FOOTNOTES:

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 Association (Sept. 1853), drawings were exhibited of the same nebulæ, 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]Thefrontispieceto this volume represents two of these Spiral Nebulæ; those denominated 51 Messier, and 99 Messier, as given by Lord Rosse in thePhil. Trans. for 1850. The former of these two has a lateral focus, besides the principal 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, andiii. p. 566). But I think the opinion that the greater part of the light is self-luminous, like the nebulæ, 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 assume 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 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.[12]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. SeeExpos. 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.)

[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 Association (Sept. 1853), drawings were exhibited of the same nebulæ, 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]Thefrontispieceto this volume represents two of these Spiral Nebulæ; those denominated 51 Messier, and 99 Messier, as given by Lord Rosse in thePhil. Trans. for 1850. The former of these two has a lateral focus, besides the principal 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, andiii. p. 566). But I think the opinion that the greater part of the light is self-luminous, like the nebulæ, 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 assume 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 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.

[12]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. SeeExpos. 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.)

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 assiduousscrutiny 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.[1]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,[2]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 centreof 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 pairswhich 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 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 starAlpha Centauri, the first star in the constellation of the Centaur, theperiod 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 astronomers61 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.[4]

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 ofcondensation, 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.[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 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 attendantplanets, 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 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 arelike 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 arelike 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?[8]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 haveactually been observed;[9]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 havebeen 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.[10]The starEta Arguschanges both its degree of light and its color; ranging, in seemingly irregular intervals of time, from the fourth to the first magnitude,[11]and from yellow to red. Several other examples of the like kind have been observed. Mr. Hind[12]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 exampleof a change of this kind already noted in ancient times. There are several others, of which the extinction is recognized by astronomers as proved.[13]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 periodis, 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.[14]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.[15]This has been supposed to be proved with regardto 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[16]in the constellationCetus, is the longest known of them; and is held to revolve in 831 days. From the curious phenomena now spoken of, it has been calledMira Ceti.[17]Algol, the second star (Beta) ofPerseus, called alsoCaput 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 starDelta, in the constellationCephus, is another, with a period of 5 days 9 hours. The starBetain theLyre, has a period of 6 days 10 hours, or perhaps 12 days 21 hours, one revolution having been taken for two. Another such star isEta Aquilæ, with a period of 7 days 4 hours. These five are all the periodical stars of which astronomers can speak with precision.[18]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; whichcannot 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.[19]The staris 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 31/2hours; is at its lowest brightness for a quarter of an hour; and then, in 31/2hours 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 71/4hours out of 69. And therefore the space during which the eclipsing body obscures the central one, must be about onesixthof thediameterof 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 suppositionof 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 beunlikeour 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 appearsto 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,[20]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 acheck 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 inquiriesand 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'sDialogues 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 orà prioriarguments; 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, shouldnotbe 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 proofhowitwasinhabited. 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?

28. It should be recollected that there must be some system which is the most complete of all systems, some planet which is the largest of all planets. And if that largest planet, in the most complete system, be, after being for ages tenanted by irrational creatures, at last, and alone of all, occupied by a rational race, that race must necessarily have the power of asking such questions as these: Why they should be alone rational? Why their planet should be alone thus favored? If the case be ours, we may hope to be then able to answer these questions, when we can explain the most certain fact which theyinvolve; Why the Earth was occupied so long by irrational creatures, before the rational race was placed upon it? The mere power of asking such questions can prove or disprove nothing; for it is a power which must equally subsist, whether the human inhabitants of the earth be or be not the only rational population which the universe contains. If there be a race thus favored by the Creator, they must, at that stage of their knowledge in which man now is, be able to doubt, as man does, of the extent and greatness of the privilege which they enjoy.

