Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
A single underscore after a symbol indicates a
subscript.
Carat symbol “^” designates a superscript.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Typographical errors have been silently corrected.
The numerical references to the NOTES, e. g. [1] have been changed to
“(see NOTE x)” in order to avoid confusion with the true footnotes.
THE EVOLUTION OF WORLDS
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[Illustration: SATURN—PHOTOGRAPHED AT THE LOWELL OBSERVATORY
BY MR. E. C. SLIPHER. SEPTEMBER, 1909.]
THE EVOLUTION OF WORLDS
BY
PERCIVAL LOWELL, A.B., LL.D.
AUTHOR OF “MARS AND ITS CANALS,” “MARS AS THE
ABODE OF LIFE,” ETC.
DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT
PROFESSOR OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF
TECHNOLOGY; FELLOW OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES;
MEMBRE DE LA SOCIÉTÉ ASTRONOMIQUE DE FRANCE; MEMBER OF THE
ASTRONOMICAL AND ASTROPHYSICAL SOCIETY OF AMERICA; MITGLIED
DER ASTRONOMISCHE GESELLSCHAFT; MEMBRE DE LA SOCIÉTÉ
BELGE D’ASTRONOMIE; HONORARY MEMBER OF THE SOCIEDAD
ASTRONOMICA DE MEXICO; JANSSEN MEDALLIST OF THE
SOCIÉTÉ ASTRONOMIQUE DE FRANCE, 1904, FOR
RESEARCHES ON MARS; MEDALLIST OF THE
SOCIEDAD ASTRONOMICA DE MEXICO FOR
STUDIES ON MARS, 1908
_ILLUSTRATED_
New York
THE MACMILLAN COMPANY
1909
_All rights reserved_
COPYRIGHT, 1909,
BY THE MACMILLAN COMPANY.
Set up and electrotyped. Published December, 1909.
Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.
TO
THE PRESIDENT OF THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
TO MY COLLEAGUES THERE
AND TO ITS STUDENT BODY
TO WHOSE INTEREST AND ATTENTION THESE
LECTURES ARE INDEBTED
THEY ARE APPRECIATIVELY INSCRIBED
“Si je n’étais pas devenu général en chef et l’instrument du sort
d’un grand people, j’aurais couru les bureaux et les salons pour me
mettre dans la dépendance de qui que ce fût, en qualité de ministre
ou d’ambassadeur? Non, non! je me serais jeté dans l’étude des
sciences exactes. J’aurais fait mon chemin dans la route des
Galilée, des Newton. Et puisque j’ai réussi constamment dans mes
grandes entreprises, eh bien, je me serais hautement distingué aussi
par des travaux scientifiques. J’aurais laissé le souvenir de belles
découvertes. Aucune autre gloire n’aurait pu tenter mon ambition.”
—NAPOLEON Iᴱᴿ, QUOTED BY ARAGO.
The substance of the following pages was written and presented in
a university course of lectures before the Massachusetts Institute
of Technology—in February and March of this year. The kind interest
with which the lectures were received, not only by the students and
professional bodies, but by the public, was followed by an immediate
request from The Macmillan Company to issue them in book form, and as
such they now appear.
PERCIVAL LOWELL.
BOSTON, MASS., May 29, 1909.
CONTENTS
CHAPTER PAGE
I. BIRTH OF A SOLAR SYSTEM 1
II. EVIDENCE OF THE INITIAL CATASTROPHE IN OUR OWN CASE 31
III. THE INNER PLANETS 58
IV. THE OUTER PLANETS 94
V. FORMATION OF PLANETS 127
VI. A PLANET’S HISTORY—SELF-SUSTAINED STAGE 155
VII. A PLANET’S HISTORY—SUN-SUSTAINED STAGE 182
VIII. DEATH OF A WORLD 213
NOTES
1. METEOR ORBITS 241
2. DENSITIES OF THE PLANETS 243
3. VARIATION IN SPECTROSCOPIC SHIFT 243
4. ON THE PLANETS’ ORBITAL TILTS 244
5. PLANETS AND THEIR SATELLITE SYSTEMS 245
6. ON THE INDUCED CIRCULARITY OF ORBITS THROUGH COLLISION 250
7. CAPTURE OF SATELLITES 251
INDEX 253
LIST OF ILLUSTRATIONS
PLATES
I. Saturn _Frontispiece_
OPPOSITE PAGE
II. The Moving Nebula surrounding Nova Persei, 1901-1902 14
III. Representative Stellar Spectra 24
IV. Spectra of the Major Planets 52
V. Venus, 1896-1897 82
VI. Asteroids: Major Axes of Orbits 98
VII. Saturn—A Drawing showing Agglomerations 108
VIII. Spectrogram of Jupiter, Moon Comparison 152
IX. Spectrogram showing Water-vapor in Atmosphere of Mars 160
X. Tree Fern 176
XI. Ten Views of Mercury, showing Effect of Libration 222
XII. Spectrogram of Saturn 232
CUTS APPEARING IN TEXT
PAGE
Algol and its Dark Companion 4
Nova Persei 11
Spectrum of Nova Persei 12
The Moving Nebula surrounding Nova Persei, 1901 13
Great Nebula in Orion 17
Great Nebula in Andromeda 18
Nebula M. 100 Comæ 19
Nebula ♅ I. 226 Ursæ Majoris 20
Nebula ♅ V. 24 Comæ. Showing Globular Structure 21
Nebula M. 101 Ursæ Majoris 23
The Radiant of a Meteoric Shower 37
