Kitabı oku: «The Plurality of Worlds», sayfa 9

CHAPTER VII

THE NEBULÆ

1. I have attempted to show that, even if we suppose the other bodies of the universe to resemble the Earth, so far as to seem, by their materials, forms, and motions, no less fitted than she is to be the abodes of life; yet that, knowing what we do of man, we can believe that the Earth is tenanted by a race who are the special objects of God's care. Even if the tendency of the analogies of creation were, to incline us to suppose that the other planets are as well suited as our globe, to have inhabitants, still it would require a great amount of evidence, to make us believe that they have such inhabitants as we are; while yet such evidence is altogether wanting. Even if we knew that the stars were the centres of revolving systems, we should have an immense difficulty in believing that an Earth, with such a population as ours, revolves about any of them. If astronomy made a plurality of worlds probable, we have strong reasonings, drawn from other subjects, to think that the other worlds are not like ours.

2. The admirers of astronomical triumphs may perhaps be disposed to say, that when so much has been discovered, we may be allowed to complete the scheme by the exercise of fancy. I have attempted to show that we are not in such a state of ignorance, when we look at other relations of the earth and of man, as to allow us to do this. But now we may go a little onwards in our argument; and may ask, whether Astronomy really does what is here claimed for her:—whether she carries us so securely to the bounds of the visible universe, that our Fancy may take up the task, and people the space thus explored:—whether the bodies which Astronomy has examined, be really as fitted as our Earth, to sustain a population of living things:—whether the most distant objects in the universe do really seem to be systems, or the beginnings of systems:—whether Astronomy herself may not incline in favor of the condition of man, as being the sole creature of his kind?

3. In making this inquiry, it will of course be understood, that I do so with the highest admiration for the vast discoveries which Astronomy has really made; and for the marvellous skill and invention of the great men who have, in all ages of the world, and not least, in our time, been the authors of such discoveries. From the time when Galileo first discovered the system of Jupiter's satellites, to the last scrutiny of the structure of a nebula by Lord Rosse's gigantic telescope, the history of the telescopic exploration of the sky, has been a history of genius felicitously employed in revealing wonders. In this history, the noble labors of the first and the second Herschel relative to the distribution of the fixed stars, the forms and classes of nebulæ, and the phenomena of double stars, especially bear upon our present speculations; to which we may add, the examination of the aspect of each planet, by various observers, as Schroeter, and of the moon by others, from Huyghens to Mädler and Beer. The achievements which are most likely to occur to the reader's mind are those of the Earl of Rosse; as being the latest addition to our knowledge, and the result of the greatest instrumental powers. By the energy and ingenuity of that eminent person, an eye is directed to the heavens, having a pupil of six feet diameter, with the most complete optical structure, and the power of ranging about for its objects over a great extent of sky; and thus the quantity of light which the eye receives from any point of the heavens is augmented, it may be, fifty thousand times. The rising Moon is seen from the Observatory in Ireland with the same increase of size and light, as if her solid globe, two thousand miles in diameter, retaining all its illumination, really rested upon the summits of the Alps, to be gazed at by the naked eye. An object which appears to the naked eye a single star, may, by this telescope, so far as its power of seeing is concerned, be resolved into fifty thousand stars, each of the same brightness as the obvious star. What seems to the unassisted vision a nebula, a patch of diluted light, in which no distinct luminous point can be detected, may, by such an instrument, be discriminated or resolved into a number of bright dots; as the stippled shades of an engraving are resolved into dots by the application of a powerful magnifying glass. Similar results of the application of great telescopic power had of course been attained long previously; but, as the nature of scientific research is, each step adds something to our means of knowledge; and the last addition assumes, includes, and augments the knowledge which we possessed before. The discussions in which we are engaged, belong to the very boundary region of science;—to the frontier where knowledge, at least astronomical knowledge, ends, and ignorance begins. Such discoveries, therefore, as those made by Lord Rosse's telescope, require our special notice here.

4. We may begin, at what appears to us the outskirts of creation, the Nebulæ. At one time it was conceived by astronomers in general, that these patches of diffused light, which are seen by them in such profusion in the sky, are not luminous bodies of regular terms and definite boundaries, apparently solid, as the stars are supposed to be; but really, as even to good telescopes many of them seem, masses of luminous cloud or vapor, loosely held together, as clouds and vapors are, and not capable by any powers of vision of being resolved into distinct visible elements. This opinion was for a time so confidentially entertained, that there was founded upon it an hypothesis, that these were gaseous masses, out of which suns and systems might afterwards be formed, by the concentration of these luminous vapors into a solid central sun, more intensely luminous; while detached portions of the mass, flying off, and cooling down so as to be no longer self-luminous, might revolve round the central body, as planets and satellites. This is the Nebular Hypothesis, suggested by the elder Herschel, and adopted by the great mathematician Laplace.