29. The argument that the Fixed Stars are like the Sun, and therefore the centres of inhabited systems as the Sun is, is sometimes called an argument from Analogy; and this wordAnalogyis urged, as giving great force to the reasoning. But it must be recollected, that precisely the point in question is, whether thereisan analogy. The stars, it is said, are like the Sun. In what respects? We know of none, except in being self-luminous; and this they have in common with the nebulæ, which, as we have seen, are not centres of inhabited systems. Nor does this quality of being self-luminous at all determine the degree of condensation of a star. Sirius may be less than a hundredth or a thousandth of the density of the Sun. But the Stars, it may be further urged, are like the Sun in turning on their axes. To this we reply, that we know this only of those stars in which, the very phenomenon which proves their revolution, proves also that they are unlike the Sun, in having one side darker than the other. Add to which, their revolution is not connected with the existence of planets, still less of inhabitants of planets, in any intelligible manner. The resemblance, therefore, so far as it bears upon the question, is confined to one single point, in the highest degree ambiguous and inconclusive; and any argument drawn from this onepoint of resemblance, has little claim to be termed an argument from analogy.[21]

30. On a subject on which we know so little, it is difficult to present any view which deserves to be regarded as an analogy. We see, among the stars, nebulæ more or less condensed, which are possibly, in some cases, stages of a connected progress towards a definite star; and it may be, to a star with planets in permanent orbits. We see, in our planet, evidence of successive stages of a connected series of brute animals, preceded perhaps by various stages of lifeless chaos. If the histories of the Sun, and of all the stars, are governed by a common analogy, the nebulous condensation, and the stages of animal life, may be parts of the same continued series of events; and different stars may be at different points of that series. But even on this supposition, but a few of the stars may be the seats of conscious life, and none, of intelligence. For among the stars which have condensed to a permanent form, how many have failed in throwing off a permanent planet! How many may be in some stage of lifeless chaos! We must needs suppose a vast number of stages between a nebular chaos and the lowest forms of conscious life. Perhaps as many as there are fixed stars; and far more than there are of stars which become fertile of life: so that no two systems may be at the same stage of the planetary progress. And if this be so,—our system being so complicated, that we must supposeit peculiarly developed, having the largest Sun that we know of, and our Earth being (as we shall hereafter attempt to prove) the largest solid planet that we know of,—this Earth may be the sole seat of the highest stage of planetary development.

31. The assumption that there is anything of the nature of a regular law or order of progress from nebular matter to conscious life,—a law which extends to all the stars, or to many of them,—is in the highest degree precarious and unsupported; but since it is sometimes employed in such speculations as we are pursuing, we may make a remark or two connected with it. If we suppose, on the planets of other systems, a progress in some degree analogous to that which geology shows to have occurred on the Earth, there may be, in those planets, creatures in some way analogous to our vegetables and animals; but analogy also requires that they should differ far more from the terrestrial vegetables and animals of any epoch, than those of one epoch do from those of another; since they belong to a different stellar system, and probably exist under very different conditions from any that ever prevailed on the Earth. We are forbidden, therefore, by analogy, to suppose that on any other planet there was such an anatomical progression towards the form of man, as we can discern (according to some eminent physiologists) among the tribes which have occupied the Earth. Are we to conceive that the creatures on the planets of other systems are, like the most perfect terrestrial animals, symmetrical as to right and left, vertebrate, with fore limbs and hind limbs, heads, organs of sense in their heads, and the like? Every one can see how rash and fanciful it would be to make such suppositions. Those who have, in the play of their invention, imagined inhabitants of other planets, have tried to avoid this servile imitation of terrestrial forms. Here is Sir Humphry Davy's account of the inhabitants of Saturn. "I sawmoving on the surface below me, immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to that of the morse or sea-horse, but I saw with great surprise that they moved from place to place by six extremely thin membranes, which they used as wings. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant, than to anything else I can imagine, occupying what I supposed to be the upper parts of the body."[22]The attendant Genius informs the narrator, that though these creatures look like zoophytes, they have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of the Earth. If we were to reason upon a work of fancy like this, we might say, that it was just as easy to ascribe superior sensibility and intelligence to zoophyte-formed creatures upon the Earth, as in Saturn. Even fancy cannot aid us in giving consistent form to the inhabitants of other planets.

