The Plurality of Worlds Part 5

19. This would be so, we say, if the luminous matter moved in a greatly resisting medium. But what is the measure of _great_ resistance? It is, as we have already said, that the resistance which opposes the motion shall bear a considerable proportion to the force which deflects the motion. But what is that force? Upon the theory of the universal gravitation of matter, on which theory we here proceed, the force which deflects the motions of the parts of each system into curves, is the mutual attraction of the parts of the system; leaving out of the account the action of other systems, as comparatively insignificant and insensible. The condition, then, for the production of such spiral figures as I have spoken of, amounts really to this; that the mutual attraction of the parts of the luminous matter is slight; or, in other words, that the matter itself is very thin and rare. In that case, indeed, we can easily see that such a result would follow. A cloud of dust, or of smoke, which was thin and light, would make but a little way through the air, and would soon fall downwards; while a metal bullet shot horizontally with the same velocity, might fly for miles. Just so, a loose and vaporous ma.s.s of cometic matter would be pulled rapidly inwards by the attraction to the centre; and supposing it also drawn into a long train, by the different density of its different parts, it would trace, in lines of light, a circular or elliptical spiral converging to the centre of attraction, and resembling one of the branches of the spiral nebulae. And if several such cometic ma.s.ses thus travelled towards the centre, they would exhibit the wheel-like figure with bent spokes, which is seen in the spiral nebulae. And such a figure would all the more resemble some of these nebulae, as seen through Lord Rosse's telescope, if the spirals were accompanied by exterior branches of thinner and fainter light, which nebulous matter of smaller density might naturally form. Perhaps too, such matter, when thin, may be supposed to cool down more rapidly from its state of incandescence; and thus to become less luminous. If this were so, a great optical power would of course be required, to make the diverging branches visible at all.

20. There is one additional remark, which we may make, as to the resemblance of cometary[9] and nebular matter. That cometary matter is of very small density, we have many reasons to believe:--its transparency, which allows us to see stars through it undimmed;--the absence of any mechanical effect, weight, inertia, impulse, or attraction, in the nearest appulses of comets to planets and satellites:--and the fact that, in the recent remarkable event in the cometic history, the separation of Biela's comet into two, the two parts did not appear to exert any perceptible attraction on each other, any more than two volumes of dust or of smoke would do on earth. Luminous cometary matter, then, is very light, that is, has very little weight or inertia. And luminous nebulous matter is also very light in this sense: if our account of the cause of spiral nebulae has in it any truth. But yet, if we suppose the nebulae to be governed by the law of universal gravitation, the attractive force of the luminous matter upon itself, must be sufficient to bend the spirals into their forms. How are we to reconcile this; that the matter is so loose that it falls to the centre in rapid spirals, and yet that it attracts so strongly that there is a centre, and an energetic central force to curve the spirals thither? To this, the reply which we must make is, that the size of the nebular s.p.a.ce is such, that though its rarity is extreme, its whole ma.s.s is considerable. One part does not perceptibly attract another, but the whole does perceptibly attract every part. This indeed need the less surprise us, since it is exactly the case with our earth. One stone does not visibly attract another. It is much indeed for man, if he can make perceptible the attraction of a mountain upon a plumb-line; or of a stratum of rock a thousand feet thick upon the going of a pendulum; or of large ma.s.ses of metal upon a delicate balance. By such experiments men of science have endeavored to measure that minute thing, the attraction of one portion of terrestrial matter upon another; and thus, to weigh the whole ma.s.s of the earth. And equally great, at least, may be the disproportion between the mutual attraction of two parts of a nebulous system, and the total central attraction; and thus, though the former be insensible, the latter may be important.

