A broad and ample road, whose dust is gold,And pavement stars, as stars to thee appearSeen in the Galaxy, that Milky WayWhich nightly as a circling zone thou seestPowder'd with stars.
A broad and ample road, whose dust is gold,
And pavement stars, as stars to thee appear
Seen in the Galaxy, that Milky Way
Which nightly as a circling zone thou seest
Powder'd with stars.
Milton, P. L.vii, 580.
Were the earth transparent as crystal, so that we could see downward through it and outward in all directions to the celestial sphere, the Galaxy or Milky Way would appear as a belt or zone of cloudlike luminosity extending all the way round the heavens. As the horizon cuts the celestial sphere in two, we see at anyone time only one-half of the Milky Way, spanning the dome of the sky as a cloudlike arch.
As the general plane of the Galaxy makes a large angle with our equator, the Milky Way is continually changing its angle with the horizon, so that it rises at different elevations. One-half of the Milky Way will always be below our horizon, and a small region of it lies so near the south pole of the heavens that it can never be seen from medium northern latitudes.
Galileo was the first to explain the fundamental mystery of this belt, when he turned his telescope upon it and found that it was not a continuous sheet of faint light, as it seemed to be, but was made up of countless numbers of stars, individually too faint to be visible to the naked eye, but whose vast number, taken in the aggregate, gave the well-known effect which we see in the sky. In some regions, as Perseus, the stars are more numerous than in others, and they are gathered in close clusters. The larger the telescope we employ, the greater the number of stars that are seen as we approach the Galaxy on either side; and the farther we recede from the Galaxy and approach either of its poles fewer and fewer stars are found. Indeed, if all the stars visible in a 12-inch telescope could be conceived as blotted out, nearly all the stars that are left would be found in the Galaxy itself.
The naked eye readily notes the variations in breadth and brightness of the galactic zone. Nearly a third of it, from Scorpio to Cygnus, is split into two divisions nearly parallel. In many regions its light is interrupted, especially in Centaurus, where a dark starless region exists, known as the "coal sack." Sir John Herschel, who followed up the stellar researches of his father, Sir William, in great detail, places the north pole of the Galactic plane in declination 37 degrees N., and right ascension 12 h. 47 m. This makes the plane of the Milky Way lie at an angle of about 60 degrees with the ecliptic, which it intersects not far from the solstices.
Now Kant, in view of the two great facts about the Galaxy known in his time, (1) that it wholly encircles the heavens, and (2) that it is composed of countless stars too faint to be individually visible to the naked eye, drew the safe conclusions that the system of the stars must extend much farther in the direction of the Milky Way than in other directions.
This theory of Kant was next investigated from an observational standpoint by Sir William Herschel, the ultimate goal of whose researches was always a knowledge of the construction of the heavens. The present conclusion is that we may regard the stellar bodies of the sidereal universe as scattered, without much regard to uniformity, throughout a vast space having in general the shape of a thick watch, its thickness being perhaps one-tenth its diameter. On both sides of this disk of stars, and clustered about the poles of the sidereal system are the regions occupied by vast numbers of nebulæ. The entire visible universe, then, would be spheroidal in general shape. The plane of the Milky Way passes through the middle of this aggregation of stars and nebulæ, and the solar system is near the center of the Milky Way. Throughout the watch-form space the stars are clustered irregularly, in varied and sometimes fantastic forms, but without approach to order or system. If we except some of the star groups and star clusters and consider only the naked-eye stars, we find them scattered with fair approach to uniformity.
Star Clouds and Black Holes in Sagittarius.The dark rifts and lanes resemble those in the nearby Milky Way. (Photo, Yerkes Observatory.)
Star Clouds and Black Holes in Sagittarius.The dark rifts and lanes resemble those in the nearby Milky Way. (Photo, Yerkes Observatory.)
The Great Nebula of Andromeda, Largest (Apparently) of all the Spiral Nebulæ.This nebula can be seen very faintly with the naked eye, but no telescope has yet resolved it into separate stars. (Photo, Yerkes Observatory.)
The Great Nebula of Andromeda, Largest (Apparently) of all the Spiral Nebulæ.This nebula can be seen very faintly with the naked eye, but no telescope has yet resolved it into separate stars. (Photo, Yerkes Observatory.)
The watch-shaped disk is not to be understood as representing the actual form of the stellar system, but only in general the limits within which it is for the most part contained.
A vigorous attack on the problem of the evolution and structure of the stellar universe as a whole is now being conducted by cooperation of many observatories in both hemispheres. It is known as the Kapteyn "Plan of Selected Areas," embracing 206 regions which are distributed regularly over the entire sky. Besides this a special plan includes forty-six additional regions, either very rich or extremely poor in stars, or to which other interest attaches.
Of all investigators Kapteyn has gone into the question of our precise location in the Milky Way most thoroughly, concluding that the solar system lies, not at the center in the exact plane, but somewhat to the north of the Galaxy. Discussing the Sirian stars he finds that if stars of equal brightness are compared, the Sirians average nearly three times more distance from the sun than those of the solar type. So, probably, the Sirians far exceed the Solars in intrinsic brightness. Farther, Kapteyn concludes that the Galaxy has no connectionwith our solar system, and is composed of a vast encircling annulus or ring of stars, far exceeding in number the stars of the great central solar cluster, and everywhere exceedingly remote from these stars, as well as differing from them in physical type and constitution. So it would be mainly the mere element of distance that makes them appear so faint and crowded thickly together into that gauzy girdle which we call the Galaxy.
