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- Herausgeber: Science & Technology Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Stellar Evolution and Its Relations to Geological Time by Sir George Howard Darwin is a seminal work that explores the profound connections between the life cycles of stars and the vast epochs of geological history on Earth. Drawing from his expertise in astronomy and mathematics, Darwin delves into the processes governing the birth, development, and eventual demise of stars, and examines how these cosmic events are intricately linked to the evolution of our planet. The book discusses the physical laws underlying stellar formation, the transformation of matter and energy in the universe, and the implications of these processes for the age and development of the Earth. Darwin addresses the interplay between astronomical phenomena and geological changes, such as the cooling of the Earth, the formation of the solar system, and the timescales required for the emergence of life. He also considers the broader philosophical and scientific questions raised by the study of cosmic and terrestrial evolution, offering insights into the origins and destiny of both stars and planets. Rich in historical context and scientific analysis, Stellar Evolution and Its Relations to Geological Time stands as a foundational text for anyone interested in the intersection of astronomy, geology, and the history of science.
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STELLAR EVOLUTION
Climate and Time in their Geological Relations: A Theory of Secular Changes of the Earth’s Climate. By James Croll, of H. M. Geological Survey of Scotland. With Maps and Illustrations. 12mo. Cloth, $2.50.
Discussions on Climate and Cosmology. By James Croll, LL.D., F.R.S. With Chart. 12mo. Cloth, $2.00.
There are two, and only two, conceivable sources from which the prodigious amount of energy possessed by our sun and solar system can possibly have been derived. Not only are these two sources radically distinct in their essential nature, but both are admitted to be real and not merely hypothetical sources of energy. The one source is gravitation; the other, the source discussed in the present volume, a source to which attention was directed some twenty years ago. A most important distinction between these two sources is this: the amount of energy available from the former can be accurately determined, but such is not the case in regard to the latter. We can tell with tolerable certainty the greatest amount of energy which gravitation could possibly have conferred on the sun and solar system; but we have, at present, no means of assigning a limit to the possible amount which might have been derived from the other source. It may have been equal to that which gravitation could afford, or it may have been twofold, fourfold, or even tenfold that amount.
We have evidently in this case a means of determining which of the two sources will ultimately have to be adopted as the source to which the energy of our solar system must be referred. For if it can be proved from the admitted facts of geology, biology, and other sciences, that the amount of energy in the form of heat which has been radiated into space by the sun during geological time is far greater than the amount which could possibly have been derived from gravitation, this will undoubtedly show that gravitation cannot account for the energy originally possessed by our system.
The First Part of the volume is devoted to the consideration of what I believe to be the probable origin of meteorites, comets, and nebulæ, and of the real source from which our sun derived his energy. The facts which support the theory here advocated, together with the light which that theory appears to cast upon those facts, are next considered; and it will be found, I think, that the theory has been very much strengthened by the recent important spectroscopic researches of Mr. Lockyer and others in reference to the constitution of nebulæ. The Second Part of the work deals with the evidence in support of the theory derived from the testimony of geology and biology as to the age of the sun’s heat. The Third, and last, Part has been devoted to questions relating to the pre-nebular condition of the universe, and the bearing which these have on theories of stellar evolution. Several subjects introduced in this part are only very briefly treated. These will, however, be considered at greater length in a future volume, “Determinism, not Force, the Foundation-stone of Evolution,” a work of a more general and abstract character, which was commenced many years ago.
Perth: January 2, 1889.
PART I.
THE IMPACT THEORY OF STELLAR EVOLUTION.
