of a diffused luminous fluid, which by its eventual aggregation, produced stars. A telescopic power much exceeding that used by Herschel, has enabled Lord Rosse to resolve some of the nebulae previously unresolved; and, returning to the conclusion which Herschel first formed on similar grounds but afterwards rejected, many astronomers have a.s.sumed that, under sufficiently high powers, every nebula would be decomposed into stars--that the resolvability is solely a question of distance. The hypothesis now commonly entertained is, that all nebulae are galaxies more or less like in nature to that immediately surrounding us; but that they are so inconceivably remote, as to look, through an ordinary telescope, like small faint spots. And not a few have drawn the corollary, that by the discoveries of Lord Rosse the Nebular Hypothesis has been disproved.
Now, even supposing that these inferences respecting the distances and natures of the nebulae are valid, they leave the Nebular Hypothesis substantially as it was. Admitting that each of those faint spots is a sidereal system, so far removed that its countless stars give less light than one small star of our own sidereal system; the admission is in no way inconsistent with the belief, that stars and their attendant planets have been formed by the aggregation of nebulous matter. Though, doubtless, if the existence of nebulous matter now in course of concentration be disproved, one of the evidences of the Nebular Hypothesis is destroyed; yet the remaining evidences remain just as they were. It is a perfectly tenable position, that though nebular condensation is now nowhere to be seen in progress, yet it was once going on universally. And, indeed, it might be argued that the still-continued existence of diffused nebulous matter is scarcely to be expected; seeing that the causes which have resulted in the aggregation of one ma.s.s, must have been acting on all ma.s.ses, and that hence the existence of ma.s.ses not aggregated would be a fact calling for explanation. Thus, granting the immediate conclusions suggested by these recent disclosures of the six-feet reflector, the corollary which many have drawn is inadmissible.
But we do not grant these conclusions. Receiving them though we have, for years past, as established truths, a critical examination of the facts has convinced us that they are quite unwarrantable. They involve so many manifest incongruities, that we have been astonished to find men of science entertaining them even as probable hypotheses. Let us consider these incongruities.
In the first place, mark what is inferable from the distribution of nebulae.
”The s.p.a.ces which precede or which follow simple nebulae,” says Arago, ”and, _a fortiori_, groups of nebulae, contain generally few stars. Herschel found this rule to be invariable. Thus, every time that, during a short interval, no star approached, in virtue of the diurnal motion, to place itself in the field of his motionless telescope, he was accustomed to say to the secretary who a.s.sisted him, 'Prepare to write; nebulae are about to arrive.'”
How does this fact consist with the hypothesis that nebulae are remote galaxies? If there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of s.p.a.ce, as to agree in direction with a starless spot in our own sidereal system. If there were but two nebulae, and both were so placed, the coincidence would be excessively strange. What, then, shall we say on finding that there are thousands of nebulae 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 next to impossible.
Still more manifest does the impossibility of it become when we consider the general distribution of nebulae. Besides again showing itself in the fact that ”the poorest regions in stars are near the richest in nebulae,”
the law above specified applies to the heavens as a whole. In that zone of celestial s.p.a.ce where stars are excessively abundant, nebulae are rare; while in the two opposite celestial s.p.a.ces that are furthest removed from this zone, nebulae are abundant. Scarcely any nebulae lie near the galactic circle (or plane of the Milky Way); and the great ma.s.s of them lie round the galactic poles. Can this also be mere coincidence? When to the fact that the general ma.s.s of nebulae are ant.i.thetical in position to the general ma.s.s of stars, we add the fact that local regions of nebulae are regions where stars are scarce, and the further fact that single nebulae are habitually found in comparatively starless spots; does not the proof of a physical connexion become overwhelming? Should it not require an infinity of evidence to show that nebulae are not parts of our sidereal system? Let us see whether any such infinity of evidence is a.s.signable. Let us see whether there is even a single alleged proof which will bear examination.
”As seen through colossal telescopes,” says Humboldt, ”the contemplation of these nebulous ma.s.ses leads us into regions from whence a ray of light, according to an a.s.sumption not wholly improbable, requires millions of years to reach our earth--to distances for whose measurement the dimensions (the distance of Sirius, or the calculated distances of the binary stars in Cygnus and the Centaur) of our nearest stratum of fixed stars scarcely suffice.”