The Milky Way reveals irregularities of stellar density and star clustering on a large scale, with deep rifts between great clouds of stars. Modern photographs, particularly those of Barnard in Sagittarius, make this very apparent. Within the Milky Way, nearly in its plane and almost central, is what Eddington terms the inner stellar system, near the center of which is the sun. Surrounding it and near its plane are the masses of star clouds which make up the Milky Way. Whether these star clouds are isolated from the inner system or continuous with it, is not yet ascertained.
The vast masses of the Milky Way stars are very faint, and we know nothing yet as to their proper motions, their radial motions, or their spectra. Probably a few stars as bright as the sixth magnitude are actually located in the midst of the Milky Way clusters, the fainter ninth magnitude stars certainly begin the Milky Way proper, while the stars of the twelfth or thirteenth magnitude carry us into the very depths of the Galaxy.
It is now pretty generally believed that many of the dark regions of the Milky Way are due not to actual absence of stars so much as to the absorption of light by intervening tracts of nebulous matter on the hither side of the Galactic aggregationsand, probably in fact, within the oblate inner stellar system itself. Easton has made many hundred counts of stars in galactic regions of Cygnus and Aquila where the range of intensity of the light is very marked; in fact, the star density of the bright patches of the Galaxy is so far in excess of the density adjacent and just outside the Milky Way, that the conclusion is inevitable that this excess is due to the star clouds.
Of the distance of the Milky Way we have very little knowledge. It is certainly not less than 1,000 parsecs, and more likely 5,000 parsecs, a distance over which light would travel in about 16,000 years. Quite certainly all parts of the Galaxy are not at the same distance, and probably there are branches in some regions that lie behind one another. While the general regions of the nebulæ are remote from the Galactic plane, the large irregular nebulæ, as the Trifid, the Keyhole, and the Omega nebulæ, are found chiefly in the Milky Way.
In addition to the irregular nebulæ many types of stellar objects appear to be strongly condensed toward the Milky Way, but this may be due to the inner stellar system, rather than a real relation to Galactic formations. Quite different are the Magellanic clouds, which contain many gaseous nebulæ and are unique objects of the sky, having no resemblance to the true spiral nebulæ which, as a rule, avoid the Galactic regions. Worthy of note also is the theory of Easton that the Milky Way has itself the form of a double-branched spiral, which explains the visible features quite well, but is incapable of either disproof or verification. The central nucleus he locates in the rich Galactic region of Cygnus, with the sun well outside the nucleus itself. By combiningthe available photographs of the Galaxy, he has produced a chart which indicates in a general way how the stellar aggregations might all be arrayed so as to give the effect of the Galaxy as we see it.
Shapley, at Mount Wilson, has studied the structure of the Galactic system, in which he has been aided by Mrs. Shapley. An interesting part of this work relates to the distribution of the spiral nebulæ, and to certain properties of their systematic recessional motion, suggesting that the entire Galactic system may be rapidly moving through space. Apparently the spiral nebulæ are not distant stellar organizations or "island universes," but truly nebular structures of vast volume which in general are actively repelled from stellar systems. A tentative cosmogonic hypothesis has been formulated to account for the motions, distribution, and observed structure of clusters and spiral nebulæ.
An additional great problem of the Galaxy is a purely dynamical one. Doubtless it is in some sort of equilibrium, according to Eddington, that is to say, the individual stars do not oscillate to and fro across the stellar system in a period of 300 million years, but remain concentrated in clusters as at present. Poincaré has considered the entire Milky Way as in stately rotation, and on the assumption that the total mass of the inner stellar system is 1,000,000,000 times the sun's mass, and that the distance of the Milky Way is 2,000 parsecs, the angular velocity for equilibrium comes out 0".5 per century. That is to say, a complete revolution would take place in about 250 million years.
From star clusters to nebulæ, only a century ago, the transition was thought to be easy and immediate. Accuracy in determining the distances of stars was just beginning to be reached, the clusters were obviously of all degrees of closeness following to the verge of irresolvability, and it was but natural to jump to the conclusion that the mystery of the nebulæ consisted in nothing but their vaster distance than that of clusters, and it was believed that all nebulæ would prove resolvable into stars whenever telescopes of sufficiently great power could be constructed.
But the development of the spectroscope soon showed the error of this hypothesis, by revealing bright lines in the nebular spectra showing that many nebulæ emit light that comes from glowing incandescent gas, not from an infinitude of small stars.
In pre-telescope days nothing was known about the nebulæ. The great nebula in Andromeda, and possibly the great nebula in Orion, are alone visible to the naked eye, but as thus seen they are the merest wisps of light, the same as the larger clusters are. Galileo, Huygens and other early users of the telescope made observations of nebulæ, but long-focus telescopes were not well adapted to this work. Simon Mayer has left us the first drawing of a nebula, the Orion nebula as he saw it in 1612. Thevast light-gathering power of the reflectors built by Sir William Herschel first afforded glimpses of the structure of the nebulæ, and if his drawings are critically compared with modern ones, no case of motion with reference to the stars or of change in the filaments of the nebulæ themselves has been satisfactorily made out.
Only very recently has the distance of a nebula been determined, and the few that have been measured seem to indicate that the nebulæ are at distances comparable with the stars. Of all celestial objects the nebulæ fill the greatest angles, so that we are forced to conclude, with regard to the actual size of the greater nebulæ as they exist in space, that they far surpass all other objects in bulk.
Photography invaded the realm of the nebulæ in 1880, when Dr. Henry Draper secured the first photograph of the nebula of Orion. Theoretically photography ought to help greatly in the study of the nebulæ, and enable us in the lapse of centuries to ascertain the exact nature of the changes which must be going on. The differences of photographic processes, of plates, of exposure and development produce in the finished photograph vastly greater differences than any actual changes that might be going on, so that we must rely rather on optical drawings made with the telescope, or on drawings made by expert artists from photographs with many lengths of exposure on the same object.