PAGE
Consideration of the Facts which support the Theory, and of the Light which the Theory appears to cast upon the Facts
12
I. Probable Origin of Meteorites
12
II. Motion of the Stars; how of such different velocities, and always in straight lines
14
III. Motion of the Stars not due to their mutual attractions
14
IV. Probable Origin of Comets
17
V. Nebulæ
18
1. Origin of Nebulæ
18
2. How Nebulæ occupy so much space
18
3. Why Nebulæ are of such various shapes
19
4. Broken fragments in a Gaseous mass of an excessively high temperature the First stage of a Nebula
19
5. The Gaseous condition the Second stage of a Nebula
24
6. The Gaseous condition Essential to the Nebular Hypothesis
25
7. The mass must have possessed an excessive temperature
26
8. Gravitation could, under no possible condition, have generated the Amount of Heat required by the Nebular Hypothesis
27
9. Condensation the Third and last stage of a Nebula
30
10. How Nebulæ emit such feeble Light
30
VI. Binary Systems
32
VII. Sudden Outbursts of Stars
33
VIII. Star Clusters
34
IX. Age of the Sun’s Heat: a Crucial Test
34
PART II.
EVIDENCE IN SUPPORT OF THE THEORY
FROM THE AGE OF THE SUN’S HEAT.
Testimony of Geology and Biology as to the Age of the Sun’s Heat
37
Testimony of Geology: Method employed
39
The Average Rate of Denudation in the Past probably not much greater than at the Present
44
How the Method has been applied
47
Method as applied by Professor Haughton
50
Method as applied by Mr. Alfred R. Wallace
51
Method as applied directly
52
Evidence from “faults”
53
Time required to effect the foregoing amount of Denudation
62
Age of the Earth as determined by the Date of the Glacial Epoch
64
Testimony of Biology
65
PART III.
EVIDENCE IN SUPPORT OF THE THEORY
FROM THE PRE-NEBULAR CONDITION OF
THE UNIVERSE.
Professor A. Winchell on the pre-nebular condition of matter
71
Mr. Charles Morris on the pre-nebular condition of matter
75
Sir William R. Grove on the pre-nebular condition of matter
78
Evolution of the Chemical Elements, and its Relations to Stellar Evolution
80
Sir Benjamin Brodie on the pre-nebular condition of matter
84
Dr. T. Sterry Hunt on the pre-nebular condition of matter
85
Professor Oliver Lodge on the pre-nebular condition of matter
87
Mr. William Crookes on the pre-nebular condition of matter
90
Professor F. W. Clarke on the pre-nebular condition of matter
98
Dr. G. Johnstone Stoney on the pre-nebular condition of matter
99
The Impact Theory in relation to the foregoing Theories of the Pre-nebular Condition of Matter
102
The Theories do not account for the Motion of the Stars
105
The Theories do not account for the Amount of Heat required
106
Evolution of Matter
107
Objection considered
109
Can we on Scientific grounds trace back the Evolution of the Universe to an Absolute First condition?
110
PART I.
Upwards of twenty years ago[1] the theory—or, I should rather say, the hypothesis—was advanced[2] that our sun was formed from a hot gaseous nebula produced by the colliding of two dark stellar masses; and that, as the stars are suns like our own, they in all likelihood had a similar origin. The probability of this theory has been very much strengthened by the facts, both astronomical and physical, which have accumulated since the theory was enunciated. Before proceeding to the consideration of these facts, and the conclusions to which they lead, it will be necessary to give a statement of the fundamental principles of the theory.
In the theory here discussed the truth of the nebular hypothesis, which begins by assuming the existence of a solar nebulous mass, is taken for granted. The present theory deals not so much with the nebulous mass itself as with the formation of the nebula, and with those causes which led to its formation. For convenience of reference, and to prevent confusion, I have called it the “Impact Theory,” by which name it may be distinguished, on the one hand, from the nebular theory, and, on the other hand, from the meteoric theory, and all other theories which regard gravitation as the primary source of the solar energy.
The theory starts with the assumption that the greater part of the energy possessed by the universe exists or is stored up in the form of the motion of stellar masses. The amount of energy which may thus be stored up is startling to contemplate. Thus a mass equal to that of the sun, moving with a velocity of 476 miles per second, would possess, in virtue of that motion, energy sufficient, if converted into heat, to maintain the present rate of the sun’s radiation for 50,000,000 years.[3] There is nothing extravagant in the assumption of such a velocity. A comet, for example, having an orbit extending to the path of the planet Neptune, approaching so near the sun as to almost graze his surface in passing, would have a velocity within 86 miles of what we have assumed. Twice this assumed velocity would give 200,000,000 years’ heat; four times the velocity would give 800,000,000 years’ heat; and so on.
