The Story of Evolution/Chapter III
The greater part of this volume will be occupied with the things that have happened on one small globe in the universe during a certain number of millions of years. It cannot be denied that this has a somewhat narrow and parochial aspect. The earth is, you remember, a million times smaller than the sun, and the sun itself is a very modest citizen of the stellar universe. Our procedure is justified, however, both on the ground of personal interest, and because our knowledge of the earth's story is so much more ample and confident. Yet we must preface the story of the earth with at least a general outline of the larger story of the universe. No sensible man is humbled or dismayed by the vastness of the universe. When the human mind reflects on its wonderful scientific mastery of this illimitable ocean of being, it has no sentiment of being dwarfed or degraded. It looks out with cold curiosity over the mighty scattering of worlds, and asks how they, including our own world, came into being.
We now approach this subject with a clearer perception of the work we have to do. The universe is a vast expanse of ether, and somehow or other this ether gives rise to atoms of matter. We may imagine it as a spacious chamber filled with cosmic dust; recollecting that the chamber has no walls, and that the dust arises in the ether itself. The problem we now approach is, in a word: How are these enormous stretches of cosmic dust, which we call matter, swept together and compressed into suns and planets? The most famous answer to this question is the "nebular hypothesis." Let us see, briefly, how it came into modern science.
We saw that some of the ancient Greek speculators imagined their infinite number of atoms as scattered originally, like dust, throughout space and gradually coming together, as dust does, to form worlds. The way in which they brought their atoms together was wrong, but the genius of Democritus had provided the germ of another sound theory to the students of a more enlightened age. Descartes (1596-1650) recalled the idea, and set out a theory of the evolution of stars and planets from a diffused chaos of particles. He even ventured to say that the earth was at one time a small white-hot sun, and that a solid crust had gradually formed round its molten core. Descartes had taken refuge in Sweden from his persecutors, and it is therefore not surprising that that strange genius Swedenborg shortly afterwards developed the same idea. In the middle of the eighteenth century the great French naturalist, Buffon, followed and improved upon Descartes and Swedenborg. From Buffon's work it was learned by the German philosopher Kant, who published (1755) a fresh theory of the concentration of scattered particles into fiery worlds. Then Laplace (1749-1827) took up the speculation, and gave it the form in which it practically ruled astronomy throughout the nineteenth century. That is the genealogy of the famous nebular hypothesis. It did not spring full-formed from the brain of either Kant or Laplace, like Athene from the brain of Zeus.
Laplace had one great advantage over the early speculators. Not only was he an able astronomer and mathematician, but by his time it was known that nebulae, or vast clouds of dispersed matter, actually existed in the heavens. Here was a solid basis for the speculation. Sir William Herschel, the most assiduous explorer of the heavens, was a contemporary of Laplace. Laplace therefore took the nebula as his starting-point.
A quarter of an ounce of solid matter (say, tobacco) will fill a vast space when it is turned into smoke, and if it were not for the pressure of the atmosphere it would expand still more. Laplace imagined the billions of tons of matter which constitute our solar system similarly dispersed, converted into a fine gas, immeasurably thinner than the atmosphere. This nebula would be gradually drawn in again by gravitation, just as the dust falls to the floor of a room. The collisions of its particles as they fell toward the centre would raise its temperature and give it a rotating movement. A time would come when the centrifugal force at the outer ring of the rotating disk would equal the centripetal (or inward) pull of gravity, and this ring would be detached, still spinning round the central body. The material of the ring would slowly gather, by gravitation, round some denser area in it; the ring would become a sphere; we should have the first, and outermost, planet circling round the sun. Other rings would successively be detached, and form the rest of the planets; and the sun is the shrunken and condensed body of the nebula.
So simple and beautiful a theory of the solar system could not fail to captivate astronomers, but it is generally rejected to-day, in the precise form which Laplace gave it. What the difficulties are which it has encountered, and the modifications it must suffer, we shall see later; as well as the new theories which have largely displaced it. It will be better first to survey the universe from the evolutionary point of view. But I may observe, in passing, that the sceptical remarks one hears at times about scientific theories contradicting and superseding each other are frivolous. One great idea pervades all the theories of the evolution of worlds, and that idea is firmly established. The stars and their planets are enormous aggregations of cosmic dust, swept together and compressed by the action of gravitation. The precise nature of this cosmic dust—whether it was gas, meteorites and gas, or other particles—is open to question.