The great work on nebulæ and star clusters recently concluded by Bigourdan of the Paris Observatory and published in five volumes received the award of the gold medal of the Royal Astronomical Society. While D'Arrest measured about 2,000 nebulæ, and Sir John Herschel about double thatnumber in both hemispheres, Bigourdan has measured about 7,000. His work forms an invaluable lexicon of information concerning the nebulæ.
Classification of the nebulæ is not very satisfactory, if made by their shapes alone. There are perhaps fifteen thousand nebulæ in all that have been catalogued, described, and photographed. Dreyer's new general catalogue (N.G.C.) is the best and most useful. Many of the nebulæ, especially the large ones, can only be classified as irregular nebulæ. The Orion nebula is the principal one of this class, revealing an enormous amount of complicated detail, with exceptional brilliancy of many regions and filaments. An extraordinary multiple star, Theta Orionis, occupies a very prominent position in the nebula, and photographs by Pickering have brought to light curved filaments, very faint and optically invisible, in the outlying regions which give the Orion nebula in part a spiral character. But the delicate optical wisps of this nebula are well seen, even in very small telescopes. Its spectrum yields hydrogen, helium and nitrogen. The Orion nebula is receding from the earth about eleven miles in every second. Keeler and Campbell have shown that nearly every line of the nebular spectrum is a counterpart of a prominent dark line in the spectrum of the brighter stars of the constellation of Orion. A recent investigator of the distribution of luminosity in the great nebula of Orion finds that radiations from nebulium are confined chiefly to the Huygenian region of the nebula and its immediate neighborhood.
Photography has revealed another extraordinary nebula or group of nebulæ surrounding the stars in the Pleiades, which the deft manipulation ofBarnard has brought to light. All the stars and the nebula are so interrelated that they are obviously bound together physically, as the common proper motion of the stars also appears to show. Also in the constellation Cygnus, Barnard has discovered very extensive nebulosities of a delicate filmy cloudlike nature which are wholly invisible with telescopes, but very obvious on highly sensitive plates with long exposures.
Another class of these objects are the annular and elliptic nebulæ which are not very abundant. The southern constellation Grus, the crane, contains a fine one, but by far the best example is in the constellation Lyra. It is a nearly perfect ring, elliptic in figure, exceedingly faint in small telescopes; but large instruments reveal many stars within the annulus, one near the center which, although very faint to the eye, is always an easy object on the photographic plate, because it is rich in blue and violet rays. The parallax of the ring nebula in Lyra comes out only one-sixth of that of the planetary nebulæ, and the least greatest diameters of this huge continuous ring are 250 and 330 times the orbit of Neptune.
Planetary nebulæ and nebulous stars are yet another class of nebulæ, for the most part faint and small, resembling in some measure a planetary disk or a star with nebulous outline. Practically all are gaseous in composition, and have large radial velocities. Probably they are located within our own stellar system. The parallaxes of several of them have been measured by Van Maanen: one of the very small angle 0".023, which enables us to calculate the diameter of this faint but interesting object as equal to nineteen times the orbit of Neptune.
Last and most important of all are the spiral nebulæ. The finest example is in the constellation Canes Venatici, and its spiral configuration was first noted by Lord Rosse, an epoch-making discovery. The convolutions of its spiral are filled with numerous starlike condensations, themselves engulfed in nebulosity. Photography possesses a vast advantage over the eye in revealing the marvelous character of this object, an inconceivably vast celestial whirlpool. Naturally the central regions of the whorl would revolve most swiftly, but no comparison of drawings and photographs, separated by intervals of many years, has yet revealed even a trace of any such motion.
The number of large spiral nebulæ is not very great; the largest of all is the great nebula of Andromeda, whose length stretches over an arc of seven times the breadth of the moon, and its width about half as great. This nebula is a naked-eye object near Eta Andromedæ, and it is often mistaken for a comet. Optically it was always a puzzle, but photographs by Roberts of England first revealed the true spiral, with ringlike formations partially distinct, and knots of condensing nebulosity as of companion stars in the making. While its spectrum shows the nongaseous constitution of this nebula, no telescope has yet resolved it into component stars.
Systematic search for spiral nebulæ by Keeler, and later continued by Perrine, at the Lick Observatory, with the 36-inch Crossley reflector, disclosed the existence of vast numbers of these objects, in fact many hundreds of thousands by estimation; so that, next to the stars, the spiral nebulæ are by far the most abundant of all objects in the sky. They present every phase according to the angle of their plane with the line of sight, and the convolutions of the open ones are very perfectly marked. Many are filled with stars in all degrees of condensation, and the appearance is strongly as if stars are here caught in every step of the process of making.
The vast multitude of the spiral nebulæ indicates clearly their importance in the theory of the cosmogony, or science of the development of the material universe. Curtis of the Lick Observatory has lately extended the estimated number of these objects to 700,000. He has also photographed with the Crossley reflector many nebulæ with lanes or dark streaks crossing them longitudinally through or near the center. These remarkable streaks appear as if due to opaque matter between us and the luminous matter of the nebula beyond. Perhaps a dark ring of absorptive or occulting matter encircles the nebula in nearly the same plane with the luminous whorls. Duncan has employed the 60-inch Mount Wilson reflector in photographing bright nebulæ and star clusters in the very interesting regions of Sagittarius. One of these shows unmistakable dark rifts or lanes in all parts of the nebula, resembling the dark regions of the neighboring Milky Way.
