Popular Science Monthly/Volume 2/April 1873/The Nebular Hypothesis
|THE NEBULAR HYPOTHESIS.|
PROFESSOR OF PHYSICS IN THE UNIVERSITY OF CALIFORNIA.
THE speculative views of Lambert and Kant led them to the adoption of a Nebular Hypothesis, and to the idea of a perpetual development in the regions of space. Sir William Herschel, after long hesitation, was ultimately led, by the surer path of observation and cautious induction, to the adoption of similar views, in relation to the existence of a self-luminous substance of a highly-attenuated nature, distributed through the celestial realms. At a later period, in 1811, he communicated to the Royal Society an exposition of his famous hypothesis of the transformation of nebulae into stars.
Sir William Herschel made no attempt to extend his hypothesis to a cosmogony of our solar system. If, therefore, the "Nebular Hypothesis" is restricted to the theory which professes to explain the genesis of our solar system, it is only analogically related to the loftier speculations of Sir William Herschel, in regard to the processes of star-formation going on in the stellar realms. In this restricted sense, the "Nebular Hypothesis" is due to Laplace. This illustrious mathematician, with a modesty and diffidence befitting a true philosopher, endeavored to lay rational foundations for a cosmogony of the solar system. This sublime speculation has been egregiously misunderstood and misrepresented alike in itself and in its tendencies.
The lecturer proposed to disconnect Laplace's Nebular Hypothesis from the question of the general diffusion of cosmical vapor in the celestial regions. Indeed, the origin of Laplace's hypothesis did not lie in Herschel's speculations in relation to the transformation of nebula? into stars and clusters of stars. In contemplating our solar system, he discerned numerous harmonies and adjustments, which were not accounted for by the law of gravitation, which induced him to infer that all its members were of one family of a common origin. The Nebular Hypothesis was framed to explain and coordinate these facts, and, if possible, to refer them to established mechanical principles. Under this view, the lecturer considered the Nebular Hypothesis in two aspects—viz.: As a pure hypothesis, framed to explain the arrangements of the solar system; and as a physical reality, indicating the actual process by which the phenomena were evolved or produced.
Notwithstanding the number of orbs of which the solar system is composed, and the consequent almost infinite variety of their possible dispositions, the following coincidences wholly independent of the! law of gravitation—are found to obtain: 1. The sun rotates on his axis from west to east. 2. All the planets (now 104 in number) revolve about the sun from west to east. 3. All the planets (as far as known rotate on their axes from west to east. 4. All the satellites (excepting those of Uranus and Neptune) revolve about their primaries from west to east. 5. All the satellites (as far as known) rotate on their axes in the same direction in which their primaries turn on their axes. 6. All the planets (with the exception of a few minute asteroids) revolve about the sun, nearly in the plane of the solar equator. 7. All the satellites (as far as known) revolve about their primaries nearly in the planes of the equator of their respective planets. 8. All the planets (with the exception of a few asteroids) have orbits of small eccentricity. 9. All the satellites have, in like manner, orbits of small eccentricity.
These nine independent coincidences in the arrangements of more than 127 separate bodies, cannot be supposed to be fortuitous—they naturally suggest the existence of some grand and comprehensive law, pervading the whole solar system. That they are not consequences of the law of gravitation, is evident from the fact that the comets transgress every one of these laws which could be applicable to them. According to the laws of probability, the chances against the concurrence of so many unconnected phenomena are almost infinite. Laplace estimated that the chances were four millions of millions to one, that these were not arbitrary accidental phenomena. Since his time, facts of a similar bearing have largely accumulated, and the chances against their fortuitous concurrence are now almost beyond the power of numbers to express. "The coördination of these divers and unconnected phenomena—the grouping them into one coherent and harmonious scheme—the referring them to one common cause and origin, and thereby imparting to this fair work of the Eternal the semblance of a Unity worthy of a Divine Idea"—these were the sublime and lofty aims of the famous "Nebular Hypothesis" of Laplace. He imagined "that this consummate fabric—this gorgeous planetary scheme—like the blossom, had a bud—and deeper yet, that it had a mysterious germ, within which rested the necessities of its present glorious unfolding! . . . . He sought, by penetrating the deep recesses of the past, to reveal the mystery of its development, and conceived the bold thought of portraying the modus operandi of the genesis of our Solar System."
