The Plurality of Worlds

28. Nor does this result (that the bright element of the nebulæ 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 masses, 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 nebulæ, and the circumstances, 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 masses;[18 - Humboldt, whom nothing relative to the history of science escapes, quotes from Seneca a passage 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.] 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 retardation 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.[19 - Laplace has proved that the masses of comets are very small. He reckons their mean mass as very much less than 1-100000th of the Earth's mass. And hence, considering their great size, we see how rare they must be. See Expos. du Syst. du Monde.] 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 masses. In the Nebulæ, we have loose matter of a thin and vaporous constitution, differing as more or less rare, more or less luminous, in a small degree; diffused over enormous spaces, 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.[20 - Humboldt repeatedly expresses his conviction that our Solar System contains a greater variety of forms than other systems. (Cosmos, iii. 373 and 587.)] The spiral nebulæ, 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 masses, 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 nebulæ, which spread out patches and paths of various degrees of brightness; and shoot out, into surrounding space, 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 nebulæ, 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 nebulæ; 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 nebulæ.

34. Nebulæ which assume 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 nebulæ, 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 nebulæ, 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 nebulæ are vast masses of incoherent or gaseous matter, of immense tenuity, diffused in forms more or less irregular, but all of them destitute 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 Uranus, 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 nebulæ 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 Nebulæ 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 assumption than we are, when we stock the Nebulæ with inhabitants, and call them in that sense, "distant worlds."

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 Nebulæ 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 asserted analogy 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 assiduous 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 Nebulæ, the cases which may most naturally engage our attention, are Clusters of stars. The cases, indeed, in which these clusters 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 nebulæ, except in the degree of optical power which is required to resolve them. We may, therefore, it would seem, apply to such clusters, what we have said of resolvable nebulæ: that when they are thus, by the application of telescopic power, resolved into bright points, it seems to be a very bold assumption to assume, 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 assumption appears to be felt by our wisest astronomers.[21 - Herschel, 866.] That several of the clusters 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,[22 - Ibid. 866.] that if we suppose a globular space 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 clusters. The motions will be swift or slow, according to the total mass of the groups. If, for instance, our Sun were thus broken into fragments, so as to fill the sphere girdled by the earth's orbit, all the fragments would revolve round the centre in a year. Now, there is no symptom, in any cluster, 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 space so filled with fragments of the sun. The slowness of the motions, in this case, as in the nebulæ, is evidence of the weakness of the forces, and therefore, of the rarity of the mass; 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 unassisted 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;[23 - Herschel, 846.] thus leaving no room for doubting that their motions are really of this gyratory kind. Not only the fact, but the law of this orbital 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 establishing the reality of that Law, for several Binary Systems, with as complete evidence as that which carries its operations to the orbits of Uranus 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 masses 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 destitute 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 masses 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 space, 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 orbit; that is, an orbit greater than that of Saturn, and approaching to that of Uranus. 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 Uranus, 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 masses of the two stars together are considerably less than the mass 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 space, while the distance of the two stars is 15 seconds. Here therefore we have an orbit 45 times the size of the Earth's orbit; 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 masses of the two stars which make up this pair is about one-third of the mass of our Sun.[24 - Herschel, 848.]

6. These results give some countenance to the opinion, that the quantity of luminous matter, in other systems, does not differ very considerably from the mass 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 orbit; and in that case, would probably not be more dense than the tail of a comet.[25 - That these systems have not condensed to one centre, appears to imply a less complete degree of condensation than exists in those systems which have done so.] 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,[26 - Herschel, 850.] but the smaller individual being in each case of a deeper tint. This might suggest to us the conjecture that the smaller mass 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 assumption 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.[27 - Herschel, 847.] 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 passage round their own, might carry them off, or whirl them into orbits utterly inconsistent with the existence of their inhabitants." To assume 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. Clustered 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 clusters, 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 assumption 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 shine with an independent light, not with a borrowed light, as the planets shine. 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, shine 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 masses?[28 - The density of the sun is about as great as the density of water.] Or are they, or many of them, luminous masses in a far more diffused state; visually contracted to points, by the immense distance from us at which they are?