Pease of Mount Wilson has recently employed the 60-inch and the 100-inch reflectors of the Mount Wilson Observatory to good advantage in photographingseveral hundred of the fainter nebulæ. Many of these are spirals, and others present very intricate and irregular forms. A search was made for additional spirals among the smaller nebulæ along the Galaxy, but without success. Several of the supposedly variable nebulæ are found to be unchanging. Many nights in each month when the moon is absent are devoted to a systematic survey of the smaller nebulæ and their spectra by photography. The visible spiral figure of all these objects is a double-branched curve, its two arms joining on the nucleus in opposing points, and coiling round in the same geometrical direction. The spiral nebulæ, as to their distribution, are remote from the Galaxy, and the north Galactic polar region contains a greater aggregation than the south. The distances of the spiral nebulæ are exceedingly great. They lie far beyond the planetary and irregular gaseous nebulæ, like that of Orion, which are closely related to the stars forming part of our own system. Possibly the spiral nebulæ are exterior or separate "island universes." If so, they must be inconceivably vast in size, and would develop, not into solar systems, but into stellar clusters. The enormous radial velocities of the spiral nebulæ, averaging 300 to 400 kilometers per second, or twenty-fold that of the stars, tend to sustain the view that they may be "island universes," each comparable in extent with the universe of stars to which our sun belongs.
Recent spectroscopic observations of the nebulæ applying the principle of Doppler have revealed high velocities of rotation. Slipher of the Lowell Observatory made the first discovery of this sort and Van Maanen of Mount Wilson has detected in the great Ursa Major spiral, No. 101 in Messier's catalogue,a speed of rotation at five minutes of arc from the center that would correspond to a complete period in 85,000 years. As was to be expected, the nebula does not rotate as a rigid body, but the nearer the center the greater the angular velocity, and Van Maanen finds evidence of motion along the arms and away from the center.
These great velocities appear to belong to the spiral nebulæ as a class, and not to other nebulæ. Thirteen nebulæ investigated by Keeler are as a whole almost at rest relatively to our system, as are the large irregular objects in Orion, and the Trifid nebula. This would seem to indicate that the spiral nebulæ form systems outside our own and independent of it.
Quite different from the spirals in their distribution through space are the planetary nebulæ. The spirals follow the early general law of nebulæ arrangement, that is, they are concentrated toward the poles of the Galaxy; but the planetary nebulæ, on the other hand, are very few near the poles and show a marked frequency toward the Galactic plane. Campbell and Moore have found spectroscopic evidence of internal rotatory motion in a large proportion of the planetary nebulæ.
The distribution of the nebulæ throughout space, like that of the stars, is still under critical investigation, but the location of vast numbers of the more compact nebulæ on the celestial sphere is very extraordinary. The Milky Way appears to be the determining plane in both cases; the nearer we approach it the more numerous the stars become, whereas this is the general region of fewest nebulæ and they increase in number outward in both directions from the Galaxy, and toward both poles of theGalactic circle. Obviously this relation, or contra-relation of stars and nebulæ on such a vast scale is not accidental, and it also must be duly accounted for in the true theory of the cosmogony. The nebulæ which are found principally in and near the Milky Way are the large irregular nebulæ, and vast nebulous backgrounds, like those photographed by Barnard in Scorpio, Taurus and elsewhere, as well as the Keyhole, Omega, and Trifid nebulæ. Allied to these backgrounds are doubtless some of the dark Galactic spaces, radiating little or no intrinsic light, and absorbing the light of the fainter stars beyond them. A peculiar veiled or tinted appearance has been remarked in some cases visually, and examination of the photographs strongly confirms the existence of absorbing nebulosity.
The spiral nebulæ are so abundant, and so much attention is now being given to them, both by observers and mathematicians, that their precise relation to the stellar systems must soon be known; that is, whether they are comparatively small objects belonging to the stellar system, or independent systems on the borders of the stellar system, or as seems more likely, vast and exceedingly remote galaxies comparable with that of the Milky Way itself. Our knowledge of the motions of the spirals, both radial and angular, is increasing rapidly, and must soon permit accurate general conclusions to be drawn.
Down to the middle of the last century and later, it was commonly believed that in the beginning the cosmos came into being by divine fiat substantially as it is. Previously the earth had been "without form and void," as in the Scripture. Had it not been for the growth and gradual acceptance of the doctrine of evolution, and its reactionary effect upon human thought, it is conceivable that the early view might have persisted to the present day; but now it is universally held that everything in the heavens above and the earth beneath is subject more or less to secular change, and is the result of an orderly development throughout indefinite past ages, a progressive evolution which will continue through indefinite aeons of the future.
In the writings of the Greek philosophers, and down through the Middle Ages we find the idea of an original "chaos" prevailing, with no indication whatever of the modern view of the process by which the cosmos came to be what they saw it and as it is to-day. If we go still farther back, there is no glimmer of any ideas that will bear investigation by scientific method, however interesting they may be as purely philosophical conceptions. Many ancient philosophers, among them Anaxagoras, Democritus, and Anaximenes, regarded the earth as the productof diffused matter in a state of the original chaos having fallen together haphazard, and they even presumed to predict its future career and ultimate destiny.
In Anaximander and Anaximenes alone do we find any conception of possible progress; their thought was that as the world had taken time to become what it is, so in time it would pass, and as the entire universe had undergone alternate renewal and destruction in the past, that would be its history in the future. Aristotle, Ptolemy, and others appear to have held the curious notion that although everything terrestrial is evanescent, nevertheless the cosmos beyond the orbit of the moon is imperishable and eternal.