21. It has been shown by Newton, that if any ma.s.s of matter be distributed in a uniform sphere, or in uniform concentric spherical sh.e.l.ls, the total attraction on a point without the sphere, will be the same as if the whole ma.s.s were collected in that single point, the centre. Now, proceeding upon the supposition of such a distribution of the matter in a nebula, (which is a reasonable average supposition,) we may say, that if our sun were expanded into a nebula reaching to the extreme bounds of the known solar system, namely, to the newly-discovered planet Neptune, or even hundreds of times further; the attraction on an external point would remain the same as it is, while the attraction on points within the sphere of diffusion would be less than it is; according to some law, depending upon the degree of condensation of the nebular matter towards the centre; but still, in the outer regions of the nebula, not differing much from the present solar attraction. If we could discover a ma.s.s of luminous matter, descending in a spiral course towards the centre of such a nebula, that is, towards the sun, we should have a sort of element of the spiral nebulae which have now attracted so much of the attention of astronomers. But, by an extraordinary coincidence, recent discoveries have presented to us such an element. Encke's comet, of which we have just spoken, appears to be describing such a spiral curve towards the sun. It is found that its period is, at every revolution, shorter and shorter; the amplitude of its sweep, at every return within the limits of our observation, narrower and narrower; so that in the course of revolutions and ages, however numerous, still, not such as to shake the evidence of the fact, it will fall into the sun.

22. Here then we are irresistibly driven to calculate what degree of resemblance there is, between the comet of Encke, and the luminous elements of the spiral nebulae, which have recently been found to exist in other regions of the universe. Can we compare its density with theirs? Can we learn whether the luminous matter in such nebulae is more diffused or less diffused, than that of the comet of Encke? Can we compare the mechanical power of getting through s.p.a.ce, as we may call it, that is, the ratio of the inertia to the resistance, in the one case, and in the other? If we can, the comparison cannot fail, it would seem, to be very curious and instructive. In this comparison, as in most others to which cosmical relations conduct us, we must expect that the numbers to which we are led, will be of very considerable amount. It is not equality in the density of the two luminous ma.s.ses which we are to expect to find; if we can mark their proportions by thousands of times, we shall have made no small progress in such speculations.

23. The comet of Encke describes a spiral, gradually converging to the sun; but at what rate converging? In how many revolutions will it reach the sun? Of how many folds will its spire consist, before it attains the end of its course? The answer is:--Of very many. The r.e.t.a.r.dation of Encke's Comet is very small: so small, that it has tasked the highest powers of modern calculation to detect it. Still, however, it is there: detected, and generally acknowledged, and confirmed by every revolution of the comet, which brings it under our notice; that is, commonly, about every three years. And having this fact, we must make what we can of it, in reasoning on the condition of the universe. No accuracy of calculation is necessary for our purpose: it is enough, if we bring into view the kind of scale of numbers to which calculation would lead us.

24. Encke's comet revolves round the sun in 1,211 days. The period diminishes at present, by about one-ninth of a day every revolution.

This amount of diminution will change, as the orbit narrows; but for our purpose, it will be enough to consider it unchangeable. The orbit therefore will cease to exist in a number of periods expressed by 9 times 1,211; that is, in something more that 10,000 revolutions; and of course sooner than this, in consequence of its coming in contact with the body of the sun. In 30,000 years then, it may be, this comet will complete its spiral, and be absorbed by the central ma.s.s. This long time, this long series of ten thousand revolutions, are long, because the resistance is so small, compared with the inertia of the moving ma.s.s. However thin, and rare, and unsubstantial the comet may be, the medium which resists it is much more so.

25. But this spiral, converging to its pole so slowly that it reaches it only after 10,000 circuits, is very different indeed from the spirals which we see in the nebulae of which we have spoken. In the most conspicuous of those, there are only at most three or four circular or oval sweeps, in each spiral, or even the spiral reaches the centre before it has completed a single revolution round it. Now, what are we to infer from this? How is it, that the comet has a spiral of so many revolutions, and the nebulae of so few? What difference of the mechanical conditions is indicated by this striking difference of form? Why, while the Comet thus lingers longer in the outer s.p.a.ce, and approaches the sun by almost imperceptible degrees, does the Nebular Element rush, as it were, headlong to its centre, and show itself unable to circulate even for a few revolutions?

26. Regarding the question as a mechanical problem, the answer must be this:--It is so, because the nebula is so much more rare than the matter of the comet, or the resisting medium so much more dense; or combining the two suppositions, because in the case of the comet, the luminous matter has _much_ more inertia, more mechanical reality and substance, than the medium through which it moves; but in the nebula very _little_ more.

