COSMOGONY
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
product of 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
cosmogony with 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 nebulae 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 nebulae, not as a
possible means of accounting for the birth and development of the solar
system, which constitutes Herschel's chief contribution to the nebular
hypothesis. Classifying the nebulae 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, when a 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 primal solar 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 of a 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 nebulae, 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 nebulae 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 nebulae
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 of such 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; and lesser 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 nebulae 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 progressive development 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 formulae, 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 mass was 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 small dimensions. 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 nebulae, originally single, but separated by a process allied to
that known as "fission" in protozoans. Indeed, Poincare 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 nebulae, 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 of the 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 multiple stars. 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."