By tracing the history of the intellectual development of Europe we may find why it was that scientific speculation on the cosmogony was delayed until the 18th century, and then undertaken quite independently by three philosophers in three different countries. Swedenborg, the theologian, set down in due form many of the principles that underlie the modern nebular hypothesis. Thomas Wright of Durham whose early theory of the arrangement of stars in the Galaxy we have already mentioned, speculated also on the origin and development of the universe, and his writings were known to Kant, who is now regarded as the author of the modern nebular hypothesis. This presents a definite mechanical explanation of the development and formation of the heavenly bodies, and in particular those composing the solar system.
Kant was illustrious as a metaphysician, but he was a great physicist or natural philosopher as well, and he set down his ideas regarding the cosmogonywith precision. Learned in the philosophy of the ancients, he did not follow their speculative conceptions, but merely assumed that all the materials from which the bodies of the solar system have been fashioned were resolved into their original elements at the beginning, and filled all that part of space in which they now move. True, this is pretty near the chaos of the Greeks, but Kant knew of the operation of the Newtonian law of gravitation, which the Greeks did not.
As a natural result of gravitative processes, Kant inferred that the denser portions of the original mass would draw upon themselves the less dense portions, whirling motions would be everywhere set up, and the process would continue until many spherical bodies, each with a gaseous exterior in process of condensation, had taken the place of the original elements which filled space. In this manner Kant would explain the sameness in direction of motion, both orbital and axial, of all the planets and satellites of our system. But many philosophers are of the opinion that Kant's hypothesis would result, not in the formation of such a collection of bodies as the solar system is, but rather in a single central sun formed by common gravitation toward a single center.
From quite another viewpoint the work of the elder Herschel is important here. No one knew the nebulæ from actual observation better than he did; but, while his ideas about their composition were wrong, he nevertheless conceived of them as gradually condensing into stars or clusters of stars. And it was this speculative aspect of the nebulæ, not as a possible means of accounting for the birth and development of the solar system, which constitutesHerschel's chief contribution to the nebular hypothesis. Classifying the nebulæ which he had carefully studied with his great telescopes, it seemed obvious to him that they were actually in all the different stages of condensation, and subsequent research has strongly tended to substantiate the Herschelian view.
Then came Laplace, who took up the great hypothesis where Kant and Herschel had left it, added new and important conceptions in the light of his mature labors as mathematician and astronomer, and put the theory in definitive form, such that it has ever since been known under the name of Laplacian nebular hypothesis. For reasons like those that prevailed with Kant, he began the evolution of the solar system with the sun already formed as the center, but surrounded by a vast incandescent atmosphere that filled all the space which the sun's family of planets now occupy. This entire mass, sun, atmosphere, and all, he conceived to have a stately rotation about its axis. With rotation of the mass and slow reduction of temperature in its outer regions, there would be contraction toward the solar center, and an increase in velocity of rotation until the whole mass had been much reduced in diameter at its poles and proportionately expanded at its equator.
When the centrifugal force of the outer equatorial masses finally became equal to the gravitational forces of the central mass, then these conjoined outer portions would be left behind as a ring, still revolving at the velocity it had acquired when detached. The revolution of the entire inner mass goes on, its velocity accelerating until a similar equilibration of forces is again reached, whena second rotating ring is left behind. Laplace conceived the process as repeated until as many rings had been detached as there are individual planets, all central about the sun, or nearly so.
In all, then, we should have nine gaseous rings; the outer ones preceding the inner in formation, but not all existing as rings at the same time. Radiation from the ring on all sides would lead to rapid contraction of its mass, so that many nuclei of condensation would form, of various sizes, all revolving round the central sun in practically the same period. Laplace conceived the evolution of the ring to proceed still farther till the largest aggregation in it had drawn to itself all the other separate nuclei in the ring.
This, then, was the planet in embryo, in effect a diminutive sun, a secondary incandescent mass endowed with axial rotation in the same direction as the parent nebula. With reduction of temperature by radiation, polar contraction and equatorial expansion go on, and planetary rings are detached from this secondary mass in exactly the same way as from the original sun nebula. And these planetary rings are, in the Laplacian hypothesis, the embryo moons or planetary satellites, all revolving round their several planets in the same direction that the planets revolve about the sun.
In the case of one of the planetary rings, its formation was so nearly homogeneous throughout that no aggregation into a single satellite was possible; all portions of the ring being of equal density, there was no denser region to attract the less dense regions, and in this manner the rings of Saturn were formed, in lieu of condensation into a separate satellite. Similarly in the case of the primalsolar ring that was detached next after the Jovian ring; there was such a nice balancing of masses and densities that, instead of a single major planet, we have the well-known asteroidal ring, composed of innumerable discrete minor planets.
This, then, in bare outline, is the Laplacian nebular hypothesis, and it accounted very well for the solar system as known in his day; the fairly regular progression of planetary distances; their orbits round the sun all nearly circular and approximately in a single plane; the planetary and satellite revolutions in orbit all in the same direction; the axial rotations of planets in the same direction as their orbital revolutions; and the plane of orbital revolution of the satellites practically coinciding with the plane of the planet's axial rotation. But the principle of conservation of energy was, of course, unknown to Laplace, nor had the mechanical equivalence of heat with other forms of energy been established in his day.