In its original form, the Nebular Hypothesis required three assumptions, viz.: 1. An agglomerated nebulous mass; 2. That this mass be rotating about its centre of gravity; and 3. That it be incandescent from excessive heat. The successors of Laplace have endeavored to simplify the hypothesis, by showing that rotation of the nebulous mass, and perhaps also its incandescence, may be simple consequences of the processes of aggregation.
The lecturer proceeded to show that if we suppose the great nebulous mass to have been a continuous gas or fluid, symmetrical in form, and either homogeneous or heterogeneous in structure, provided the component strata were homogeneous in themselves—"the fundamental principles of mechanics assure us that the process of cooling and condensation by contraction, would not generate a motion of rotation." Nay, more; he thought it was very difficult to conceive the mechanical possibility of a continuous gaseous or liquid mass of any form—whether homogeneous or otherwise—acquiring a motion of rotation by the internal motions of its several parts. The mass being isolated, and out of the sphere of external forces—all of its parts being in intercommunication through fluid continuity—how can we reconcile a generation of rotation in the whole mass, with the fundamental principles of the equality of action and reaction? Any force, that begins and ends in a body—whose parts are in continuous material connection cannot impart motion to the whole mass—whatever relative movements might be communicated to portions of it.
The lecturer was disposed to look for the origin of the primitive rotation of the nebulous mass in the discontinuous structure of the primordial nebulous matter. Adopting Sir John Herschel's idea, that the nebulous condition indicates not the gaseous, but the cloudy form of matter, we must look upon nebulous masses as consisting of discrete portions of matter—of various density and bulk—aggregated into special forms, under the influence of mutual attraction. By the gradual subsidence and condensation of these discrete particles by the effect of gravity, a central aggregation or nucleus would be formed—the germ of our sun. Let us hasten to see what light this view throws upon the physical cause of the rotation of the primitive solar nebula. It is obvious that a crowd of aggregating bodies, animated by independent and partially-opposing impulses, must produce collision, destruction of velocity, and a subsidence toward the centre of attraction. It is also evident that those impulses which conspire or remain outstanding after such conflicts, must ultimately give rise to circulation or rotation of a permanent character about some axis. It will be observed that the causes imparting motion to the central mass are, in this view, entirely exterior to it. For the subsiding and conflicting bodies, being discrete and independent of each other, act like the impinging of a comet, or any cosmical mass, on the central nucleus. Under this aspect, the Nebular Hypothesis becomes identical with Sir John Herschel's "Theory of Sidereal Aggregation;" the only difference consisting in the magnitudes of the aggregating bodies.
Instead of supposing that the primitive or chaotic condition of matter was intensely hot, is it not more rational to suppose that it was originally deficient in heat or cold, and that the high temperature was subsequently developed during the processes which brought about its organization? According to the preceding view of the structure and constitution of a nebulous mass, the idea of the chaotic matter being maintained in a diffused and attenuated condition through the agency of heat, is by no means necessary. Indeed, the assumption that the primitive matter of the universe existed in a diffused gaseous condition, through the agency of excessive heat, is itself, prima facie, improbable. If it were absolutely universal, what became of the heat, and how did the cooling and condensation commence? Even if we suppose that the chaotic matter existed in enormous detached masses, what an inconceivable amount of heat must have been created, merely to be dissipated throughout the infinitudes of space! Such a view ill accords with our conceptions of the economy of the Creator's operations.
According to the views previously announced, the original concentration of the nebulous matter about a central nucleus was not the result of cooling and contraction, but of a gradual process of aggregation of discrete bodies under the action of mutually-attractive forces. Now, in the collisions and frictions necessarily incident to this process of aggregation, we have an indefinite supply of heat. The establishment of the "Dynamical Theory of Heat," on the sure basis of experiment and observation, assures us that when motion is checked or arrested, it is transformed into heat. Hence, we see that the collisions and destruction of velocity, incident to the process of aggregation, while imparting a motion of rotation to the nebulous mass, at the same time evolved heat, more or less, throughout its structure—and especially toward the nucleus, where the bodies, whose velocities had been checked, were gradually subsiding. The larger portion of the "dynamical energy" of the crowd of bodies aggregating toward the nucleus, was thus transformed into heat—a smaller portion remaining in the motion of rotation of the solar nebula. This view makes the heat and light-producing process continuous and gradual, and the true gaseous and fused conditions of the nebula, subsequent states, induced by the evolution of intense heat.