27. The numbers of revolutions of the spiral, in the two cases, may not exactly represent the difference of the proportions; but, as I have said, they may serve to show the scale of them; and thus we may say, that if Encke's comet, approaching the centre by 10,000 revolutions, is 100,000 times as dense as the surrounding medium, the elements of the nebula, which reach the centre in a single revolution, are only ten times as dense as the medium through which they have to move.[10]

28. Nor does this result (that the bright element of the nebulae is so few times denser than the medium in which it moves) offer anything which need surprise us: for, in truth, in a diffused nebula, since we suppose that its parts have mechanical properties, the nebula itself is a resisting medium. The rarer parts, which may very naturally have cooled down in consequence of their rarity, and so, become non-luminous, will resist the motions of the more dense and still-luminous portions. If we recur to the supposition, which we lately made, that the Sun were expanded into a nebulous sphere, reaching the orbit of Neptune, the diffused matter would offer a far greater resistance to the motions of comets than they now experience. In that case, Encke's comet might be brought to the centre after a few revolutions; and if, while it were thus descending, it were to be drawn out into a string of luminous ma.s.ses, as Biela's comet has begun to be, these comets, and any others, would form separate luminous spiral tracks in the solar system; and would convert it into a spiral nebula of many branches, like those which are now the most recent objects of astronomical wonder.

29. It seems allowable to regard it as one of those coincidences, in the epochs of related yet seeming unconnected discoveries, which have so often occurred in the history of science; that we should, nearly at the same time, have had brought to our notice, the prevalence of spiral nebulae, and the circ.u.mstances, in Biela's and in Encke's comets, which seem to explain them: the one by showing the origin of luminous broken lines, one part drifting on faster than another, according to its different density, as is usual in incoherent ma.s.ses;[11] and the other by showing the origin of the spiral form of those lines, arising from the motion being in a resisting medium.

30. But though I have made suppositions by which our Solar System might become a spiral nebula, undoubtedly it is at present something very different; and the leading points of difference are very important for us to consider. And the main point is, that which has already been cursorily noticed: that instead of consisting of matter all nearly of the same density, and a great deal of it luminous, our Solar System consists of kinds of matter immensely different in density, and of large and regular portions which are not luminous. Instead of a diffused nebula with vaporous comets trailing spiral tracks through a medium little rarer than themselves; we have a central sun, and the dark globes of the solid planets rolling round him, in a medium so rare, that in thousands of revolutions not a vestige of r.e.t.a.r.dation can be discovered by the most subtle and persevering researches of astronomers. In the solar system, the luminous matter is collected into the body of the sun; the non-luminous matter, into the planets. And the comets and the resisting medium, which offer a small exception to this account, bear a proportion to the rest which the power of numbers scarce suffices to express.

31. Thus with regard to the density of matter in the solar system; we have supposed, as a mode of expression, that the density of a comet, Encke's comet for instance, is 100,000 times that of the resisting medium. Probably this is greatly understated; and probably also we greatly understate the matter, when we suppose that the tail of a comet is 100,000 times rarer than the matter of the sun.[12] And thus the resisting medium would be, at a very low calculation, 10,000 millions of times more rare than the substance of the sun.

32. And thus we are not, I think, going too far, when we say, that our Solar System, compared with spiral nebulous systems, is a system completed and finished, while they are mere confused, indiscriminate, incoherent ma.s.ses. In the Nebulae, we have loose matter of a thin and vaporous const.i.tution, differing as more or less rare, more or less luminous, in a small degree; diffused over enormous s.p.a.ces, in straggling and irregular forms; moving in devious and brief curves, with no vestige of order or system, or even of separation of different kinds of bodies. In the Solar System, we have the luminous separated from the non-luminous, the hot from the cold, the dense from the rare; and all, luminous and non-luminous, formed into globes, impressed with regular and orderly motions, which continue the same for innumerable revolutions and cycles.[13] The spiral nebulae, compared with the solar system, cannot be considered as other than a kind of chaos; and not even a chaos, in the sense of a state preceding an orderly and stable system; for there is no indication, in those objects, of any tendency towards such a system. If we were to say that they appear mere shapeless ma.s.ses, flung off in the work of creating solar systems, we might perhaps disturb those who are resolved to find everywhere worlds like ours; but it seems difficult to suggest any other reason for not saying so.

33. The same may be said of the other very irregular nebulae, which spread out patches and paths of various degrees of brightness; and shoot out, into surrounding s.p.a.ce, faint branches which are of different form and extent, according to the optical power with which they are seen.