In 1870, Lane of Washington first demonstrated the remarkable law that a gaseous sphere, in process of losing heat by radiation and contraction because of its own gravity, actually grows hotter instead of cooler, as long as it continues to be gaseous, and not liquid or solid. So there is no need of postulating with Laplace an excessively high temperature of the original nebula. The chief objection to Laplace's hypothesis by modern theorists is that the detachment of rings, though possible, would likely be a rare occurrence; protuberances or lumps on the equatorial exterior of a swiftly revolving mass would be more likely, and it is much easier to see how such masses would ultimately become planets than it is to follow the disruption ofa possible ring and the necessary steps of the process by which it would condense into a final planet. The continued progress of research in many departments of astronomy has had important bearing on the nebular hypothesis, and we may rest assured that this hypothesis in somewhat modified form can hardly fail of ultimate acceptance, though not in every essential as its great originator left it.
Lord Rosse's discovery of spiral nebulæ, followed up by Keeler's photographic search for these bodies, revealing their actual existence in the heavens by the hundreds of thousands, has led to another criticism of the Laplacian theory. Could Laplace have known of the existence of these objects in such vast numbers, his hypothesis would no doubt have been suitably modified to account for their formation and development. It is generally considered that the ring of Saturn suggested to Laplace the ring feature in his scheme of origin of planets and satellites; so far as we know, the Saturnian ring is unique, the only object of its kind in the heavens. Whereas, next to the star itself, the spiral nebula is the type object which occurs most frequently. A theory, therefore, which will satisfactorily account for the origin and development of spiral nebulæ must command recognition as of great importance in the cosmogony.
Such a theory has been set forth by Chamberlin and Moulton in their planetesimal hypothesis, according to which the genesis of spiral nebulæ happens when two giant suns approach each other so closely that tide-producing effects take place on a vast scale. These suns need not be luminous; they may perhaps belong to the class of dark or extinguished suns. The evidences of the existence ofsuch in vast numbers throughout the universe is thought to be well established.
Now, on close approach, what happens? There will be huge tides, and the nearer the bodies come to each other, the vaster the scale on which tides will be formed. If the bodies are liquid or gaseous, they will be distorted by the force of gravitation, and the figure of both bodies will become ellipsoidal; and at last under greater stress, the restraining shell of both bodies will burst asunder on opposite sides in streams of matter from the interior. In this manner the arms of the spiral are formed.
As Chamberlin puts it: "If, with these potent forces thus nearly balanced, the sun closely approaches another sun, or body of like magnitude … the gravity which restrains this enormous elastic power will be reduced along the line of mutual attraction. At the same time the pressure transverse to this line of relief will be increased. Such localized relief and intensified pressure must bring into action corresponding portions of the sun's elastic potency, resulting in protuberances of corresponding mass and high velocity."
Only a fraction of one per cent of the sun's mass ejected in this fashion would be sufficient to generate the entire planetary system. Nuclei or knots in the arms of the spiral gradually grew by accretion, the four interior knots forming Mercury, Venus, the Earth, and Mars. The earth knot was a double one, which developed into the earth-moon system. The absence of a dominating nucleus beyond Mars accounts for the zone of the asteroids remaining in some sense in the original planetesimal condition. The vaster nuclei beyond Mars gradually condensed into Jupiter, Saturn, Uranus, and Neptune; andlesser nuclei related to the larger ones form the systems of moons or satellites.
The orbits of the planetesimals and the planetary and satellite nuclei would be very eccentric, forming a confusion of ellipses with frequently crossing paths. Collisions would occur, and the nuclei would inevitably grow by accretion. Each planet, then, would clear up the planetesimals of its zone; and Moulton shows that this process would give rise to axial revolution of the planet in the same direction as its orbital revolution. The eccentricities would finally disappear, and the entire mass would revolve in a nearly circular orbit.
Rotation twists the streams into the spiral form, and the huge amounts of wreckage from the near-collision are thrown into eddies. The fragments or particles (planetesimals) which have given the name to the theory, begin their motion round their central sun in elliptical paths as required by gravitation. The form of the spiral is preserved by the orbital motion of its particles. There is a gradual gathering together of the planetesimals at points or nodes of intersection, and these become aggregations of matter, nuclei that will perhaps become planets, though more likely other stars. The appulse or near approach is but one of the methods by which the spiral nebulæ may have come into existence. The planetesimal hypothesis would seem to account for the formation of many of these objects as we see them in the sky, though perhaps it is hardly competent to replace entirely the Laplacian hypothesis of the formation of the solar system, which would appear to be a special case by itself.
It will be observed that while the Laplacian hypothesis is concerned in the main with the progressivedevelopment of the solar system, and systems of a like order surrounding other stellar centers, whose existence is highly probable, the origin and development of the stellar universe is a vaster problem which can only be undertaken and completed in its broadest bearings when the structure of the stellar universe has been ascertained.
Darwin's important investigations in 1877-1878 on tidal friction may be here related. Before his day acceptance of the ring-theory of development of the moon from the earth had scarcely been questioned; but his recondite mathematical researches on the tidal reaction between a central yielding mass and a body revolving round it brought to light the unsuspected effect of tides raised upon both bodies by their mutual attraction. The type of tides here meant is not the usual rise and fall of the waters of the ocean, but primeval tides in the plastic material of which the earth in its early history was composed. The Newtonian law of gravitation afforded a complete explanation of the rise and fall of the waters of the oceans, but as applied to the motions of planets and satellites by the Lagrangian formulæ, it presupposed that all these bodies are rigid and unyielding. However, mutual tides of phenomenal height in their early plastic substances must have been a necessary consequence of the action of the Newtonian law, and they gradually drew upon the earth's rotational moment of momentum.