We thus reach a lofty point of view. Given diffused or chaotic matter, and mutual attraction, and the whole machinery of the Nebular Hypothesis is set in action! The "star-dust," or "world-stuff" begins to aggregate—heat is evolved—rotation is imparted—and all the apparatus required for the formation of suns, planets, and satellites, is established! Assuming that the processes of aggregation and heat-evolution had so far progressed that the rotating spheroid consisted of a more or less continuous mass of liquid or gas, extending far beyond the orbit of Neptune, and we are furnished with all the conditions assumed by Laplace.
It is unnecessary to follow the lecturer in his exposition of Laplace's reasoning, by which it was shown, upon mechanical principles, that, as the rotating spheroid slowly contracted and condensed by the gravitation of its parts toward the centre, and the process of cooling at its (surface, the rotation must necessarily be accelerated, and consequently the centrifugal force augmented, particularly at its equatorial parts. In fact, this could not be done-without the use of illustrative diagrams. Suffice it to state, that the final result would be, the development of a system of planets, revolving in a common direction around a vast central solar mass, with subordinate systems of satellites circulating in a like direction around their primaries. These are precisely the arrangements which are found to exist in our solar system.
Assuming that the primitive solar nebula rotated on its axis, as the sun does, from west to east, the following consequences were deduced from the theory, viz.: 1. All the planets should move around the sun from west to east. 2. All the planets should rotate on their axes from west to east. 3. All the satellites should rotate on their axes from west to east. 4. All the planets should revolve about the sun in orbits nearly coincident with the plane of the solar equator. 5. All the satellites should revolve about their primaries nearly in the planes of the equators of their respective planets. 6. All the planets should revolve in orbits of small eccentricity. 7. All the satellites should revolve in orbits of small eccentricity. 8. The central mass—the sun—should rotate on his axis in less time than any of the planets revolve about him in their orbits. 9. The primary planets should revolve on their axes in less time than any of their satellites revolve around them; and 10. The central mass, left after the process of genesis was completed, should contain a much larger quantity of matter than the sum of the masses separated. All of these arrangements (with a few unimportant deviations), were shown to exist in the solar system. Recapitulating these coincidences, we obtain the following significant results, viz.:
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"We thus see that there are no less than 390 independent phenomena—of which the law of gravitation gives no account—which are simple consequences of the Nebular Hypothesis. In the aggregate, they imply a very large number of facts—complex—diverse—unconnected with each other—having no mutual dependence—all accounted for by a simple supposition, and the aid of the known laws of matter and motion." It can hardly be denied that, regarded as a pure hypothesis, framed to account for a certain set of facts, its remarkable success in explaining them invests it with a high degree of probability.
It was admitted that the theory had encountered some apparent difficulties—some want of coincidences—the most serious one being the retrograde direction of revolution of the satellites of Uranus. It was shown that this anomaly might be reconciled with the Nebular Hypothesis during the first stages of planet-formation.
But, it has been asked, may not these coincident phenomena be explained by other means than the Nebular Hypothesis? May they not be arrangements instituted by the Creator, for the purpose of giving perpetuity to our solar system, and making the planets suitable habitations for organized beings? And do we not transgress the legitimate domain of scientific research in attempting their explanation?
In reply to this, it was urged that such a view implies a total misconception of the doctrine of final causes. In such inquiries, "we are not to assume that we know the object of the Creator's design, and put this assumed purpose in the place of a physical cause." In these provinces of speculation, the principle of final causes is no longer the basis and guide, but the sequel and result of our physical reasonings. . . . As physical science advances, final causes do not disappear. The principle of design changes its mode of application, but loses none of its force; it is merely transferred from the region of facts to that of laws." We do not consider the sun as less intended to warm and vivify the tribes of plants and animals, because we find evidences that the earth and other planets were developed in the vast periods of past ages, from a common nebulous mass! We are rather, by the discovery of so general a law, led into a scene of wider design—of deeper contrivance—of more comprehensive adjustments. "The object of such views is not to lead to physical truth, but to connect such truth—obtained by its proper processes and methods—with our views of God—the Master of the universe."