These irregular forms are incapable of being permanent according to the laws of mechanics. They are not figures of equilibrium; and, therefore, must change by the attraction of the matter upon itself. But if the tenuity of the matter is extreme, and the resistance of the medium in which it floats considerable, this tendency to change and to condensation may be almost nullified; and the bright specks may long keep their straggling forms, as the most fantastically shaped clouds of a summer-sky often do. It is true, it may be said that the reason why we see no change in the form of such nebulae, is that our observations have not endured long enough; all visible changes in the stars requiring an immense time, according to the gigantic scale of celestial mechanism.

But even this hypothesis (it is no more) tends to establish the extreme tenuity of the nebulae; for more solid systems, like our solar system, require, for the preservation of their form, motions which are perceptible, and indeed conspicuous, in the course of a month; namely, the motions of the planets. All, therefore, concurs to prove the extreme tenuity of the substance of irregular nebulae.

34. Nebulae which a.s.sume a regular, for instance, a circular or oval shape, with whatever variation of luminous density from the inner to the outer parts, may have a form of equilibrium, if their parts have a proper gyratory motion. Still, we see no reason for supposing that these differ so much from irregular nebulae, as to be denser bodies, kept in their forms by rapid motions. We are rather led to believe that, though perhaps denser than the spiral nebulae, they are still of extremely thin and vaporous character. It would seem very unlikely that these vast clouds of luminous vapor should be as dense as the tail of a comet; since a portion of luminous matter so small as such a tail is, must have cooled down from its most luminous condition; and must require to be more dense than nebular matter in order to be visible at all by its own light.

35. Thus we appear to have good reason to believe that nebulae are vast ma.s.ses of incoherent or gaseous matter, of immense tenuity, diffused in forms more or less irregular, but all of them dest.i.tute of any regular system of solid moving bodies. We seem, therefore, to have made it certain that _these_ celestial objects at least are not inhabited. No speculators have been bold enough to place inhabitants in a comet; except, indeed, some persons who have imagined that such a habitation, carrying its inmates alternately into the close vicinity of the sun's surface, and far beyond the orbit of Ura.n.u.s, and thus exposing them to the fierce extremes of heat and cold, might be the seat of penal inflictions on those who had deserved punishment by acts done in their life on one of the planets. But even to give coherence to this wild imagination, we must further suppose that the tenants of such prison-houses, though still sensible to human suffering from extreme heat and cold, have bodies of the same vaporous and unsubstantial character as the vehicle in which they are thus carried about the system; for no frame of solid structure could be sustained by the incoherent and varying volume of a comet. And probably, to people the nebulae with such thin and fiery forms, is a mode of providing them with population, that the most ardent advocates of the plurality of worlds are not prepared to adopt.

36. So far then as the Nebulae are concerned, the improbability of their being inhabited, appears to mount to the highest point that can be conceived. We may, by the indulgence of fancy, people the summer-clouds, or the beams of the aurora borealis, with living beings, of the same kind of substance as those bright appearances themselves; and in doing so, we are not making any bolder a.s.sumption than we are, when we stock the Nebulae with inhabitants, and call them in that sense, "distant worlds."

FOOTNOTES:

[1] Herschel, _Outl. of Astr._ Art. 893.

[2] Herschel, _Outl. of Astr._ Art. 874, and Plate 11, Fig. 3.

[3] Ibid. Art. 897.

[4] Hersch. 874.

[5] Ibid. 881-8.

[6] At the recent meeting of the British a.s.sociation (Sept. 1853), drawings were exhibited of the same nebulae, as seen through Lord Rosse's large telescope, and through a telescope of three feet aperture. With the smaller telescopic power, all the characteristic features were lost.

The spiral structure (see next Article but one) has been almost entirely brought to light by the large telescope.

[7] See monthly Notices of the Royal Astronomical Society, Dec. 13, 1850.

[8] The frontispiece to this volume represents two of these Spiral Nebulae; those denominated 51 Messier, and 99 Messier, as given by Lord Rosse in the _Phil. Trans. for 1850_. The former of these two has a lateral focus, besides the princ.i.p.al focus or pole.