12. We have seen that some of those stars, which we have the best means of examining, are, in mass, one third, or less, of our Sun. If such a mass, at the distance of the fixed stars, were diffused through a sphere equal in radius to the earth's orbit, it would still appear to us as a point; as is evident by this, that the fixed stars, for the most part, have no discoverable annual parallax; that is, the earth's orbit appears to them a point. If one of the fixed stars, Sirius, for instance, be in this diffused condition, such a circumstance will not, mechanically speaking, prevent his having planets revolving round him; for, as we have said, the attraction of his whole mass, in whatever state of spherical diffusion, will be the same as if it were collected at the centre. But such a state of diffusion will make him so unlike our Sun, as much to break the force of the presumption that he must have planets because our Sun has. If the luminous matter of the stars gradually cools, grows dark, and solidifies, such diffusion would imply that the time of solidification is not yet begun; and therefore that the solid planets which accompany the luminous central body are not yet brought into being. If there be any truth in this hypothetical account of the changes, through which the matter of the stars successively passes; and if, by such changes, planetary systems are formed; how many of the fixed stars may never yet have reached the planetary state! how many, for want of some necessary mechanical condition, may never give rise to permanent orbits at all!

13. And that the matter of the stars does go through changes, we have evidence, in many such changes which have actually been observed;[29 - Herschel, 827-832.] and perhaps in the different colors of different stars; which may, not improbably, arise from their being at different stages of their progress. That planetary systems, once formed, go through mighty changes, we have evidence in the view which geology gives us of the history of this earth; and in that view, we see also, how unique, and how far elevated in its purpose, the last period of this history may be, compared with the preceding periods; and, up to the present time at least, how comparatively brief in its duration. If, therefore, stellar globes can become planetary systems in the progress of ages, it will not be at all inconsistent with what we know of the order of nature, that only a few, or even that only one, should have yet reached that condition. All the others, but the one, may be systems yet unformed, or fragments struck off in the forming of the one. If any one is not satisfied with this account of the degree of resemblance between the fixed stars and the sun, but would make the likeness greater than this; we have only to say, that the proof that it is so lies upon him. Such a resemblance as we have supposed, is all that the facts suggest. That the stars are independent luminaries, we see; but whether they are as dense as the sun, or globes a hundred or a thousand times as rare, we have no means whatever of knowing. And, to assume that besides these luminous bodies which we see, there are dark bodies which we do not see, revolving round the others in permanent orbits, which require special mechanical conditions; and to suppose this, in order that we may build upon this assumption a still larger one, that of living inhabitants of these dark bodies; is a hypothetical procedure, which it seems strange that we should have to combat, at the present stage of the history of science, and in dealing with those whose minds have been disciplined by the previous events in the progress of astronomy.

14. Let us consider, however, further, how far astronomy authorizes us to regard the Fixed Stars as being, like our Sun, the centres of systems of Planets. Those who hold this, consider them as having a permanent condition of brightness, as our Sun has had for an indefinite period, so far as we have any knowledge on the subject. Yet, as we have said, no small number of the stars undergo changes of brightness; and some of them undergo such changes, in a manner which is not discernibly periodical; and which must therefore be regarded as progressive. This phenomenon countenances the opinion of such a progress from one material condition to another; which, we have seen, is suggested by the analogy of the probable formation of our own solar system. The very star which is so often taken as the probable centre of a system, Sirius, has, in the course of the last 2,000 years, changed its light from red to white. Ptolemy notes it as a red star: in Tycho's time it was already, as it is now, a white one.[30 - Cosmos, iii. 169, 205, and 641.] The star Eta Argus changes both its degree of light and its color; ranging, in seemingly irregular intervals of time, from the fourth to the first magnitude,[31 - Ibid., iii. 172 and 252.] and from yellow to red. Several other examples of the like kind have been observed. Mr. Hind[32 - Astron. Soc. Notices, Dec. 13, 1850.] gives an example in which he has, quite recently, observed in two years a star change its color from very red to bluish. These variable unperiodical stars are probably very numerous. Also, some stars, observed of old, are now become invisible. "The lost Pleiad," by the loss of which the cluster, called the Seven Stars, offers now only six to the naked eye, is an example of a change of this kind already noted in ancient times. There are several others, of which the extinction is recognized by astronomers as proved.[33 - See Grant's Hist. of Physical Astronomy, p. 538.] In other cases, new stars have appeared, and have then seemed to die away and vanish. The appearance of a new star in the time of the Greek astronomer Hipparchus, induced him to construct his famous Catalogue of the Stars. Others are recorded to have appeared in the middle ages. The first which was observed by modern astronomers was the celebrated star seen by Tycho Brahe in 1572. It appeared suddenly in the constellation Cassiopeia, was fixed in its place like the neighboring stars, had no nebula or tail, exceeded in splendor all other stars, being as bright as Venus when she is nearest the earth. It soon began to diminish in brightness, and passing through various diminishing degrees of magnitude, vanished altogether after seventeen months. This star also passed through various colors; being first white, then yellow, then red. In like manner, in 1604, a new star of great magnitude blazed forth in the constellation Serpentarius; and was seen by Kepler. And this also, like that of 1572, after a few months, declined and vanished.

