The Plurality of Worlds

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

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 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 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 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 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 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 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 and from yellow to red. Several other examples of the like kind have been observed. Mr. Hind32 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 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 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 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. Omicron36 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 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 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 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 31/2 hours; is at its lowest brightness for a quarter of an hour; and then, in 31/2 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 71/4 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 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.