But even admitting this application of the principle of final causes, it was shown that the conditions of stability of the solar system, and its adaptability to living beings, are totally insufficient to account for all the observed coincidences. There are many other phenomena in the arrangements of our system, which have no relations to these ends or purposes. It was shown that there are no less than seven sets of phenomena, of which the principle of final causes affords, as far as we can see, no explanation.
The Nebular Hypothesis not only accounts for and coördinates all the arrangements of the solar system, but the conditions of stability and adaptability to living beings are simple consequences of its mode of genesis! Does not the cheering doctrine of final causes—of design and purpose—become strengthened and invigorated by leading us to a view so comprehensive? "How simple the means—how multiform the effects—how far-reaching and grand the design!" How deeply they impress us with the wisdom, power, and glory of the Creator and Governor of the universe!
We now come to consider the physical reality of the fundamental assumption of the Nebular Hypothesis. Have nebulous masses a real existence in the universe? Is the Star-dust—the World-stuff—a physical reality, or a mere figment of the brain of the theorist? If the actual existence of self-luminous nebulous matter—the chaotic elements of future worlds and suns—can be established—the fundamental assumption of Laplace loses the character of a pure hypothesis: his conception becomes a physical theory, which, in proportion as it is verified by phenomena, approaches the domain of fact—a vera causa.
It was shown that the highly-diffused and attenuated matter constituting comets, as well as that constituting the zodiacal light—while affording some suggestive analogies to nebulous masses—do not furnish examples in all respects identical with the supposed nebula of Laplace. We are, therefore, compelled to fall back on Sir William Herschel's opinion, that there are numerous nebulæ which really consist—not of clusters of stars, but of a diffused, self-luminous, vaporiform matter. Such bodies are, beyond all question, self-luminous, but the question is, Are they clusters of stars or true nebulæ? In other terms, are they optically or physically nebulous?
For a long time, this question was keenly discussed, and opinions fluctuated in regard to the tenability of the fundamental assumption of the nebular hypothesis. It is well known that, since 1846, the tendency of telescopic observations, as revealed by the magnificent instruments of Lord Rosse, and corroborated by the splendid achromatic of Harvard University, has been to break down Sir William Herschel's distinction between stellar clusters and true nebulæ. After the sword-handle of Orion was broken into glittering fragments, shining with separate and distinct lustre, Sir John Herschel himself was disposed to abandon the opinion of his illustrious father.
But the development of a new and wonderful branch of physical science has recently furnished the most satisfactory proofs of the reality of such bodies. We allude to the application of Spectrum Analysis to the study of the celestial bodies. The well-matured speculations of Sir William Herschel, and the mathematical theory of Laplace, have been vindicated from the doubt under which they have been laboring, and the early nebulous condition of the cosmical matter has been demonstrated. The accomplished Sir John Herschel has been permitted to witness the complete verification of the previsions of his illustrious father; to see the link connecting the past with the present in the cosmogony of the universe—which seemed to have been almost ruptured by the extension of telescopic vision—restored and strengthened by this new branch of physical investigation.
Until recently the light from the heavenly bodies, even when collected by the largest telescopes, conveyed to us but very meagre information. With regard to the moon, sun, and some of the planets, in addition to their form and size, we have been able, by this means, to obtain some slight knowledge of their physical structure. But, with reference to the myriads of stars, clusters, and nebulæ, which people the depths of space, the telescope reveals little more than variety in color, brightness, and shape. (In relation to the nebulæ, this was illustrated by diagrams contrasting the appearance presented by the same objects when viewed in the telescopes of Sir John Herschel and of Lord Rosse.)
The discovery of "Spectrum Analysis"—the optical Analysis of Light—enables us to interpret symbols and indications hidden within the light itself. Wherever the tiny waves of light—the swift messengers of the celestial realms—can penetrate, they bear with them intelligence of their origin! "Bodies, so remote that astronomers fail to give us an idea of their distance, are brought, as it were, into our grasp, and are analyzed with certainty! We recognize in them the same elements which compose the soil we tread—the water we drink—the air we breathe!"