15. These appearances led Tycho to frame an hypothesis like that which Sir William Herschel afterwards proposed, that the stars are formed by the condensation of luminous nebulous matter. Nor is it easy to think of such phenomena (of which several others have been observed, though none so conspicuous as these), without regarding them as showing that the matter of the fixed stars, occasionally at least, passes through changes of consistence as great as would be the condensation and extinction of a luminous vapor. And if such changes have been but few within the recorded period of man's observation of the stars, we must recollect how small that period is, compared with the period during which the stars have existed. The stars themselves give us testimony of their having been in being for millions of years. For according to the best estimates we can form of their distances, the time which light would employ in reaching us from the most remote of them, would be millions of years; and, therefore, we now see those remote stars by means of the light emitted from them millions of years ago. And if, in the 2,000 years during which such observations are recorded, only 200 stars have undergone such changes in a degree visible to the earth's inhabitants; in a million of years, change going on at the same rate, 100,000 stars would exhibit visible progressive change, showing that they had not yet reached a permanent condition. And how much of change may go on in any star without its being in any degree perceptible to the most exact astronomical scrutiny!

16. The tendency of these considerations is, to lead us to think that the fixed stars are not generally in that permanent condition in which our sun is; and which appears to be alone consistent with the existence of a system such as the solar system.[34 - I am aware of certain speculations, and especially of some recent ones, tending to show that even our Sun is wasting away by the emission of light and heat; but these opinions, even if established, do not much affect our argument one way or the other.] These views, therefore, fall in with that which we have been led to by this consideration of the Nebulæ: that the Solar System is in a more complete and advanced state, as a system, than many at least of the stellar systems can be; it may be, than any other.

17. It has been alleged, as a proof of the likeness of the Fixed Stars to our Sun, that like him, they revolve upon their axes.[35 - Chalmers' Astron. Disc. p. 39.] This has been supposed to be proved with regard to many of them, by their having periodical recurrences of fainter and brighter lustre; as if they were revolving orbs, with one side darkened by spots. Such facts are not very numerous or definite in the heavens. Omicron[36 - Hersch. 820.] in the constellation Cetus, is the longest known of them; and is held to revolve in 831 days. From the curious phenomena now spoken of, it has been called Mira Ceti.[37 - The periodical character of this star was discovered by David Fabricius, a parish priest in East Friesland, the father of John Fabricius, who discovered the solar spots. (Cosmos, iii. 234.)]Algol, the second star (Beta) of Perseus, called also Caput Medusæ, is another, with a period of 2 days 21 hours; and in this case, the obscuration of the light, and the restoration of it, are so sudden, that from the time when it was first remarked, (by Goodricke, in 1782,) it suggested the hypothesis of an opaque body revolving round the star. The star Delta, in the constellation Cephus, is another, with a period of 5 days 9 hours. The star Beta in the Lyre, has a period of 6 days 10 hours, or perhaps 12 days 21 hours, one revolution having been taken for two. Another such star is Eta Aquilæ, with a period of 7 days 4 hours. These five are all the periodical stars of which astronomers can speak with precision.[38 - Hersch. 825. In Humboldt's Cosmos, iii. 243, Argelander, who has most carefully observed and studied these periodical stars, has given a catalogue containing 24, with the most recent determinations of their periods.] But about thirty more are supposed to be subject to such change, though their periods, epochs, and phases of brightness, cannot at present be given exactly.