As we saw in the first chapter, the universe has the word evolution written, literally, in letters of fire across it. The stars are of all ages, from sturdy youth to decrepit age, and even to the darkness of death. We saw that this can be detected on the superficial test of colour. The colours of the stars are, it is true, an unsafe ground to build upon. The astronomer still puzzles over the gorgeous colours he finds at times, especially in double stars: the topaz and azure companions in beta Cygni, the emerald and red of alpha Herculis, the yellow and rose of eta Cassiopeiae, and so on. It is at the present time under discussion in astronomy how far these colours are objective at all, or whether, if they are real, they may not be due to causes other than temperature. Yet the significance of the three predominating colours—blue-white, yellow, and red—has been sustained by the spectroscope. It is the series of colours through which a white-hot bar of iron passes as it cools. And the spectroscope gives us good ground to conclude that the stars are cooling.
When a glowing gas (not under great pressure) is examined by the spectroscope, it yields a few vertical lines or bars of light on a dark background; when a glowing liquid or solid is examined, it gives a continuous rainbow-like stretch of colour. Some of the nebulae give the former type of spectrum, and are thus known to be masses of luminous gas; many of the nebulae and the stars have the latter type of spectrum. But the stretch of light in the spectrum of a star is crossed, vertically, by a number of dark lines, and experiment in the laboratory has taught us how to interpret these. They mean that there is some light-absorbing vapour between the source of light and the instrument. In the case of the stars they indicate the presence of an atmosphere of relatively cool vapours, and an increase in the density of that atmosphere—which is shown by a multiplication and broadening of the dark lines on the spectrum—means an increase of age, a loss of vitality, and ultimately death. So we get the descending scale of spectra. The dark lines are thinnest and least numerous in the blue stars, more numerous in the yellow, heavy and thick in the red. As the body of the star sinks in temperature dense masses of cool vapour gather about it. Its light, as we perceive it, turns yellow, then red. The next step, which the spectroscope cannot follow, will be the formation of a scum on the cooling surface, ending, after ages of struggle, in the imprisonment of the molten interior under a solid, dark crust. Let us see how our sun illustrates this theory.
It is in the yellow, or what we may call the autumnal, stage. Miss Clerke and a few others have questioned this, but the evidence is too strong to-day. The vast globe, 867,000 miles in diameter, seems to be a mass of much the same material as the earth—about forty elements have been identified in it—but at a terrific temperature. The light-giving surface is found, on the most recent calculations, to have a temperature of about 6700 degrees C. This surface is an ocean of liquid or vaporised metals, several thousand miles in depth; some think that the brilliant light comes chiefly from clouds of incandescent carbon. Overlying it is a deep layer of the vapours of the molten metals, with a temperature of about 5500 degrees C.; and to this comparatively cool and light-absorbing layer we owe the black lines of the solar spectrum. Above it is an ocean of red-hot hydrogen, and outside this again is an atmosphere stretching for some hundreds of thousands of miles into space.
The significant feature, from our point of view, is the "sun-spot"; though the spot may be an area of millions of square miles. These areas are, of course, dark only by comparison with the intense light of the rest of the disk. The darkest part of them is 5000 times brighter than the full moon. It will be seen further, on examining a photograph of the sun, that a network or veining of this dark material overspreads the entire surface at all times. There is still some difference of opinion as to the nature of these areas, but the evidence of the spectroscope has convinced most astronomers that they are masses of cooler vapour lying upon, and sinking into, the ocean of liquid fire. Round their edges, as if responding to the pressure of the more condensed mass, gigantic spurts and mountains of the white-hot matter of the sun rush upwards at a rate of fifty or a hundred miles a second, Sometimes they reach a height of a hundred, and even two hundred, thousand miles, driving the red-hot hydrogen before them in prodigious and fantastic flames. Between the black veins over the disk, also, there rise domes and columns of the liquid fire, some hundreds of miles in diameter, spreading and sinking at from five to twenty miles a second. The surface of the sun—how much more the interior!—is an appalling cauldron of incandescent matter from pole to pole. Every yard of the surface is a hundred times as intense as the open furnace of a Titanic. From the depths and from the surface of this fiery ocean, as, on a small scale, from the surface of the tropical sea, the vapours rise high into the extensive atmosphere, discharge some of their heat into space, and sink back, cooler and heavier, upon the disk.