Before proceeding to explain the manner in which this new method of investigation decides the question of the existence of true nebulous-masses in the regions of space, it is necessary to recall certain well-known and long-established principles in optical science. In 1675, the immortal Newton demonstrated the composite nature of solar light. When a ray of sunlight is made to pass through a glass prism, it is refracted and spread out into a fan-like band, so as to exhibit exquisite gradations of color, from red at one end to violet at the other. This constitutes the Prismatic or Solar Spectrum. In 1802, Wollaston discovered that this spectrum is not continuous, but is interrupted by a number of dark lines. In 1815, Fraunhofer, by great improvements in the optical arrangements employed, rediscovered these lines—ascertained that their relative distances from each other were fixed for sunlight—and succeeded in mapping no less than fifty of them as belonging to the solar spectrum. Since that time, the number of these lines has been increased to thousands. The sagacious Fraunhofer traced these same dark fixed lines in reflected as well as in direct solar light: he found them quite unaltered in position, in the spectrum of moonlight and Venus-light. He, likewise, discovered, that the spectra of the fixed stars contained dark lines differing from those seen in the solar spectrum. He thence drew the important conclusion that these lines have their origin in the luminary. Fraunhofer thus opened the inquiry; but the explanation and import of these lines were reserved for a subsequent epoch.
Modern investigations have established the existence of three orders of spectra depending upon the source of the light: 1. A continuous spectrum—uninterrupted by lines—is produced, when the light emanating from solid and liquid incandescent bodies is passed through a prism. 2. A spectrum interrupted by bright lines is produced when the light emanates from flames or ignited vapors and gases. 3. A spectrum interrupted by dark lines is produced, when light emanating from a source giving a continuous spectrum, is passed through gaseous or vaporous matter giving spectra of the second order.
Now, it has been found that, when various elements are volatilized in the flame of a lamp, the light gives a spectrum interrupted by bright lines—whose character and position are different for different elements. It has also been discovered that the dark lines of spectra of the third order correspond precisely in position with the bright lines in spectra of the second order: they thus indicate the existence of elements which are volatilized in the ignited vapors or gases. The coincidence of position of these bright and dark lines was first observed and described by Foucault, of Paris, in 1849; but their real significance was first indicated in 1859, by Kirchhoff, of Heidelberg. These delicate lines carry across the immeasurable abysses of the celestial spaces evidences of their origin!
The numerous lines of the spectrum are separated from one another—the fan of light is opened out—its entire pattern is brought distinctly under view—and all of its minute details are revealed—by transmitting the light through a succession of prisms: this constitutes the Spectroscope. (This was illustrated by a diagram.)
By means of the spectroscope, no less than fourteen terrestrial elements have been identified as existing in the sun's atmosphere. Mr. William Huggins and Prof. W. A. Miller, by ingenious modifications of this instrument, have been able to extend spectrum analysis to more than sixty of the brighter fixed stars. Like our sun, they give spectra with dark lines; thus indicating that the stars (as the sun) must have intensely heated solid or liquid nuclei, surrounded by ignited gaseous atmospheres.
Encouraged by his success with the fixed stars, Mr. William Huggins applied the potent method of spectrum analysis to the examination of the nebula?. He was rewarded by a most important discovery in relation to the physical constitution of these wonderful objects. On the 29th of August, 1864, he applied his spectroscope to a planetary nebula in Draco. He was astonished to find that there was no appearance of a band of colored light, such as a star would give; but, in place of this, there were three isolated bright lines on a dark ground—a true gaseous or vaporous spectrum. In other words, the object was not a cluster of stars, but a true nebula. Mr. Huggins was not slow in following up this line of investigation. During the two years succeeding his first observation, he examined the spectra of more than sixty nebulæ and clusters. Of this number, about twenty gave spectra with bright lines; that is, were gaseous bodies. The remaining forty gave stellar spectra. Among the true nebulæ may be mentioned, the Annular Nebula in Lyra; the Dumb-bell Nebula; and the great Nebula in the Sword-handle of Orion—concerning the nature of which there has been so much discussion,
These spectrum investigations afford tangible and unmistakable evidence that there are in space, masses of ignited gaseous or vaporous matter of prodigious extent, shining by their own light, and resembling the vast nebula which the Nebular Hypothesis declares to have been the original condition of our solar system. The nebulous matter, assumed as the basis of the hypothesis, is no figment of the theorist!