18. That these periodical changes in certain of the fixed stars are a curious and interesting astronomical fact, is indisputable. Nothing can be more probable also, than that it indicates, in the stellar masses, a revolution on their axes; which cannot surprise us, seeing that revolution upon an axis is, so far as we know, a universal law of all the large compact masses of matter which exist in the universe; and may be conceived to be a result derived from their origin, and a condition of any permanent or nearly permanent figure. But this can prove little or nothing as to their being like the sun, in any way which implies their having inhabitants, in themselves or in accompanying planets. The rotation of our Sun is not, in any intelligible way, connected with its having near it the inhabited Earth.

19. If we were to suppose some of the stars to be centres of planetary systems, we can hardly suppose it likely that these alone rotate, and that the others stand still. Probably all the stars rotate, more or less regularly, according as they are permanent or variable in form; but the most regular may still have no planets; and if they have, those planets may be as blank of inhabitants as our moon will be proved to be.

20. The revolution of Algol seems to approach the nearest to a fact in favor of a star being the centre of a revolving system; and from the first, as we have said, the periodical change, and the sudden darkening and brightening of this luminary, suggested the supposition of an opaque body revolving about it. But this body cannot be a planet. The planets which revolve about our Sun are not, any of them, nor all of them together, large enough to produce a perceptible obscuration of his light, to a spectator outside the system. But in Algol, the phenomena are very different from this.[39 - Hersch. 821. Humboldt (Cosmos, iii. 238 and 246,) gives the period as 68 hours 49 minutes, and says that it is 7 or 8 hours in its less bright state. If we could suppose the times of the warning, and of the greatest eclipse, given by Herschel, to be exactly determined, as 3

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and

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, that is, in the proportion of 14 to 1, the darkening body must have its effective breadth

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of that of the star. But this is on the supposition that the orbit of the darkening body has the spectator's eye in its plane; if this be not so, the darkening body may be much larger.] The star is usually visible as a star of the second magnitude; but during each period of 2 days 21 hours, (or 69 hours,) it suffers a kind of eclipse, which reduces it to a star of the fourth magnitude. During this eclipse, the star diminishes in splendor for 3

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hours; is at its lowest brightness for a quarter of an hour; and then, in 3

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hours more, is restored to its original splendor. According to these numbers, if the obscuration be produced by a dark body revolving round a central luminary, and describing a circular orbit, as the regular recurrence of the obscuration implies, the space of the orbit during which the eclipsing body is interposed must be about one-ninth of the circumference; for the obscuration occupies 7

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hours out of 69. And therefore the space during which the eclipsing body obscures the central one, must be about one sixth of the diameter of its orbit. But in order that the revolving body may, through this space, obscure the central one, the latter must extend over this space, namely, one sixth of the diameter of the orbit. But we may remark that there is no proof, in the phenomena, that the darkening body is detached from the bright mass. The effect would be the same if the dark mass were a part of the revolving star itself. It may be that the star has not yet assumed a spherical form, but is an oblong nebular mass with one part (perhaps from being thinner in texture) cooled down and become opaque. And the amount of obscuration, reducing the star from the second to the fourth magnitude, implies that the obscuring mass is large (perhaps one half the diameter, or much more) compared with the luminous mass. If this be a probable hypothesis to account for the phenomena, they are much more against than for the supposition of the star being the centre of seats of habitation. And even if we have a planet nearly as large as its sun, revolving at the distance of only six of the sun's radii, how unlike is this to the solar system!