We are at perfect liberty to begin by assuming the existence of stellar masses in motion; for we are not called upon to explain how the masses obtained their motion, any more than we have to explain how they came to have their existence. If the masses were created, they may as likely have been created in motion as at rest; and if they were eternal, they may as likely have been eternally in motion as eternally at rest.
Eternal motion is just as warrantable an assumption as eternal matter. When we reflect that space is infinite—at least in thought—and that, for aught we know to the contrary, bodies may be found moving throughout its every region, we see that the amount of energy may be perfectly illimitable.
But, illimitable as the amount of the energy may be, it could be of no direct service while it existed simply as the motion of stellar masses. The motion, to be available, must be transformed into heat: the motion of translation into molecular, or some other form of motion. This can be done in no other way than by arresting the motion of the masses. But how is such motion to be arrested? How are bodies as large as our earth, moving at the rate of hundreds of miles per second, to have their motion stopped? According to the theory this is effected by collision: by employing the motion of the one body to arrest that of the other.
Take the case of the formation of our sun according to the theory. Suppose two bodies, each one-half of the mass of the sun, moving directly towards each other with a velocity of 476 miles per second. These bodies would, in virtue of that velocity, possess 4149 × 1038 foot-pounds of energy, which is equal to 100,000,000,000 foot-pounds per pound of the mass; and this, converted into heat by the stoppage of their motions, would suffice to maintain, as was previously stated, the present rate of the sun’s radiation for a period of 50,000,000 years. It must be borne in mind that, while 476 miles per second is the velocity at the moment of collision, more than one-half of this would be derived from the mutual attraction of the two bodies in their approach to each other.
Coming in collision with such a velocity, the result would inevitably be that the two bodies would shatter each other to pieces. But, although their onward motions would thus be stopped, it is absolutely impossible that the whole of the energy of their motions could be at once converted into heat; and it is equally impossible that it could be annihilated. Physical considerations enable us to trace, though in a rough and general way, the results which would necessarily follow. The broken fragments, now forming one confused mass, would rebound against one another, breaking up into smaller fragments, and flying off in all directions. As these fragments receded from the centre of dispersion they would strike against each other, and, by their mutual impact, become shivered into still smaller fragments, which would in turn be broken up into fragments yet smaller, and so on as they proceeded outwards. This is, however, only one part of the process, and a part which would certainly take place, though no heat were generated by the collisions.
A far more effective means of dispersing the fragments and shattering them to pieces would be the expansive force of the enormous amount of incandescent gas almost instantaneously generated by the heat of collision. The general breaking up of the two masses and the stoppage of their motions would be the work of only a few minutes, or a few hours at most. The heat evolved by the arrested motion would, in the first instance, be mainly concentrated on the surface layers of the broken blocks. The layers would be at once transformed into the gaseous condition, thus enveloping the blocks and filling the interspaces. It is difficult to determine what the temperature and expansive force of this gas would at the moment be, but evidently it would be excessive; for, were the whole of the heat of the arrested motion distributed over the mass, it would, as has been stated, amount to 100,000,000,000 foot-pounds per pound of the mass—an amount sufficient to raise 264,000 tons of iron 1° C. Thus, if we assume the specific heat of the gas to be equal to that of air (viz. ·2374), it would have a temperature of about 300,000,000° C. or more than 140,000 times that of the voltaic arc.
I hardly think it will be deemed extravagant to assume that at the moment after impact the temperature of the evolved gas would be at least as great as here stated. If we assume it to be so, it is obvious that the broken mass would, by the expansive force of the generated gas, be dispersed in all directions, breaking up into fragments smaller and smaller as they knocked against one another in their progress outwards from the centre of dispersion; and these fragments would, at the same time, become gradually converted into the gaseous state, and gradually come to occupy a space as large as that embraced in our solar system. In the course of time the whole would assume the gaseous condition, and we should then have a perfect nebula—intensely hot, but not very luminous. As its temperature diminished, the nebulous mass would begin to condense, and ultimately, according to the well-known nebular hypothesis, pass through all the different phases of rings, planets, and satellites into our solar system as it now exists.