What great results have been achieved by the power of means apparently the most trivial! Immense objects, seemingly unattainable, have been grasped by the smallest conceivable handle! A little instrument, which is scarcely any thing more than a small triangular piece of glass, solves questions which hundreds of thousands of dollars expended in telescopes, and years of observation, could not have settled! Penetrating into the illimitable depths of space, it reveals to us something of the physical and chemical constitution of stellar clusters and nebulæ, so remote, that the light which the spectroscope analyzes, must have left them thousands, perhaps millions, of years ago!
The lecturer concluded with the following reflections, which are given without abridgment:
In contemplating the vastness of the sidereal universe, every person, in every age and country, must recognize as irresistibly natural, the train of thought expressed by the Hebrew Psalmist, when he exclaims: "When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained; what is man, that thou art mindful of him? and the son of man, that thou visitest him?" (Psalm viii. 3, 4.)
How incalculably has this withering sense of insignificance been augmented by modern telescopic excursions into the remote recesses of the stellar universe! When, by measurements, in which the evidence of the method advances pari passu with the precision of the results, the volume of the earth is reduced to less than one-millionth part of the volume of the sun; when the sun himself, transported to the region of the stars, takes up a very modest place among the thousand of millions of those bodies revealed to us by the telescope; when the ninety-five millions of miles which separate the earth from the sun, by reason of their comparative smallness, have become a base totally insufficient for ascertaining the dimensions of the visible universe, when even the swiftness of light barely suffices for the common valuations of science; when, in short, by a chain of irresistible proofs, certain stars and nebulæ have retired to distances that light could not traverse in less than millions of years we feel as if annihilated by the immensity of the scale of the universe! In assigning to man, and to the planet he inhabits, so small—so insignificant—a position in the material world, science seems only to have made progress to humiliate and to humble us!
Let us accept the lesson of humiliation, with a proper sense of reverence! But, while humbling ourselves in the presence of the overwhelming vastness of God's creation, let us not degrade ourselves: let us not imagine that so insignificant—so ephemeral a being—groping about on so minute a speck in the universe—is totally unworthy of a Creator's care; or entertain the debasing idea that there is no life—no hope—beyond this transient state of existence! Such a view is not the legitimate result of the proper sense of humility which true science demands. She teaches us that grand humility which annihilates self, and places the soul as a child-like learner in the face of God's universe! Like the sacred Shepherd, with unsandalled feet, we advance with reverential awe upon the holy ground, and receive assurances that our minute sphere is benignly noticed by the eye of Omniscience; that, amid the surrounding grandeur, man is not overlooked!
But let us not forget, that there is another aspect under which such contemplations may be viewed, which is calculated to exalt man in the scale of creation. When we reflect on the extreme feebleness of the natural means by the help of which so many great problems have been attacked and solved: if we ask ourselves how such results have been attained; how have we been enabled to assure ourselves of this stupendous scale of creation—of the resplendent glories of the illimitable realms of space—the feeble being resumes all his wonted dignity! By the side of such wonderful achievements of the mind, what signifies the weakness and fragility of our body; what signifies the dimensions of the planet—our residence—the grain of sand on which it has happened to us to appear for a few moments!
From this point of view, man is exalted to his true dignity, through his spiritual and intellectual nature. A mind capable of accomplishing such results must indeed be an emanation from Deity! We must have within us some feeble spark of Divinity! Yes, there is a life and a hope beyond and above this transient existence!
"'Tis the Divinity that stirs within us,
'Tis Heaven itself that points out an hereafter,
And intimates Eternity to man."
Yes, the lofty aspirations of humanity are not delusions; they are realities. They link us with a purer order of existence, which makes us heirs of immortality. We repose under a confident and unwavering assurance that, in God's own time, these earth-mists will be dispersed, and the dim twilight of conjecture will yield to the glorious, unclouded noonday of knowledge.—The California Teacher—Abstract of a Lecture.