21. In fact, all these periodical stars, in so far as they are periodical, are proved, not to be like, but to be unlike our sun. It is true that the sun has spots, by means of which his rotation has been determined by astronomers. But these spots, besides being so small that they produce no perceptible alteration in his brightness, and are never, or very rarely, visible to the naked eye, are not permanent. A star with a permanent dark side would be very unlike our sun. The largest known of these stars, Mira, as the old astronomers called it, becomes invisible to the naked eye for 5 months during a period of 11 months. It must, therefore, have nearly one half its surface quite dark. This is very unlike the condition of the sun; and is a condition, it would seem, very little fitted to make this star the centre of a planetary system like ours.

22. But there are other remarkable phenomena respecting these periodical stars, which have a bearing on our subject. Their periods are not quite regular, but are subject to certain variations. Thus it has been supposed that the period of Mira is subject to a cyclical fluctuation, embracing 88 of its periods; that is, about 80 years. But this notion of a cycle of so long a duration, requires confirmation; the fact of fluctuation in the period is alone certain. In like manner, Algol's periods are not quite uniform. All these facts agree with our suggestion, that the periodical stars are bodies of luminous matter which have not yet assumed a permanent form; and which, therefore, as they revolve about their axes, and turn to us their darker and their brighter parts, do so at intervals, and in an order somewhat variable. And this suggestion appears to be remarkably confirmed, by a result which recent observations have discovered relative to this star, Algol; namely, that its periods become shorter and shorter. For if the luminous matter, which is thus revolving, be gradually gathering into a more condensed form;—becoming less rare, or more compact; as, for instance, it would do, if it were collecting itself from an irregular, or elongated, into a more spherical form; such a shortening of the period of revolution would take place; for a mass which contracts while it is revolving, accelerates its rate of revolution, by mechanical principles. And thus we do appear to have, in this observed acceleration of the periods of Algol, an evidence that that luminous mass has not yet reached its final and permanent condition.

23. It is true, it has been conjectured, by high authority,[40 - Hersch. Outl. Astr. 821. Another explanation of the variable period of Algol, is that the star is moving towards us, and therefore the light occupies less and less time to reach us.] that this accelerated rapidity of the periods of Algol will not continue; but will gradually relax, and then be changed to an increase; like many other cyclical combinations in astronomy. But this conjecture seems to have little to support it. The cases in which an acceleration of motion is retarded, checked, and restored, all belong to our Solar System; and to assume that Algol, like the solar system, has assumed a permanent and balanced condition, is to take for granted precisely the point in question. We know of no such cycles among the fixed stars, at least with any certainty; for the cycle proposed for Mira must be considered as greatly needing confirmation; considering how long is the cycle, and how recent the suggestion of its existence.

24. And even in the solar system, we have accelerated motions, in which no mathematician or astronomer looks for a check or regress of the acceleration. No one expects that Encke's comet will cease to be accelerated, and to revolve in periods continually shorter; though all the other motions hitherto observed in the system are cyclical. In the case of a fixed star, we have much less reason to look for such a cycle, than we have in Encke's comet. But further: with regard to the existence of such a cycle of faster and slower motion in the case of Algol, the most recent observed facts are strongly against it; for it has been observed by Argelander, that not only there is a diminution of the period, but that this diminution proceeds with accelerated rapidity; a course of events which, in no instance, in the whole of the cosmical movements, ends in a regression, retardation, and restoration of the former rate. We are led to believe, therefore, that this remarkable luminary will go on revolving faster and faster, till its extreme point of condensation is attained. And in the meantime, we have very strong reasons to believe that this mutable body is not, like the sun, a permanent centre of a permanent system; and that any argument drawn from its supposed likeness to the sun, in favor of the supposition that the regions which are near it are the seats of habitation, is quite baseless.