Diagram explaining Proportionate Visibility of Meteors 38
The Mart Iron 41
Section of Meteorite showing Widmannstättian Lines 42
Meteorite, Toluca 43
Nebula ♅ V. 14 Cygni 45
Nebula N.G.C. 1499 Persei 46
Nebula N.G.C. 6960 in Cygnus 47
Nebula M. 51 Canum Venaticorum 48
Orbits of the Inner Planets 59
Sulla Rotazione di Mercurio.—Di G. V. Schiaparelli 64
Map of Mercury. Lowell 69
Venus. October, 1896-March, 1897 78
Venus. April 12, 1909. 79
Diagram: Convection Currents in Atmosphere of Venus 81
Diagram: Shift in Central Barometric Depression 81
Spectrogram of Venus, showing its Long Day 87
Spectrogram of Jupiter, giving the Length of its Day by the
Tilt of its Spectral Lines 89
Orbits of the Outer Planets 95
Drawing of Jupiter showing its Ellipticity 103
Two Drawings of Jupiter and its Wisps 105
Photograph of Jupiter, 1909 107
Diagram of Saturn’s Rings 113
The Tores of Saturn 114
Chart showing increasing Tilts of the Major Planets 131
Orbital Tilts and Eccentricities of Satellites 133
Masses of Planets and Satellites 136
Two Drawings of Jupiter and its “Great Red Spot” 164
Sun Spots 165
Photograph of a Sun Spot 166
The Volcano Colima, Mexico, March 24, 1903 169
Jukes Butte, a Denuded Laccolith, as seen from the Northwest 170
Ideal Section of a Laccolith 170
Earth as seen from above.—Photographed at an Altitude of
5500 Feet 183
Tracks of Sauropus Primævus 188
Adventures of a Heat Ray 193
Polar Caps of Mars at their Maxima and Minima 198
Glacial Map of Eurasia 200
Map showing the Glaciated Area of North America 201
Photograph of the Moon 205
Petrified Bridge, Third Petrified Forest, near Adamana,
Arizona 210
Three Views of Venus, showing Agreement at Different
Distances 220
Diagram of Libration in Longitude due to Rotation 222
Moon,—Full and Half 225
Diagram illustrating Molecular Motion in a Gas 227
Distribution of Molecular Velocities in a Gas 229
THE EVOLUTION OF WORLDS
CHAPTER I
BIRTH OF A SOLAR SYSTEM
Astronomy is usually thought of as the study of the bodies visible in
the sky. And such it largely is when the present state of the universe
alone is considered. But when we attempt to peer into its past and
to foresee its future, we find ourselves facing a new side of the
heavens—the contemplation of the invisible there. For in the evolution
of worlds not simply must the processes be followed by the mind’s
eye, so short the span of human life, but they begin and end in what
we cannot see. What the solar system sprang from, and what it will
eventually become, is alike matter devoid of light. Out of darkness
into darkness again: such are the bourns of cosmic action.
The stars are suns; past, present, or potential. Each of those diamond
points we mark studding the heavens on a winter’s night are globes
comparable with, and in many cases greatly excelling, our own ruler of
the day. The telescope discloses myriads more. Yet these self-confessed
denizens of space form but a fraction of its occupants. Quite as
near, and perhaps much nearer, are orbs of which most of us have no
suspicion. Unimpressing our senses and therefore ignored by our minds,
bodies people it which, except for rare occurrences, remain forever
invisible. For dark stars in countless numbers course hither and
thither throughout the universe at speeds as stupendous as the lucent
ones themselves.
Had we no other knowledge of them, reasoning would suffice to
demonstrate their existence. It is the logic of unlimited subtraction.
Every self-shining star is continually giving out light and heat.
Now such an expenditure cannot go on forever, as the source of its
replenishing by contraction, accretion, or disintegration is finite.
Long to our measures of time as the process may last, it must
eventually have an end and the star finally become a cold dark body,
pursuing as before its course, but in itself inert and dead; an orb
grown _orbéd_, in the old French sense. So it must remain unless some
cosmic catastrophe rekindle it to life. The chance of such occurrence
in a given time compared with the duration of the star’s light-emitting
career will determine the number of dark stars relative to the lucent
ones. The chance is undoubtedly small, and the number of dark bodies
in space proportionally large. Reasoning, then, informs us first that
such bodies must exist all about us, and second that their multitude
must be great.