This is a star in its yellow age, as are Capella and Arcturus and other stars. The red stars carry the story further, as we should expect. The heavier lines in their spectrum indicate more absorption of light, and tell us that the vapours are thickening about the globe; while compounds like titanium oxide make their appearance, announcing a fall of temperature. Below these, again, is a group of dark red or "carbon" stars, in which the process is carried further. Thick, broad, dark lines in the red end of the spectrum announce the appearance of compounds of carbon, and a still lower fall of temperature. The veil is growing thicker; the life is ebbing from the great frame. Then the star sinks below the range of visibility, and one would think that we can follow the dying world no farther. Fortunately, in the case of Algol and some thirty or forty other stars, an extinct sun betrays its existence by flitting across the light of a luminous sun, and recent research has made it probable that the universe is strewn with dead worlds. Some of them may be still in the condition which we seem to find in Jupiter, hiding sullen fires under a dense shell of cloud; some may already be covered with a crust, like the earth. There are even stars in which one is tempted to see an intermediate stage: stars which blaze out periodically from dimness, as if the Cyclops were spending his last energy in spasms that burst the forming roof of his prison. But these variable stars are still obscure, and we do not need their aid. The downward course of a star is fairly plain.
When we turn to the earlier chapters in the life of a star, the story is less clear. It is at least generally agreed that the blue-white stars exhibit an earlier and hotter stage. They show comparatively little absorption, and there is an immense preponderance of the lighter gases, hydrogen and helium. They (Sirius, Vega, etc.) are, in fact, known as "hydrogen stars," and their temperature is generally computed at between 20,000 and 30,000 degrees C. A few stars, such as Procyon and Canopus, seem to indicate a stage between them and the yellow or solar type. But we may avoid finer shades of opinion and disputed classes, and be content with these clear stages. We begin with stars in which only hydrogen and helium, the lightest Of elements, can be traced; and the hydrogen is in an unfamiliar form, implying terrific temperature. In the next stage we find the lines of oxygen, nitrogen, magnesium, and silicon. Metals such as iron and copper come later, at first in a primitive and unusual form. Lastly we get the compounds of titanium and carbon, and the densely shaded spectra which tell of the thickly gathering vapours. The intense cold of space is slowly prevailing in the great struggle.
What came before the star? It is now beyond reasonable doubt that the nebula—taking the word, for the moment, in the general sense of a loose, chaotic mass of material—was the first stage. Professor Keeler calculated that there are at least 120,000 nebulae within range of our telescopes, and the number is likely to be increased. A German astronomer recently counted 1528 on one photographic plate. Many of them, moreover, are so vast that they must contain the material for making a great number of worlds. Examine a good photograph of the nebula in Orion. Recollect that each one of the points of light that are dotted over the expanse is a star of a million miles or more in diameter (taking our sun as below the average), and that the great cloud that sprawls across space is at least 10,000 billion miles away; how much more no man knows. It is futile to attempt to calculate the extent of that vast stretch of luminous gas. We can safely say that it is at least a million times as large as the whole area of our solar system; but it may run to trillions or quadrillions of miles.
Nearly a hundred other nebulae are known, by the spectroscope, to be clouds of luminous gas. It does not follow that they are white-hot, and that the nebula is correctly called a "fire-mist." Electrical and other agencies may make gases luminous, and many astronomers think that the nebulae are intensely cold. However, the majority of the nebulae that have been examined are not gaseous, and have a very different structure from the loose and diffused clouds of gas. They show two (possibly more, but generally two) great spiral arms starting from the central part and winding out into space. As they are flat or disk-shaped, we see this structure plainly when they turn full face toward the earth, as does the magnificent nebula in Canes Venatici. In it, and many others, we clearly trace a condensed central mass, with two great arms, each apparently having smaller centres of condensation, sprawling outward like the broken spring of a watch. The same structure can be traced in the mighty nebula in Andromeda, which is visible to the naked eye, and it is said that more than half the nebulae in the heavens are spiral. Knowing that they are masses of solid or liquid fire, we are tempted to see in them gigantic Catherine-wheels, the fireworks of the gods. What is their relation to the stars?