In its very early history, before there was any moon to produce tides, the earth rotated much more rapidly, that is, the day was very much shorter than now, probably about five or six hours long. And with the rapid whirling, it was not a Laplacian ring that was detached, but a huge globular masswas separated from the plastic earth's equator. Darwin shows that the gravitative interaction of the two bodies immediately began to raise tides of extraordinary height in both, therefore tending to slow down the rotational periods of both bodies. Action and reaction being equal, the reaction at once began driving the moon away from the earth and thereby lengthening its period of revolution. So small was the mass of the moon and so near was it to the earth, that its relative rotational energy was in time completely used up, and the moon has ever since turned her constant face toward us. Tides of sun and moon in the plastic earth, acting through the ages, slowed down the earth's rotation to its present period, or the length of the day.
Moulton, however, has investigated the tidal theory of the origin of the moon in the light of the planetesimal hypothesis, concluding that the moon never was part of the earth and separated therefrom by too rapid rotation of the earth, but that the distance of the two bodies has always been the same as now. The more massive earth has in its development throughout time robbed the less massive moon in the gradual process of accretion. So the moon has never acquired either an ocean or atmosphere, and this view is acceptable to geologists who have studied the sheer lunar surface, Shaler of Harvard among the first, and laid the foundations for a separate science of selenology.
Tidal friction has also been operant in producing sun-raised tides upon the early plastic substances which composed the planets: more powerfully in the case of planets nearer the sun; less rapidly if the planet's mass is large; also less completely if the planet has solidified earlier on account of its smalldimensions. So Darwin would account for the present rotation periods of all the planets: both Mercury and Venus powerfully acted on by the sun on account of their nearness to him, and their rotational energy completely exhausted, so that they now and for all time turn a constant face toward him, as the moon does to the earth; earth and possibly Mars even yet undergoing a very slight lengthening of their day; Jupiter and Saturn, also Uranus and probably Neptune, still exhibiting relatively swift axial rotation, because of their great mass and great original moment of momentum, and also by reason of their vast distances from the central tide-raising body, the sun.
By applying to stellar systems the principles developed by Darwin, See accounted for the fact, to which he was the first to direct attention, that the great eccentricity of the binary orbits is a necessary result of the secular action of tidal friction. The double stars, then, were double nebulæ, originally single, but separated by a process allied to that known as "fission" in protozoans. Indeed, Poincaré proved mathematically that a swiftly revolving nebula, in consequence of contraction, first undergoes distortion into a pear-shaped or hour-glass figure, the two masses ultimately separating entirely; and the observations of the Herschels, Lord Rosse and others, with the recent photographic plates at the Lick and Mount Wilson observatories, afford immediate confirmation in a multitude of double nebulæ, widely scattered throughout the nebular regions of the heavens.
Jeans of Cambridge, England, among the most recent of mathematical investigators of the cosmogony, balances the advantages and disadvantages ofthe differing cosmogonic systems as follows, in his "Problems of Cosmogony and Stellar Dynamics": "Some hundreds of millions of years ago all the stars within our Galactic universe formed a single mass of excessively tenuous gas in slow rotation. As imagined by Laplace, this mass contracted owing to loss of energy by radiation, and so increased its angular velocity until it assumed a lenticular shape…. After this, further contraction was a sheer mathematical impossibility and the system had to expand. The mechanism of expansion was provided by matter being thrown off from the sharp edge of the lenticular figure, the lenticular center now forming the nucleus, and the thrown-off matter forming the arms, of a spiral nebula of the normal type. The long filaments of matter which constituted the arms, being gravitationally unstable, first formed into chains of condensation about nuclei, and ultimately formed detached masses of gas. With continued shrinkage, the temperature of these masses increased until they attained to incandescence, and shone as luminous stars. At the same time their velocity of rotation increased until a large proportion of them broke up by fission into binary systems. The majority of the stars broke away from their neighbors and so formed a cluster of irregularly moving stars—our present Galactic universe, in which the flattened shape of the original nebula may still be traced in the concentration about the Galactic plane, while the original motion along the nebular arms still persists in the form of 'star-streaming.' In some cases a pair or small group of stars failed to get clear of one another's gravitational attractions and remain describing orbits about one another as wide binaries or multiplestars. The stars which were formed last, the present B-type stars, have been unusually immune from disturbance by their neighbors, partly because they were born when adjacent stars had almost ceased to interfere with one another, partly because their exceptionally large mass minimized the effect of such interference as may have occurred; consequently they remain moving in the plane in which they were formed, many of them still constituting closely associated groups of stars—the moving star clusters.
"At intervals it must have happened that two stars passed relatively near to one another in their motion through the universe. We conjecture that something like 300 million years ago our sun experienced an encounter of this kind, a large star passing within a distance of about the sun's diameter from its surface. The effect of this, as we have seen, would be the ejection of a stream of gas toward the passing star. At this epoch the sun is supposed to have been dark and cold, its density being so low that its radius was perhaps comparable with the present radius of Neptune's orbit. The ejected stream of matter, becoming still colder by radiation, may have condensed into liquid near its ends and perhaps partially also near its middle. Such a jet of matter would be longitudinally unstable and would condense into detached nuclei which would ultimately form planets."