32. But even if we could assent to the opinion, as probable, that there may occur, on some other planet, progressions of organized forms analogous in some way to that series of animal forms which has appeared upon the earth, we should still have no ground to assume that this series must terminate in a rational and intelligent creature like man. For the introduction of reason and intelligence upon the Earth is no part nor consequence of the series of animal forms. It is a fact of an entirely new kind. The transition from brute to man does not come within the analogy of the transition from brute to brute. The thread of analogy, even if it could lead us so far, would break here. We may conceive analogues to other animals, but we could have no analogue to man, except man. Man is not merely a higher kind of animal; he is a creature of a superior order, participating in the attributes of a higher nature;as we have already said, and as we hope hereafter further to show. Even, therefore, if we were to assume the general analogy of the Stars and of the Sun, and were to join to that the information which geology gives us of the history of our own planet; though we might, on this precarious path, be led to think of other planets as peopled with unimagined monsters; we should still find a chasm in our reasoning, if we tried, in this way, to find intelligent and rational creatures in planets which may revolve round Sirius or Arcturus.

33. The reasonable view of the matter appears to be this. The assumption that the Fixed Stars are of exactly the same nature as the Sun, was, at the first, when their vast distance and probable great size were newly ascertained, a bold guess; to be confirmed or refuted by subsequent observations and discoveries. Any appearances, tending in any degree to confirm this guess, would have deserved the most considerate attention. But there has not been a vestige of any such confirmatory fact. No planet, nor anything which can fairly be regarded as indicating the existence of a planet, revolving about a star, has anywhere been discerned. The discovery of nebulæ, of binary systems, of clusters of stars, of periodical stars, of varying and accelerated periods of such stars, all seem to point the other way. And if all these facts be held to be but small in amount, as to the information which they convey, about the larger, and perhaps nearer stars; still they leave the original assumption a mere guess, unsupported by all that three centuries of most diligent, and in other respects successful research, has been able to bring to light. That Copernicus, that Galileo, that Kepler, should believe the stars to be Suns, in every sense of the term, was a natural result of the expansion of thought which their great discoveries produced, in them and in their contemporaries. Nor are we yetcalled upon to withdraw from them our sympathy; or entitled to contradict their conjecture. But all the knowledge that the succeeding times have given us; the extreme tenuity of much of the luminous matter in the skies; the existence of gyratory motion among the stars, quite different from planetary systems; the absence of any observed motions at all resembling such systems; the appearance of changes in stars, quite inconsistent with such permanent systems; the disclosure of the history of our own planet, as one in which changes have constantly been going on; the certainty that by far the greater part of the duration of its existence, it has been tenanted by creatures entirely different from those which give an interest, and thence, a persuasiveness, to the belief of inhabitants in worlds appended to each star; the impossibility, which appears, on the gravest consideration, of transferring to other worlds such interests as belong to our own race in this world; all these considerations should, it would seem, have prevented that old and arbitrary conjecture from growing up, among a generation professing philosophical caution, and scientific discipline, into a settled belief.

34. Some of the moral and theological views which tend to encourage and uphold this belief, may be taken under our more special consideration hereafter: but here, where we are reasoning principally upon astronomical grounds, we may conclude what we have to remark about the Fixed Stars, as the centres of inhabited systems of worlds, by saying; that it will be time enough to speculate about the inhabitants of the planets which belong to such systems, when we have ascertained that there are such planets, or one such planet. When that is done, we can then apply to them any reasons which may exist, for believing that all, or many planets, are the seats of habitation of living things. What reasons of this kind can be adduced,and what is their force with regard to our own solar system, we must now proceed to discuss.[23]

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