In the first place, their mere existence has provided a solid basis for the nebular hypothesis, and their spiral form irresistibly suggests that they are whirling round on their central axis and concentrating. Further, we find in some of the gaseous nebulae (Orion) comparatively void spaces occupied by stars, which seem to have absorbed the nebulous matter in their formation. On the other hand, we find (in the Pleiades) wisps and streamers of nebulous matter clinging about great clusters of stars, suggesting that they are material left over when these clustered worlds crystallised out of some vast nebula; and enormous stretches of nebulous material covering regions (as in Perseus) where the stars are as thick as grains of silver. More important still, we find a type of cosmic body which seems intermediate between the star and the nebula. It is a more or less imperfectly condensed star, surrounded by nebular masses. But one of the most instructive links of all is that at times a nebula is formed from a star, and a recent case of this character may be briefly described.
In February, 1901, a new star appeared in the constellation Perseus. Knowing what a star is, the reader will have some dim conception of the portentous blaze that lit up that remote region of space (at least 600 billion miles away) when he learns that the light of this star increased 4000-fold in twenty-eight hours. It reached a brilliance 8000 times greater than that of the sun. Telescopes and spectroscopes were turned on it from all parts of the earth, and the spectroscope showed that masses of glowing hydrogen were rushing out from it at a rate of nearly a thousand miles a second. Its light gradually flickered and fell, however, and the star sank back into insignificance. But the photographic plate now revealed a new and most instructive feature. Before the end of the year there was a nebula, of enormous extent, spreading out on both sides from the centre of the eruption. It was suggested at the time that the bursting of a star may merely have lit up a previously dark nebula, but the spectroscope does not support this. A dim star had dissolved, wholly or partially, into a nebula, as a result of some mighty cataclysm. What the nature of the catastrophe was we will inquire presently.
These are a few of the actual connections that we find between stars and nebulae. Probably, however, the consideration that weighs most with the astronomer is that the condensation of such a loose, far-stretched expanse of matter affords an admirable explanation of the enormous heat of the stars. Until recently there was no other conceivable source that would supply the sun's tremendous outpour of energy for tens of millions of years except the compression of its substance. It is true that the discovery of radio-activity has disclosed a new source of energy within the atoms themselves, and there are scientific men, like Professor Arrhenius, who attach great importance to this source. But, although it may prolong the limited term of life which physicists formerly allotted to the sun and other stars, it is still felt that the condensation of a nebula offers the best explanation of the origin of a sun, and we have ample evidence for the connection. We must, therefore, see what the nebula is, and how it develops.
"Nebula" is merely the Latin word for cloud. Whatever the nature of these diffused stretches of matter may be, then, the name applies fitly to them, and any theory of the development of a star from them is still a "nebular hypothesis." But the three theories which divide astronomers to-day differ as to the nature of the nebula. The older theory, pointing to the gaseous nebulae as the first stage, holds that the nebula is a cloud of extremely attenuated gas. The meteoritic hypothesis (Sir N. Lockyer, Sir G. Darwin, etc.), observing that space seems to swarm with meteors and that the greater part of the nebulae are not gaseous, believes that the starting-point is a colossal swarm of meteors, surrounded by the gases evolved and lit up by their collisions. The planetesimal hypothesis, advanced in recent years by Professor Moulton and Professor Chamberlin, contends that the nebula is a vast cloud of liquid or solid (but not gaseous) particles. This theory is based mainly on the dynamical difficulties of the other two, which we will notice presently.
The truth often lies between conflicting theories, or they may apply to different cases. It is not improbable that this will be our experience in regard to the nature of the initial nebula. The gaseous nebulae, and the formation of such nebulae from disrupted stars, are facts that cannot be ignored. The nebulae with a continuous spectrum, and therefore—in part, at least—in a liquid or solid condition, may very well be regarded as a more advanced stage of condensation of the same; their spiral shape and conspicuous nuclei are consistent with this. Moreover, a condensing swarm of meteors would, owing to the heat evolved, tend to pass into a gaseous condition. On the tether hand, a huge expanse of gas stretched over billions of miles of space would be a net for the wandering particles, meteors, and comets that roam through space. If it be true, as is calculated, that our 24,000 miles of atmosphere capture a hundred million meteors a day, what would the millions or billions of times larger net of a nebula catch, even if the gas is so much thinner? In other words, it is not wise to draw too fine a line between a gaseous nebula and one consisting of solid particles with gas.