5. But the result of the optical scrutiny of the nebulæ by more modern observers, especially by Lord Rosse in Ireland, and Mr. Bond in America, has been, that many celestial objects which were regarded before as truly nebulous, have been resolved into stars; and this resolution has been extended to so many cases of nebulæ, of such various kinds, as to have produced a strong suspicion in the minds of astronomers that all the nebulæ, however different in their appearance, may really be resolved into stars, if they be attacked with optical powers sufficiently great.

6. If this were to be assumed as done, and if each of the separate points, into which the nebulæ are thus resolved, were conceived to be a star, which looks so small only because it is so distant, and which really is as likely to have a system of planets revolving about it, as is a star of the first magnitude:—we should then have a view of the immensity of the visible universe, such as I presented to the reader in the beginning of this essay. All the distant nebulæ appear as nebulæ, only because they are so distant; if truly seen, they are groups of stars, of which each may be as important as our sun, being, like it, the centre of a planetary system. And thus, a patch of the heavens, one hundredth or one thousandth part of the visible breadth of our sun, may contain in it more life, not only than exists in the solar system, but in as many such systems as the unassisted eye can see stars in the heavens, on the clearest winter night.

7. This is a stupendous view of the greatness of the creation; and, to many persons, its very majesty, derived from magnitude and number, will make it so striking and acceptable, that, once apprehended, they will feel as if there were a kind of irreverence in disturbing it. But if this view be really not tenable when more closely examined, it is, after all, not wise to connect our feelings of religious reverence with it, so that they shall suffer a shock when we are obliged to reject it. I may add, that we may entertain an undoubting trust that any view of the creation which is found to be true, will also be found to supply material for reverential contemplation. I venture to hope that we may, by further examination, be led to a reverence of a deeper and more solemn character than a mere wonder at the immensity of space and number.

8. But whatever the result may be, let us consider the evidence for this view. It assumes that all the Nebulæ are resolvable into stars, and that they appear as nebulæ only because they are more distant than the region in which they can appear as stars. Are there any facts, any phenomena in the heavens, which may help us to determine whether this is a probable opinion?

9. It is most satisfactory for us, when we can, in such inquiries, know the thoughts which have suggested themselves to the minds of those who have examined the phenomena with the most complete knowledge, the greatest care, and the best advantages; and have speculated upon these phenomena in a way both profound and unprejudiced. Some remarks of Sir John Herschel, recommended by these precious characters, seem to me to bear strongly upon the question which I have just had to ask:—Do all the nebulæ owe their nebulous appearance to their being too distant to be seen as groups of distinct stars, though they really are such groups?

10. Herschel, in the visit which he made to the Cape of Good Hope, for the purpose of erecting to his father the most splendid monument that son ever erected,—the completed survey of the vault of heaven,—had full opportunity of studying a certain pair of remarkable bright spaces of the skies, filled with a cloudy light, which lie near the southern pole; and which, having been unavoidably noticed by the first Antarctic voyagers, are called the Magellanic Clouds. When the larger of these two clouds is examined through powerful telescopes, it presents, we are told, a constitution of uncommon complexity: "large patches and tracts of nebulosity in every stage of resolution, from light, irresolvable with eighteen inches of reflecting aperture, up to perfectly separated stars like the Milky Way, and clustering groups sufficiently insulated and condensed to come under the designation of irregular, and in some cases pretty rich clusters. But besides these, there are also nebulæ in abundance, both regular and irregular; globular clusters in every stage of condensation, and objects of a nebulous character quite peculiar, and which have no analogies in any other region of the heavens."⁸ He goes on to say, that these nebulæ and clusters are far more crowded in this space than they are in any other, even the most crowded parts, of the nebulous heavens. This Nubecula Major, as it is termed, is of a round or oval form, and its diameter is about six degrees, so that it is about twelve times the apparent diameter of the moon. The Nubecula Minor is a smaller patch of the same kind. If we suppose the space occupied by the various objects which the nubecula major includes, to be, in a general way, spherical, its nearest and most remote parts must (as its angular size proves) differ in their distance from us by little more than a tenth part of our distance from its centre. That the two nubeculæ are thus approximately spherical spaces, is in the highest degree probable; not only from the peculiarity of their contents, which suggests the notion of a peculiar group of objects, collected into a limited space; but from the barrenness, as to such objects, of the sky in the neighborhood of these Magellanic Clouds. To suppose (the only other possible supposition) that they are two columns of space, with their ends turned towards us, and their lengths hundreds and thousands of times their breadths, would be too fantastical a proceeding to be tolerated; and would, after all, not explain the facts without further altogether arbitrary assumptions.