[9] I am aware that some astronomers do not consider it as proved that cometary matter is entirely self-luminous. Arago found that the light of a Comet contained a portion of polarized light, thus proving that it had been reflected (_Cosmos_, I. p. 111, and III. p. 566). But I think the opinion that the greater part of the light is self-luminous, like the nebulae, generally prevails. Any other supposition is scarcely consistent with the rapid changes of brightness which occur in a comet during its motion to and from the Sun.

[10] We a.s.sume here that the number of revolutions to the centre is greater in proportion as the relative density of the resisting medium is less; which is by no means mechanically true; but the calculation may serve, as we have said, to show the scale of the numbers involved.

[11] Humboldt, whom nothing relative to the history of science escapes, quotes from Seneca a pa.s.sage in which mention is made of a Comet which divided into two parts; and from the Chinese Annals, a notice of three "coupled Comets," which in the year 896 appeared, and described their paths together. _Cosmos_, III. p. 570, and the notes.

[12] Laplace has proved that the ma.s.ses of comets are very small. He reckons their mean ma.s.s as very much less than 1-100000th of the Earth's ma.s.s. And hence, considering their great size, we see how rare they must be. See _Expos. du Syst. du Monde_.

[13] Humboldt repeatedly expresses his conviction that our Solar System contains a greater variety of forms than other systems. (_Cosmos_, III.

373 and 587.)

CHAPTER VIII.

THE FIXED STARS.

1. We appear, in the last chapter, to have cleared away the supposed inhabitants of the outskirts of creation, so far as the Nebulae are the outskirts of creation. We must now approach a little nearer, in appearance at least, to our own system. We must consider the Fixed Stars; and examine any evidence which we may be able to discover, as to the probability of their containing, in themselves or in accompanying bodies, as planets, inhabitants of any kind. Any special evidence which we can discern on this subject, either way, is indeed slight. On the one side we have the a.s.serted a.n.a.logy of the parts of the universe; of which point we have spoken, and may have more to say hereafter. Each Fixed Star is conceived to be of the nature of our Sun; and therefore, like him, the centre of a planetary system. On the other side, it is extremely difficult to find any special facts relative to the nature of the fixed stars, which may enable us in any degree to judge how far they really are of a like nature with the Sun, and how far this resemblance goes. We may, however, notice a few features in the starry heavens, with which, in the absence of any stronger grounds, we may be allowed to connect our speculations on such questions. The a.s.siduous scrutiny of the stars which has been pursued by the most eminent astronomers, and the reflections which their researches have suggested to them, may have a new interest, when discussed under this point of view.

2. Next after the Nebulae, the cases which may most naturally engage our attention, are Cl.u.s.ters of stars. The cases, indeed, in which these cl.u.s.ters are the closest, and the stars the smallest, and in which, therefore, it is only by the aid of a good telescope that they are resolved into stars, do not differ from the resolvable nebulae, except in the degree of optical power which is required to resolve them. We may, therefore, it would seem, apply to such cl.u.s.ters, what we have said of resolvable nebulae: that when they are thus, by the application of telescopic power, resolved into bright points, it seems to be a very bold a.s.sumption to a.s.sume, without further proof, that these bright points are suns, distant from each other as far as we are from the nearest stars. The boldness of such an a.s.sumption appears to be felt by our wisest astronomers.[1] That several of the cl.u.s.ters which are visible, some of them appearing as if the component stars were gathered together in a nearly spherical form, are systems bound together by some special force, or some common origin, we may regard, with those astronomers, as in the highest degree probable. With respect to the stability of the form of such a system, a curious remark has been made by Sir John Herschel,[2] that if we suppose a globular s.p.a.ce filled with equal stars, uniformly dispersed through it, the particular stars might go on forever, describing ellipses about the centre of the globe, in all directions, and of all sizes; and all completing their revolutions in the same time. This follows, because, as Newton has shown, in such a case, the compound force which tends to the centre of the sphere would be everywhere proportional to the distance from the centre; and under the action of such a force, ellipses about the centre would be described, all the periods being of the same amount. This kind of symmetrical and simple systematic motion, presented by Newton as a mere exemplification of the results of his mechanical principles, is perhaps realized, approximately at least, in some of the globular cl.u.s.ters. The motions will be swift or slow, according to the total ma.s.s of the groups. If, for instance, our Sun were thus broken into fragments, so as to fill the sphere girdled by the earth's...o...b..t, all the fragments would revolve round the centre in a year. Now, there is no symptom, in any cl.u.s.ter, of its parts moving nearly so fast as this; and therefore we have, it would seem, evidence that the groups are much less dense than would be the s.p.a.ce so filled with fragments of the sun. The slowness of the motions, in this case, as in the nebulae, is evidence of the weakness of the forces, and therefore, of the rarity of the ma.s.s; and till we have some gyratory motion discovered in these groups, we have nothing to limit our supposition of the extreme tenuity of their total substance.