The more important question is: How do astronomers conceive the condensation of this mixed mass of cosmic dust? It is easy to reply that gravitation, or the pressure of the surrounding ether, slowly drives the particles centre-ward, and compresses the dust into globes, as the boy squeezes the flocculent snow into balls; and it is not difficult for the mathematician to show that this condensation would account for the shape and temperature of the stars. But we must go a little beyond this superficial statement, and see, to some extent, how the deeper students work out the process.[1]
Taking a broad view of the whole field, one may say that the two chief difficulties are as follows: First, how to get the whole chaotic mass whirling round in one common direction; secondly, how to account for the fact that in our solar system the outermost planets and satellites do not rotate in the same direction as the rest. There is a widespread idea that these difficulties have proved fatal to the old nebular hypothesis, and there are distinguished astronomers who think so. But Sir R. Ball (see note), Professor Lowell (see note), Professor Pickering (Annals of Harvard College Observatory, 53, III), and other high authorities deny this, and work out the newly discovered movements on the lines of the old theory. They hold that all the bodies in the solar system once turned in the same direction as Uranus and Neptune, and the tidal influence of the sun has changed the rotation of most of them. The planets farthest from the sun would naturally not be so much affected by it. The same principle would explain the retrograde movement of the outer satellites of Saturn and Jupiter. Sir R. Ball further works out the principles on which the particles of the condensing nebula would tend to form a disk rotating on its central axis. The ring-theory of Laplace is practically abandoned. The spiral nebula is evidently the standard type, and the condensing nebula must conform to it. In this we are greatly helped by the current theory of the origin of spiral nebulae.
We saw previously that new stars sometimes appear in the sky, and the recent closer scrutiny of the heavens shows this occurrence to be fairly frequent. It is still held by a few astronomers that such a cataclysm means that two stars collided. Even a partial or "grazing" collision between two masses, each weighing billions of tons, travelling (on the average) forty or fifty miles a second—a movement that would increase enormously as they approach each other—would certainly liquefy or vaporise their substance; but the astronomer, accustomed to see cosmic bodies escape each other by increasing their speed, is generally disinclined to believe in collisions. Some have made the new star plunge into the heart of a dense and dark nebula; some have imagined a shock of two gigantic swarms of meteors; some have regarded the outflame as the effect of a prodigious explosion. In one or other new star each or any of these things may have occurred, but the most plausible and accepted theory for the new star of 1901 and some others is that two stars had approached each other too closely in their wandering. Suppose that, in millions of years to come, when our sun is extinct and a firm crust surrounds the great molten ball, some other sun approaches within a few million miles of it. The two would rush past each other at a terrific speed, but the gravitational effect of the approaching star would tear open the solid shell of the sun, and, in a mighty flame, its molten and gaseous entrails would be flung out into space. It has long been one of the arguments against a molten interior of the earth that the sun's gravitational influence would raise it in gigantic tides and rend the solid shell of rock. It is even suspected now that our small earth is not without a tidal influence on the sun. The comparatively near approach of two suns would lead to a terrific cataclysm.
If we accept this theory, the origin of the spiral nebula becomes intelligible. As the sun from which it is formed is already rotating on its axis, we get a rotation of the nebula from the first. The mass poured out from the body of the sun would, even if it were only a small fraction of its mass, suffice to make a planetary system; all our sun's planets and their satellites taken together amount to only 1/100th of the mass of the solar system. We may assume, further, that the outpoured matter would be a mixed cloud of gases and solid and liquid particles; and that it would stream out, possibly in successive waves, from more than one part of the disrupted sun, tending to form great spiral trails round the parent mass. Some astronomers even suggest that, as there are tidal waves raised by the moon at opposite points of the earth, similar tidal outbursts would occur at opposite points on the disk of the disrupted star, and thus give rise to the characteristic arms starting from opposite sides of the spiral nebula. This is not at all clear, as the two tidal waves of the earth are due to the fact that it has a liquid ocean rolling on, not under, a solid bed.