11. It appears, then, that, in these groups, there are stars of various magnitudes, clusters of various forms, nebulæ regular and irregular, nebulous tracts and patches of peculiar character; and all so disposed, that the most distant of them, whichever these may be, are not more than one-tenth more distant than the nearest. If the nearest star in this space be at nine times the distance of Sirius, the farthest nebulæ, contained in the same space, will not be at more than ten times the distance of Sirius. Of course, the doctrine that nebulæ are seen as nebulæ, merely because they are so distant, requires us to assume all nebulæ to be hundreds and thousands of times more distant than the smallest stars. If stars of the eighth magnitude (which are hardly visible to the naked eye) be eight times as remote as Sirius, a nebula containing a thousand stars, which is invisible to the naked eye, must be more than eight thousand times as remote as Sirius. And thus if, in the whole galaxy, we reckon only the stars as far as the eighth magnitude, and suppose all the stars of the galaxy to form a nebula, which is visible to the spectators in a distant nebula, only as their nebula is visible to us; we must place them at eight thousand times two hundred thousand times the distance of the Sun; and, even so, we are obviously vastly understating the calculation. These are the gigantic estimates with which some astronomical speculators have been in the habit of overwhelming the minds of their listeners; and these views have given a kind of majesty to the aspect of the nebulæ; and have led some persons to speak of the discovery of every new streak of nebulous light in the starry heavens, as a discovery of new worlds, and still new worlds. But the Magellanic Clouds show us very clearly that all these calculations are entirely baseless. In those regions of space, there coexists, in a limited compass, and in indiscriminate position, stars, clusters of stars, nebulæ, regular and irregular, and nebulous streaks and patches. These, then, are different kinds of things in themselves, not merely different to us. There are such things as nebulæ side by side with stars, and with clusters of stars. Nebulous matter resolvable occurs close to nebulous matter irresolvable. The last and widest step by which the dimensions of the universe have been expanded in the notions of eager speculators, is checked by a completer knowledge and a sager spirit of speculation. Whatever inference we may draw from the resolvability of some of the nebulæ, we may not draw this inference;—that they are more distant, and contain a larger array of systems and of worlds, in proportion as they are difficult to resolve.

12. But indeed, if we consider this process, of the resolution of nebulæ into luminous points, on its own ground, without looking to such facts as I have just adduced, it will be difficult, or impossible, to assign any reason why it should lead to such inferences as have been drawn from it. Let us look at this matter more clearly. An astronomer, armed with a powerful telescope, resolves a nebula, discerns that a luminous cloud is composed of shining dots:—but what are these dots? Into what does he resolve the nebula? Into Stars, it is commonly said. Let us not wrangle about words. By all means let these dots be Stars, if we know about what we are speaking: if a Star merely mean a luminous dot in the sky. But that these stars shall resemble, in their nature, stars of the first magnitude, and that such stars shall resemble our Sun, are surely very bold structures of assumption to build on such a basis. Some nebulæ are resolvable; are resolvable into distinct points; certainly a very curious, probably an important discovery. We may hereafter learn that all nebulæ are resolvable into distinct points: that would be a still more curious discovery. But what would it amount to? What would be the simple way of expressing it, without hypothesis, and without assumption? Plainly this: that the substance of all nebulæ is not continuous, but discrete;—separable, and separate into distinct luminous elements;—nebulæ are, it would then seem, as it were, of a curdled or granulated texture; they have run into lumps of light, or have been formed originally of such lumps. Highly curious. But what are these lumps? How large are they? At what distances? Of what structure? Of what use? It would seem that he must be a bold man who undertakes to answer these questions. Certainly he must appear to ordinary thinkers to be very bold, who, in reply, says, gravely and confidently, as if he had unquestionable authority for his teaching:—"These lumps, O man, are Suns; they are distant from each other as far as the Dog-star is from us; each has its system of Planets, which revolve around it; and each of these Planets is the seat of an animal and vegetable creation. Among these Planets, some, we do not yet know how many, are occupied by rational and responsible creatures, like Man; and the only matter which perplexes us, holding this belief on astronomical grounds, is, that we do not quite see how to put our theology into its due place and form in our system."