3. Let us then go on to the cases in which we have proof of such gyratory motions in the stars; for such are not wanting. Fifty years ago, Herschel the father, had already ascertained that there are certain pairs of stars, very near each other (so near, indeed, that to the una.s.sisted eye they are seen as single stars only,) and which revolve about each other. These Binary Sidereal Systems have since been examined with immense diligence and profound skill by Herschel the son, and others; and the number of such binary systems has been found, by such observers, to be very considerable. The periods of their revolutions are of various lengths, from 30 or 40 years to several hundreds of years.

Some of those pairs which have the shortest periods, have already, since the nature of their movements was discovered, performed more than a complete revolution;[3] thus leaving no room for doubting that their motions are really of this gyratory kind. Not only the fact, but the law of this...o...b..tal motion, has been investigated; and the investigations, which naturally were commenced on the hypothesis that these distant bodies were governed by that Law of universal Gravitation, which prevails throughout the solar system, and so completely explains the minutest features of its motions, have ended in establis.h.i.+ng the reality of that Law, for several Binary Systems, with as complete evidence as that which carries its operations to the orbits of Ura.n.u.s and Neptune.

4. Being able thus to discern, in distant regions of the universe, bodies revolving about each other, we have the means of determining, as we do in our own solar system, the ma.s.ses of the bodies so revolving.

But for this purpose, we must know their distance from each other; which is, to our vision, exceedingly small, requiring, as we have said, high magnifying powers to make it visible at all. And again, to know what linear distance this small visible distance represents, we must know the distance of the stars from us, which is, for every star, as we know, immensely great; and for most, we are dest.i.tute of all means of determining how great it is. There are, however, some of these binary systems, in which astronomers conceive that they have sufficiently ascertained the value of both these elements, (the distance of the two stars from each other, and from us,) to enable them to proceed with the calculation of which I have spoken; the determination of the ma.s.ses of the revolving bodies. In the case of the star _Alpha Centauri_, the first star in the constellation of the Centaur, the period is reckoned to be 77 years; and as, by the same calculator, the apparent semi-axis of the orbit described is stated at 15 seconds of s.p.a.ce, while the annual parallax of each star is about one second, it is evident that the orbit must have a radius about 15 times the radius of the earth's...o...b..t; that is, an orbit greater than that of Saturn, and approaching to that of Ura.n.u.s. In the solar system, a revolution in such an orbit would occupy a time greater than that of Saturn, which is 30 years, and less than that of Ura.n.u.s, which is about 80 years: it would, in fact, be about 58 years. And since, in the binary star, the period is greater than this, namely 77 years, the attraction which holds together its two elements must be less than that which holds together the Sun and a planet at the same distance; and therefore the ma.s.ses of the two stars together are considerably less than the ma.s.s of our sun.

5. A like conclusion is derived from another of these conspicuous double stars, namely, the one termed by astronomers _61 Cygni_; of which the annual parallax has lately been ascertained to be one-third of a second of s.p.a.ce, while the distance of the two stars is 15 seconds. Here therefore we have an orbit 45 times the size of the Earth's...o...b..t; larger than that of the newly-discovered planet Neptune, whose orbit is 30 times as large as the earth's, and his period nearly 165 years. The period of 61 Cygni is however, it appears, probably not short of 500 years; and hence it is calculated that the sum of the ma.s.ses of the two stars which make up this pair is about one-third of the ma.s.s of our Sun.[4]