25. There are other phenomena of the Fixed Stars, and other conjectures of astronomers respecting them, which I need not notice, as they do not appear to have any bearing upon our subject. Such are the "proper motions" of the stars, and the explanation which has been suggested of some of them; that they arise from the stars revolving round other stars which are dark, and therefore invisible. Such again is the attempt to show that the Sun, carrying with it the whole Solar System, is in motion; and the further attempt to show the direction of this motion; and again, the hypothesis that the Sun itself revolves round some distant body in space. These minute inquiries and bold conjectures, as to the movements of the masses of matter which occupy the universe, do not throw any light on the question whether any part besides the earth is inhabited; any more than the investigation of the movements of the ocean, and of their laws, could prove or disprove the existence of marine plants and animals. They do not on that account cease to be important and interesting subjects of speculation; but they do not belong to our subject.

26. In Fontenelle's Dialogues on the Plurality of Worlds, a work which may be considered as having given this subject a place in popular literature, he illustrates his argument by a comparison, which it may be worth while to look at for a moment. The speaker who asserts that the moon, the planets, and the stars, are the seats of habitation, describes the person, who denies this, as resembling a citizen of Paris, who, seeing from the towers of Notre Dame the town of Saint Denis, (it being supposed that no communication between the two places had ever occurred,) denies that it is inhabited, because he cannot see the inhabitants. Of course the conclusion is easy, if we may thus take for granted that what he sees is a town. But we may modify this image, so as to represent our argument more fairly. Let it be supposed that we inhabit an island, from which innumerable other islands are visible; but the art of navigation being quite unknown, we are ignorant whether any of them are inhabited. In some of these islands, are seen masses more or less resembling churches; and some of our neighbors assert that these are churches; that churches must be surrounded by houses; and that houses must have inhabitants. Others hold that the seeming churches are only peculiar forms of rocks. In this state of the debate, everything depends upon the degree of resemblance to churches which the forms exhibit. But suppose that telescopes are invented, and employed with diligence upon the questionable shapes. In a long course of careful and skilful examination, no house is seen, and the rocks do not at all become more like churches, rather the contrary. So far, it would seem, the probability of inhabitants in the islands is lessened. But there are other reasons brought into view. Our island is a long extinct volcano, with a tranquil and fertile soil; but the other islands are apparently somewhat different. Some of them are active volcanoes, the volcanic operations covering, so far as we can discern, the whole island; others undergo changes, such as weather or earthquakes may produce; but in none of them can we discover such changes as show the hand of man. For these islands, it would seem the probability of inhabitants is further lessened. And so long as we have no better materials than these for forming a judgment, it would, surely, be accounted rash, to assert that the islands in general are inhabited; and unreasonable, to blame those who deny or doubt it. Nor would such blame be justified by adducing theological or à priori arguments; as, that the analogy of island with island makes the assumption allowable; or that it is inconsistent with the plan of the Creator of islands to leave them uninhabited. For we know that many islands are, or were long, uninhabited. And if ours were an island occupied by a numerous, well-governed, moral, and religious race, of which the history was known, and of which the relation to the Creator was connected with its history; the assumption of a history, more or less similar to ours, for the inhabitants of the other islands, whose existence was utterly unproved, would, probably, be generally deemed a fitter field for the romance-writer than for the philosopher. It could not, at best, rise above the region of vague conjecture.

27. Fontenelle, in the agreeable book just referred to, says, very truly, that the formula by which his view is urged on adversaries is, Pourquoi non? which he holds to be a powerful figure of logic. It is, however, a figure which has this peculiarity, that it may, in most cases, be used with equal force on either side. When we are asked Why the Moon, Mercury, Saturn, the system of Sirius, should not be inhabited by intelligent beings; we may ask, Why the earth in the ages previous to man might not be so inhabited? The answer would be, that we have proof how it was inhabited. And as to the fact in the other case, I shall shortly attempt to give proof that the Moon is certainly not, and Mercury and Saturn probably not inhabited. With regard to the Fixed Stars, it is more difficult to reason; because we have the means of knowing so little of their structure. But in this case also, we might easily ask on our side, Pourquoi non? Why should not the Solar System be the chief and most complete system in the universe, and the Earth the principal planet in that System? So far as we yet know, the Sun is the largest Sun among the stars; and we shall attempt to show, that the Earth is the largest solid opaque globe in the solar system. Some System must be the largest and most finished of all; why not ours? Some planet must be the largest planet; why not the Earth?