In any case, we have here a good suggestion of the origin of the spiral nebula and of its further development. As soon as the outbursts are over, and the scattered particles have reached the farthest limit to which they are hurled, the concentrating action of gravitation will slowly assert itself. If we conceive this gravitational influence as the pressure of the surrounding ether we get a wider understanding of the process. Much of the dispersed matter may have been shot far enough into space to escape the gravitational pull of the parent mass, and will be added to the sum of scattered cosmic dust, meteors, and close shoals of meteors (comets) wandering in space. Much of the rest will fall back upon the central body But in the great spiral arms themselves the distribution of the matter will be irregular, and the denser areas will slowly gather in the surrounding material. In the end we would thus get secondary spheres circling round a large primary.
This is the way in which astronomers now generally conceive the destruction and re-formation of worlds. On one point the new planetesimal theory differs from the other theories. It supposes that, since the particles of the whirling nebula are all travelling in the same general direction, they overtake each other with less violent impact than the other theories suppose, and therefore the condensation of the material into planets would not give rise to the terrific heat which is generally assumed. We will consider this in the next chapter, when we deal with the formation of the planets. As far as the central body, the sun, is concerned, there can be no hesitation. The 500,000,000 incandescent suns in the heavens are eloquent proof of the appalling heat that is engendered by the collisions of the concentrating particles.
In general outline we now follow the story of a star with some confidence. An internal explosion, a fatal rush into some dense nebula or swarm of meteors, a collision with another star, or an approach within a few million miles of another star, scatters, in part or whole, the solid or liquid globe in a cloud of cosmic dust. When the violent outrush is over, the dust is gathered together once more into a star. At first cold and attenuated, its temperature rises as the particles come together, and we have, after a time, an incandescent nucleus shining through a thin veil of gas—a nebulous star. The temperature rises still further, and we have the blue-hot star, in which the elements seem to be dissociated, and slowly re-forming as the temperature falls. After, perhaps, hundreds of millions of years it reaches the "yellow" stage, and, if it has planets with the conditions of life, there may be a temporary opportunity for living things to enjoy its tempered energy. But the cooler vapours are gathering round it, and at length its luminous body is wholly imprisoned. It continues its terrific course through space, until some day, perhaps, it again encounters the mighty cataclysm which will make it begin afresh the long and stormy chapters of its living history.
Such is the suggestion of the modern astronomer, and, although we seem to find every phase of the theory embodied in the varied contents of the heavens, we must not forget that it is only a suggestion. The spectroscope and telescopic photography, which are far more important than the visual telescope, are comparatively recent, and the field to be explored is enormous. The mist is lifting from the cosmic landscape, but there is still enough to blur our vision. Very puzzling questions remain unanswered. What is the origin of the great gaseous nebulae? What is the origin of the triple or quadruple star? What is the meaning of stars whose light ebbs and flows in periods of from a few to several hundred days? We may even point to the fact that some, at least, of the spiral nebulae are far too vast to be the outcome of the impact or approach of two stars.
We may be content to think that we have found out some truths, by no means the whole truth, about the evolution of worlds. Throughout this immeasurable ocean of ether the particles of matter are driven together and form bodies. These bodies swarm throughout space, like fish in the sea; travelling singly (the "shooting star"), or in great close shoals (the nucleus of a comet), or lying scattered in vast clouds. But the inexorable pressure urges them still, until billions of tons of material are gathered together. Then, either from the sheer heat of the compression, or from the formation of large and unstable atomic systems (radium, etc.), or both, the great mass becomes a cauldron of fire, mantled in its own vapours, and the story of a star is run. It dies out in one part of space to begin afresh in another. We see nothing in the nature of a beginning or an end for the totality of worlds, the universe. The life of all living things on the earth, from the formation of the primitive microbes to the last struggles of the superman, is a small episode of that stupendous drama, a fraction of a single scene. But our ampler knowledge of it, and our personal interest in it, magnify that episode, and we turn from the cosmic picture to study the formation of the earth and the rise of its living population.
- ↑ See, especially, Dr. P. Lowell, "The Evolution of Worlds" (1909). Professor S. Arrhenius, "Worlds in the Making" (1908), Sir N. Lockyer, "The Meteorite Hypothesis" (1890), Sir R. Ball, "The Earth's Beginning" (1909), Professor Moulton, "The Astrophysical Journal" (October, 1905), and Chamberlin and Salisbury, "Geology," Vol. II. (1903).