6. These results give some countenance to the opinion, that the quant.i.ty of luminous matter, in other systems, does not differ very considerably from the ma.s.s of our Sun. It differs in these cases as 1 to 3, or thereabouts. In what degree of condensation, however, the matter of these binary systems is, compared with that of our solar system, we have no means whatever of knowing. Each of the two stars may have its luminous matter diffused through a globe as large as the earth's...o...b..t; and in that case, would probably not be more dense than the tail of a comet.[5] It is observed by astronomers, that in the pairs of binary stars which we have mentioned, the two stars of each pair are of different colors; the stars being of a high yellow, approaching to orange color,[6] but the smaller individual being in each case of a deeper tint. This might suggest to us the conjecture that the smaller ma.s.s had cooled further below the point of high luminosity than the larger; but that both these degrees of light belong to a condition still progressive, and probably still gaseous. Without attaching any great value to such conjectures, they appear to be at least as well authorized as the supposition that each of these stars, thus different, is nevertheless precisely in the condition of our sun.

7. But, even granting that each of the individuals of this pair were a sun like ours, in the nature of its material and its state of condensation, is it probable that it resembles our Sun also in having planets revolving about it? A system of planets revolving around or among a pair of suns, which are, at the same time, revolving about one another, is so complex a scheme, so impossible to arrange in a stable manner, that the a.s.sumption of the existence of such schemes, without a vestige of evidence, can hardly require confutation. No doubt, if we were really required to provide such a binary system of suns with attendant planets, this would be best done by putting the planets so near to one sun, that they should not be sensibly affected by the other; and this is accordingly what has been proposed.[7] For, as has been well said of the supposed planets, in making this proposal, "Unless closely nestled under the protecting wing of their immediate superior, the sweep of the other sun in his perihelion pa.s.sage round their own, might carry them off, or whirl them into orbits utterly inconsistent with the existence of their inhabitants." To a.s.sume the existence of the inhabitants, in spite of such dangers, and to provide against the dangers by placing them so close to one sun as to be out of the reach of the other, though the whole distance of the two may not, and as we have seen, in some cases does not, exceed the dimensions of our solar system, is showing them all the favor which is possible. But in making this provision, it is overlooked that it may not be possible to keep them in permanent orbits so near to the selected centre: their sun may be a vast sphere of luminous vapor; and the planets, plunged into this atmosphere, may, instead of describing regular orbits, plough their way in spiral paths through the nebulous abyss to its central nucleus.

8. Cl.u.s.tered stars, then, and double stars, appear to give us but little promise of inhabitants. We must next turn our attention to the single stars, as the most hopeful cases. Indeed, it is certain that no one would have thought of regarding the individual stars of cl.u.s.ters, or of pairs, as the centres of planetary systems, if the view of insulated stars, as the centres of such systems, had not already become familiar, and, we may say, established. What, then, is the probability of that view? Is there good evidence that the Fixed Stars, or some of them, really have planets revolving round them? What is the kind of proof which we have of this?

9. To this we must reply, that the only proof that the fixed stars are the centres of planetary systems, resides in the a.s.sumption that those stars are _like the Sun_;--resemble him in their qualities and nature, and therefore, it is inferred, must have the same offices, and the same appendages. They are, as the Sun is, independent sources of light, and thence, probably, of heat; and therefore they must have attendant planets, to which they can impart their light and heat; and these planets must have inhabitants, who live under and enjoy those influences. This is, probably, the kind of reasoning on which those rely, who regard the fixed stars as so many worlds, or centres of families of worlds.

10. Everything in this argument, therefore, depends upon this: that the Stars are _like the Sun_; and we must consider, what evidence we have of the exactness of this likeness.

11. The Stars are like the Sun in this, that they s.h.i.+ne with an independent light, not with a borrowed light, as the planets s.h.i.+ne. In this, however, the stars resemble, not only the Sun, but the nebulous patches in the sky, and the tails of comets; for these also, in all probability, s.h.i.+ne with an original light. Probably it will hardly be urged that we see, by the very appearance of the stars, that they are of the nature of the Sun: for the appearance of luminaries in the sky is so far from enabling us to discriminate the nature of their light, that to a common eye, a planet and a fixed star appear alike as stars. There is no obvious distinction between the original light of the stars and the reflected light of the planets. The stars, then, being like the sun in being luminous, does it follow that they are, like the sun, definite dense ma.s.ses?[8] Or are they, or many of them, luminous ma.s.ses in a far more diffused state; visually contracted to points, by the immense distance from us at which they are?