I am glad to find that the theory, in one of its main features, has been adopted by Sir William Thomson,[4] the highest authority we have on all points relating to the source of the sun’s heat.
“We cannot,” says Sir William, “help asking the question, What was the condition of the sun’s matter before it came together and became hot? (1) It may have been two cool, solid masses, which collided with the velocity due to their mutual gravitation; or (2), but with enormously less of probability, it may have been two masses colliding with velocities considerably greater than the velocities due to their mutual gravitation.”
He adopts the first of these suppositions. “To fix the idea,” he continues, “think of two cool, solid globes, each of the same mean density as the earth, and of half the sun’s diameter, given at rest, or nearly at rest, at a distance asunder equal to twice the earth’s distance from the sun. They will fall together and collide in exactly half a year. The collision will last for about half an hour, in the course of which they will be transformed into a violently agitated incandescent fluid mass flying outward from the line of the motion before the collision, and swelling to a bulk several times greater than the sum of the original bulks of the two globes. How far the fluid mass will fly out all around from the line of collision it is impossible to say. The motion is too complicated to be fully investigated by any known mathematical method; but with sufficient patience a mathematician might be able to calculate it with some fair approximation to the truth. The distance reached by the extreme circular fringe of the fluid mass would probably be much less than the distance fallen by each globe before the collision, because the translational motion of the molecules constituting the heat into which the whole energy of the original fall of the globes becomes transformed in the first collision is probably about three-fifths of the whole amount of that energy. The time of flying out would probably be less than half a year, when the fluid mass must begin to fall in again towards the axis. In something less than a year after the first collision the fluid will again be in a state of maximum crowding round the centre, and this time probably even more violently agitated than it was immediately after the first collision; and it will again fly outward, but this time axially towards the places whence the two globes fell. It will again fall inwards, and after a rapidly subsiding series of quicker and quicker oscillations it will subside, probably in the course of two or three years, into a globular star of about the same dimensions, heat, and brightness, as our present sun, but differing from him in this, that it will have no rotation.”[5]
This is precisely what I have been contending for during the past twenty years, with the simple exception that I assume, according to his second supposition, that the “two masses collided with velocities considerably greater than the velocities due to mutual gravitation.” Sir William admits, of course, my supposition to be quite a possible one, but rejects it on the supposed ground of its improbability. His reasons for this, stated in his own words, are as follows:
“This last supposition implies that, calling the two bodies A and B for brevity, the motion of the centre of inertia of B relatively to A must, when the distance between them was great, have been directed with great exactness to pass through the centre of inertia of A; such great exactness that the rotational momentum or moment of momentum after collision was no more than to let the sun have his present slow rotation when shrunk to his present dimensions. This exceedingly exact aiming of the one body at the other, so to speak, is, on the dry theory of probability, exceedingly improbable. On the other hand, there is certainty that the two bodies A and B at rest in space if left to themselves, undisturbed by other bodies and only influenced by their mutual gravitation, shall collide with direct impact, and therefore with no motion of their centre of inertia, and no rotational momentum of the compound body after the collision. Thus we see that the dry probability of collision between two neighbours of a vast number of mutually attracting bodies widely scattered through space is much greater if the bodies be all given at rest than if they be given moving in any random directions and with any velocities considerable in comparison with the velocities which they would acquire in falling from rest into collision.”
Sir William here argues that the second supposition is far less probable than the first, because, according to it, the motion of the one body relatively to the other must, in order to strike, be directed with great exactness. The result, in such a case, is that collision will rarely occur; whereas, according to the first supposition, the two bodies starting from a state of rest will, by their mutual gravitation, inevitably collide. According to the second hypothesis they will generally miss; according to the first they will always collide.