Now, in this somewhat confused sentence there is expressed a more or less decided belief, that the distances of the nebulae from our galaxy of stars as much transcend the distances of our stars from each other, as these interstellar distances transcend the dimensions of our planetary system.
Just as the diameter of the Earth's...o...b..t, is an inappreciable point when compared with the distance of our Sun from Sirius; so is the distance of our Sun from Sirius, an inappreciable point when compared with the distance of our galaxy from those far removed galaxies const.i.tuting nebulae. Observe the consequences of this a.s.sumption.
If one of these supposed galaxies is so remote that its distance dwarfs our interstellar s.p.a.ces into points, and therefore makes the dimensions of our whole sidereal system relatively insignificant; does it not inevitably follow that the telescopic power required to resolve this remote galaxy into stars, must be incomparably greater than the telescopic power required to resolve the whole of our own galaxy into stars? Is it not certain that an instrument which can just exhibit with clearness the most distant stars of our own cl.u.s.ter, must be utterly unable to separate one of these remote cl.u.s.ters into stars? What, then, are we to think when we find that the same instrument which decomposes hosts of nebulae into stars, _fails_ to resolve completely our own Milky Way? Take a homely comparison. Suppose a man surrounded by a swarm of bees, extending, as they sometimes do, so high in the air as to be individually almost invisible, were to declare that a certain spot on the horizon was a swarm of bees; and that he knew it because he could see the bees as separate specks. Astounding as the a.s.sertion would be, it would not exceed in incredibility this which we are criticising. Reduce the dimensions to figures, and the absurdity becomes still more palpable. In round numbers, the distance of Sirius from the Earth is a million times the distance of the Earth from the Sun; and, according to the hypothesis, the distance of a nebula is something like a million times the distance of Sirius.
Now, our own ”starry island, or nebula,” as Humboldt calls it, ”forms a lens-shaped, flattened, and everywhere detached stratum, whose major axis is estimated at seven or eight hundred, and its minor axis at a hundred and fifty times the distance of Sirius from the Earth.”[I] And since it is concluded that our Solar System is near the centre of this aggregation, it follows that our distance from the remotest parts of it is about four hundred distances of Sirius. But the stars forming these remotest parts are not individually visible, even through telescopes of the highest power.
How, then, can such telescopes make individually visible the stars of a nebula which is a million times the distance of Sirius? The implication is, that a star rendered invisible by distance becomes visible if taken two thousand five hundred times further off! Shall we accept this implication?
or shall we not rather conclude that the nebulae are _not_ remote galaxies?
Shall we not infer that, be their nature what it may, they must be at least as near to us as the extremities of our own sidereal system?
[I] Cosmos. (Seventh Edition.) Vol. i. pp. 79, 80.
Throughout the above argument, it is tacitly a.s.sumed that differences of apparent magnitude among the stars, result mainly from differences of distance. On this a.s.sumption the current doctrines respecting the nebulae are founded; and this a.s.sumption is, for the nonce, admitted in each of the foregoing criticisms. From the time, however, when it was first made by Sir W. Herschel, this a.s.sumption has been purely gratuitous; and it now proves to be totally inadmissible. But, awkwardly enough, its truth and its untruth are alike fatal to the conclusions of those who argue after the manner of Humboldt. Note the alternative.
On the one hand, what follows from the untruth of the a.s.sumption? If apparent largeness of stars is not due to comparative nearness, and their successively smaller sizes to their greater and greater degrees of remoteness, what becomes of the inferences respecting the dimensions of our sidereal system and the distances of nebulae? If, as has lately been shown, the almost invisible star 61 Cygni has a greater parallax than [alpha]
Cygni, though, according to an estimate based on Sir W. Herschel's a.s.sumption, it should be about twelve times more distant--if, as it turns out, there exist telescopic stars which are nearer to us than Sirius; of what worth is the conclusion that the nebulae are very remote, because their component luminous ma.s.ses are made visible only by high telescopic powers?
Clearly, if the most brilliant star in the heavens and a star that cannot be seen by the naked eye, prove to be equidistant, relative distances cannot be in the least inferred from relative visibilities. And if so, nebulae may be comparatively near, though the starlets of which they are made up appear extremely minute.