We have seen how Wright in 1750 initiated a theory of evolution, not only of the solar system, but of all the stars and nebulæ as well; how Kant in 1752 by elaborating this theory sought to develop the details of evolution of the solar system on the basis of the Newtonian law, though weakened, as we know, by serious errors in applying physical laws; how Laplace in 1796 put forward his nebular hypothesis of origin and development of the solar system, by contraction from an original gaseous nebula in accord with the Newtonian law; how Sir William Herschel in 1810 saw in all nebulæ merely the stuff that stars are made of; how Lord Rosse in 1845 discovered spiral nebulæ; how Helmholtz in 1854 put forward his contraction theory of maintenance of the solar heat, seemingly reinforcing the Laplacian theory; how Lane in 1870 proved that a contracting gaseous star might rise in temperature; how Roche in 1873 in attempting to modify the Laplacian hypothesis, pointed out the conditions under which a satellite would be broken up by tidal strains; how Darwin in 1879 showed that the theory of tidal evolution of non-rigid bodies might account for the formation of the moon, and binary stars might originate by fission; how Keeler in 1900 discovered the vast numbers of spiral nebulæ; how Chamberlin and Moulton in 1903 put forwardthe planetesimal hypothesis of formation of the spiral nebulæ, showing also how that hypothesis might account for the evolution of the solar system; and how Jeans in 1916 advocated the median ground in evolution of the arms of the spiral nebulæ, showing that they will break up into nuclei, if sufficiently massive.
In all these theories, truth and error, or lack of complete knowledge, appear to be intermingled in varying proportions. Is it not early yet to say, either that any one of them must be abandoned as totally wrong, or on the other hand that any one of them, or indeed any single hypothesis, can explain all the evolutionary processes of the universe?
Clearly the great problems cannot all be solved by the kinetic theory of gases and the law of gravitation alone. Recent physical researches into sub-atomic energy and the structure and properties of matter, appear to point in the direction where we must next look for more light on such questions as the origin and maintenance of the sun's heat, the complex phenomena of variable stars and the progressive evolution of the myriad bodies of the stellar universe. Because we have actually seen one star turn into a nebula we should not jump to the conclusion that all nebulæ are formed from stars, even if this might seem a direct inference from the high radial velocities of planetary nebulæ.
Quite as obviously many of the spiral nebulæ are in a stage of transition into local universes of stars—even more obvious from the marvelous photographs in our day than the evolution of stars from nebulæ of all types was to Herschel in his day.
The physicist must further investigate such questions as the building up of heavy atomic elementsby gravitative condensation of such lighter ones as compose the nebulæ; and laboratory investigation must elucidate further the process of development of energy from atomic disintegration under very high pressures. This leads to a reclassification of the stars on a temperature basis.
Equally important is the inquiry into the mechanism of radiative equilibrium in sun and stars. Not impossibly the process of the earth's upper atmosphere in maintaining a terrestrial equilibrium may afford some clue. What this physical mechanism may be is very incompletely known, but it is now open to further research through recent progress of aeronautics, which will afford the investigator a "ceiling" of 50,000 feet and probably more. Beneath this level, perhaps even below 40,000 feet, lie all the strata, including the inversion layer, where the sun's heat is conserved and an equilibrium maintained.
Even ten years ago, had an astronomer been asked about the physical condition of the interior of the stars, he would have replied that information of this character could only be had on visiting the stars themselves—and perhaps not even then. But at the Cardiff meeting of the British Association in 1920, Eddington, the president of Section A, delivered an address on the internal constitution of the stars. He cites the recent investigations of Russell and others on truly gaseous stars, like Aldebaran, Arcturus, Antares and Canopus, which are in a diffuse state and are the most powerful light-givers, and thus are to be distinguished from the denser stars like our Sun. The termgiantsis applied to the former, anddwarfsto the latter, in accord with Russell's theory.
As density increases through contraction, these terms represent the progressive stages, from earlier to later, in a star's history. A red or M-type star begins its history as a giant of comparatively low temperature. Contracting, according to Lane's law, its temperature must rise until its density becomes such that it no longer behaves as a perfect gas. Much depends on the star's mass; but after its maximum temperature is attained, the star, which has shrunk to the proportions of a dwarf, goes on cooling and contracts still further.
Each temperature-level is reached and passed twice, once during the ascending stage and once again in descending—once as a giant, and once as a dwarf. Thus there are vast differences in luminosity: the huge giant, having a far larger surface than the shrunken dwarf, radiates an amount of light correspondingly greater.
The physicist recognizes heat in two forms—the energy of motion of material atoms, and the energy of ether waves. In hot bodies with which we are familiar, the second form is quite insignificant; but in the giant stars, the two forms are present in about equal proportions. The super-heated conditions of the interior of the stars can only be estimated in millions of degrees; and the problem is not one of convection currents, as formerly thought, bringing hot masses to the surface from the highly heated interior, but how can the heat of the interior be barred against leakage and reduced to the relatively small radiation emitted by the stars. "Smaller stars have to manufacture the radiant heat which they emit, living from hand to mouth; the giant stars merely leak radiant heat from their store."
So a radioactive type of equilibrium must be established, rather than a convective one. Laboratory investigations of the very short waves are now in progress, bearing on the transparency of stellar material to the radiation traversing it; and the penetrating power of the star's radiation is much like that of X-rays. The opacity is remarkably high, explaining why the star is so nearly "heat-tight."
Opacity being constant, the total radiation of a giant star depends on its mass only, and is quite independent of its temperature or state of diffuseness. So that the total radiation of a star which is measured roughly by its luminosity, may readily remain constant during the entire 'giant' stage of its history. As Russell originally pointed out, giant stars of every spectral type have nearly the same luminosity. From the range of luminosity of the giant stars, then, we may infer their range of masses: they come out much alike, agreeing well with results obtained by double-star investigation.
These studies of radiation and internal condition of the stars again bring up the question of the original source of that supply of radiant energy continually squandered by all self-luminous bodies. The giant stars are especially prodigal, and radiate at least a hundredfold faster than the sun.
"A star is drawing on some vast reservoir of energy," says Eddington, "by means unknown to us. This reservoir can scarcely be other than the sub-atomic energy which, it is known, exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well-nigh inexhaustible, if only it could be tapped. There is sufficient in the sun to maintain its output of heat for fifteen billion years."