13. In discussing such matters as these, where our knowledge and our ignorance are so curiously blended together, and where it is so difficult to make men feel that so much ignorance can lie so close to so much knowledge;—to make them believe that they have been allowed to discover so much, and yet are not allowed to discover more:—we may be permitted to illustrate our meaning, by supposing a case of blended knowledge and ignorance, of real and imaginary discovery. Suppose that there were carried from a scientific to a more ignorant nation, excellent maps of the world, finely engraved; the mountain-ranges shaded in the most delicate manner, and the sheet crowded with information of all kinds, in writing large, small, and microscopic. Suppose also, that when these maps had been studied with the naked eye, so as to establish a profound respect for the knowledge and skill of the author of them, some of those who perused them should be furnished with good microscopes, so as to carry their examination further than before. They might then find that, in several parts, what before appeared to be merely crooked lines, was really writing, stating, it may be, the amount of population of a province, or the date of foundation of a town. To exhaust all the information thus contained on the maps, might be a work of considerable time and labor. But suppose that, when this was done, a body of resolute microscopists should insist that the information which the map contained was not exhausted: that they should continue peering perseveringly at the lines which formed the shading of the mountains, maintaining that these lines also were writing, if only it might be deciphered; and should go on increasing, with immense labor and ingenuity, the powers of their microscopes, in order to discover the legend contained in these unmeaning lines. We should, perhaps, have here an image of the employment of these astronomers, who now go on looking in nebulæ for worlds. And we may notice in passing, that several of the arguments which are used by such astronomers, might be used, and would be used, by our microscopists:—how improbable it was that a person so full of knowledge, and so able to convey it, as the author of the maps was known to be, should not have a design and purpose in every line that he drew: what a waste of space it would be to leave any part of the sheet blank of information; and the like. To which the reply is to us obvious; that the design of shading the mountains was design enough; and that the information conveyed was all that was necessary or convenient. Nor does this illustration at all tend to show that such astronomical scrutiny, directed intelligently, with a right selection of the points examined, may not be highly interesting and important. If the microscopists had examined the map with a view to determine the best way in which mountains can be indicated by shading, they would have employed themselves upon a question which has been the subject of multiplied and instructive discussion in our own day.

14. But to return to the subject of Nebulæ, we may further say, with the most complete confidence, that whether or not nebulous matter be generally resolvable into shining dots, it cannot possibly be true that its being, or not being so resolvable by our telescopes, depends merely upon its smaller or greater distance from the observer. For, in the first place, that there is matter, to the best assisted eye not distinguishable from nebulous matter, which is not so resolvable, is proved by several facts. The tails of Comets often resemble nebulæ; so much so that there are several known nebulæ, which are, by the less experienced explorers of the sky, perpetually mistaken for comets, till they are proved not to be so, by their having no cometary motion. Such is the nebula in Andromeda, which is visible to the naked eye.⁹ But the tails and nebulous appendages of comets, though they alter their appearance very greatly, according to the power of the telescope with which they are examined, have never been resolved into stars, or any kind of dots; and seem, by all investigations, to be sheets or cylinders or cones of luminous vapor, changing their form as they approach to or recede from the sun, and perhaps by the influence of other causes. Yet some of them approach very near the earth; all of them come within the limits of our system. Here, then, we have (probably, at least,) nebulous matter, which when brought close to the eye, compared with the stellar nebulæ, still appears as nebulous.

15. Again, as another phenomenon, bearing upon the same question, we have the Zodiacal Light. This is a faint cone of light¹⁰ which, at certain seasons, may be seen extending from the horizon obliquely upwards, and following the course of the ecliptic, or rather, of the sun's equator. It appears to be a lens-shaped envelope of the sun, extending beyond the orbits of Mercury and Venus, and nearly attaining that of the earth; and in Sir John Herschel's view, may be regarded as placing the sun in the list of nebulous stars. No one has ever thought that this nebulous appearance was resolvable into luminous points; but if it were, probably not even the most sanguine of speculators on the multitude of suns would call these points suns.

16. But indeed the nebulæ themselves, and especially the most remote of the nebulæ, or at least those which most especially require the most powerful telescopes, offer far more decisive proofs that their resolvability or non-resolvability,—their apparent constitution as diffused and vaporous masses,—does not depend upon their distance. A remarkable fact in the irregular, and in some of the regular nebulæ¹¹ is, that they consist of long patches and streaks, which stretch out in various directions, and of which the form¹² and extent vary according to the visual power which is applied to them. Many of the nebulæ and especially of the fainter ones, entirely change their form with the optical power of the instrument by which they are scrutinized; so that, as seen in the mightier telescopes of modern times, the astronomer scarcely recognizes the figures in which the earlier observers have recorded what they saw in the same place. Parts which, before, were separate, are connected by thin bridges of light which are now detected; and where the nebulous space appeared to be bounded, it sends off long tails of faint light into the surrounding space. Now, no one can suppose that these newly-seen portions of the nebula are immensely further off than the other parts. However little we know of the nature of the object, we must suppose it to be one connected object, with all its parts, as to sense, at the same distance from us. Whether therefore it be resolvable or no, there must be some other reason, besides the difference of distance, why the brighter parts were seen, while the fainter parts were not. The obvious reason is, that the latter were not seen because they were thin films which required more light to see them. We are led, irresistibly as it seems, to regard the whole mass of such a nebula, as an aggregation of vaporous rolls and streaks, assuming such forms as thin volumes of smoke or vapor often assume in our atmosphere, and assuming, like them, different shapes according to the quantity of light which comes to us from them. If, as soon as one of these new filaments or webs of a nebula comes into view, we should say, Here we have a new array of suns and of worlds, we should judge as fantastically, as any one who should combine the like imaginations with the varying cloud-work of a summer-sky. To suppose that all the varied streaks by which the patch of nebulous light shades off into the surrounding darkness, and which change their form and extent with every additional polish which we can give to a reflecting or refracting surface, disclose, with every new streak, new worlds, is a wanton indulgence of fancy, to which astronomy gives us no countenance.¹³

17. Undoubtedly all true astronomers, taught caution and temperance of thought by the discipline of their magnificent science, abstain from founding such assumptions upon their discoveries. They know how necessary it is to be upon their guard against the tricks which fancy plays with the senses; and if they see appearances of which they cannot interpret the meaning, they are content that they should have no meaning for them, till the due explanation comes. We have innumerable examples of this wise and cautious temper, in all periods of astronomy. One has occurred lately. Several careful astronomers, observing the stars by day, had been surprised to see globes of light glide across the field of view of their telescopes, often in rapid succession and in great numbers. They did not, as may be supposed, rush to the assumption that these globes were celestial bodies of a new kind, before unseen; and that from the peculiarity of their appearance and movement, they were probably inhabited by beings of a peculiar kind. They proceeded very differently; they altered the focus of their telescopes, looked with other glasses, made various changes and trials, and finally discovered that these globes of light were the winged seeds of certain plants which were wafted through the air; and which, illuminated by the sun, were made globular by being at distances unsuited to the focus of the telescope.¹⁴

18. But perhaps something more may be founded on the ramified and straggling form which belongs to many of the nebulæ. Under the powers of Lord Rosse's telescope, a considerable number of them assume a shape consisting of several spiral films diverging from one centre, and growing broader and fainter as they diverge, so as to resemble a curled feather, or whirlpool of light.¹⁵ This form, though generally deformed by irregularities, more or less, is traceable in so many of the nebulæ, that we cannot easily divest ourselves of the persuasion that there is some general reason for such a form;—that something, in the mechanical causes which have produced the nebulæ, has tended to give them this shape. Now, when this thought has occurred to us, since mathematicians have written a great deal concerning the mechanics of the universe, it is natural to ask, whether any of the problems which they have solved give a result like that thus presented to our eyes. Do such spirals as we here see, occur in any of the diagrams which illustrate the possible motions of celestial bodies? And to this, a person acquainted with mathematical literature might reply, that in the second Book of Newton's Principia, in the part which has especial reference to the Vortices of Descartes, such spirals appear upon the page. They represent the path which a body would describe if, acted upon by a central force, it had to move in a medium of which the resistance was considerable;—considerable, that is, in comparison with the other forces which act; as for example, the forces which deflect the motion from a straight line. Indeed, that in such a case a body would describe a spiral, of which the general form would be more or less oval, is evident on a little consideration. And in this way, for instance, Encke's comet, which, if the resistance to its motion were insensible, would go on describing an ellipse about the sun, always returning upon the same path after every revolution; does really describe a path which, at each revolution, falls a little within the preceding revolution, and thus gradually converges to the centre. And if we suppose the comet to consist of a luminous mass, or a string of masses, which should occupy a considerable arc of such an orbit, the orbit would be marked by a track of light, as an oval spiral. Or if such a comet were to separate into two portions, as we have, with our own eyes, recently seen Biela's comet do; or into a greater number; then these portions would be distributed along such a spiral. And if we suppose a large mass of cometic matter thus to move in a highly resisting medium, and to consist of patches of different densities, then some would move faster and some more slowly; but all, in spirals such as have been spoken of; and the general aspect produced would be, that of the spiral nebulæ which I have endeavored to describe. The luminous matter would be more diffused in the outer and more condensed in the central parts, because to the centre of attraction all the spirals converge.

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