Valid as this reasoning is, however, we are not left to inference
for our knowledge of them. There is a certain star amid the polar
constellations known as Algol,—el Ghoul, the Arabs called it, or The
Dæmon. The name shows they noticed how it winked its eye and recognized
something sarcastically sinister in its intent. For once in two days
and twenty hours its light fades to one-third of its usual amount,
remains thus for about twenty minutes, and then slowly regains its
brightness. Seemingly unmoved itself, its steady blinking from the time
man first observed it took on an uncanniness he felt. To untelescoped
man it certainly seemed demoniacal, this punctual recurrent wink.
Spectroscoped man has learnt its cause.
Goodricke in 1795 divined it, and research since has confirmed his keen
intuition. Its loss of light is occasioned by the passing in front of
it of a dark companion almost of its own size revolving about it in
a close elliptic orbit. That this is the explanation of its strange
behavior, the shift of its spectral lines makes certain, by showing
that the bright star is receding from us at twenty-seven miles a second
seventeen hours before the eclipse and coming towards us at about the
same rate seventeen hours after it; its dark companion, therefore,
doing the reverse.
Algol is no solitary specimen of a mind-seen invisible star. Many
eclipsing binaries of the same class are now known; and considering
that the phenomenon could not be disclosed unless the orbital plane
of the pair traversed the observer’s eye, an unlikely chance in a
fortuitous distribution, we perceive how many such in truth there must
be which escape recognition for their tilt.
[Illustration: ALGOL AND ITS DARK COMPANION,
AS SEEN FROM THE EARTH,]
[Illustration: AS SEEN FROM ABOVE ORBIT.]
But if dark stars exist in connection with lucent ones, there must be
many more that travel alone. Our own Sun is an instance in embryo. If
he live long enough, he will become such a solitary shrouded tramp in
his old age. For he has no companion to betray him. The only way in
which we could become cognizant of these wanderers would be by their
chance collision with some other star, dark or lucent as the case might
be. The impact of the catastrophe would generate so much light and heat
that the previously dark body would be converted into a blazing sun and
a new star make its advent in the sky.
Star births of the sort have actually been noted. Every now and then a
new star suddenly appears in the firmament—a nova as it is technically
called. These apparitions date from the dawn of astronomic history.
The earliest chronicled is found in the Chinese Annals of 134 B.C. It
shone out in Scorpio and was probably the new star which Pliny tells us
incited Hipparchus, “The Father of Astronomy,” to make his celebrated
catalogue of stars. From this time down we have recorded instances of
like character.
One of the most famous was the “Pilgrim Star” of Tycho Brahe. That
astronomer has left us a full account of it. “While I was living,” he
tells us, “with my uncle in the monastery of Hearitzwadt, on quitting
my chemical laboratory one evening, I raised my eyes to the well-known
vault of heaven and observed, with indescribable astonishment, near the
zenith, in Cassiopeia, a radiant fixed star of a magnitude never before
seen. In my amazement I doubted the evidence of my senses. However,
to convince myself that it was no illusion, and to have the testimony
of others, I summoned my assistants from the laboratory and inquired
of them, and of all the country people that passed by, if they also
observed the star that had thus suddenly burst forth. I subsequently
heard that in Germany wagoners and other common people first called the
attention of astronomers to this great phenomenon in the heavens,—a
circumstance which, as in the case of non-predicted comets, furnished
fresh occasion for the usual raillery at the expense of the learned.”
The new star, he informs us, was just like all other fixed stars, but
as bright as Venus at her brightest. Those gifted with keen sight could
discern it in the daytime and even at noon. It soon began to wane.
In December, 1572, it resembled Jupiter, and a year and three months
later had sunk beyond recognition to the naked eye. It changed color
as it did so, passing from white through yellow to red. In May, 1573,
it returned to yellow (“the hue of Saturn,” he expressly states), and
so remained till it disappeared from sight, scintillating strongly in
proportion to its faintness.
Thirty-two years later another stranger appeared and was seen by
Kepler, who wrote a paper about it entitled “The New Star in the Foot
of the Serpent.” It shone out in the same sudden manner and faded in
the same leisurely way.
Since 1860 there have been several such apparitions, and since 1876
it has been possible to study them with the spectroscope, which has
immensely increased our knowledge of their constitution. Indeed, this
instrument of research has really opened our eyes to what they are.
Nova Cygni, in 1876, Nova Aurigæ, in 1892, and Nova Persei, in 1901,
besides several others found by Mrs. Fleming on the Arequipa plates,
were excellent examples, and all agreed in their main features,
showing that novæ constitute a type of stars by themselves, whose
appearing in the first place and whose behavior afterwards prove them
to have started from like cause and to have pursued parallel lines of
development.
As a typical case we may review the history of Nova Aurigæ. On February
1, 1892, an anonymous post-card was received by Dr. Copeland of the
Royal Observatory, Edinburgh, that read as follows: “Nova in Aurigæ.
In Milky Way, about 2° south of χ Aurigæ, preceding 26 Aurigæ. Fifth
magnitude slightly brighter than χ.” The observatory staff at once
looked for the nova and easily found it with an opera glass. They then
examined it through a prism placed before their 24-inch reflector and
found its spectrum. It proved to be that of a “blaze star.”
Dr. Thomas D. Anderson turned out to be the writer of the anonymous
post-card—his name modestly self-obliterated by the nova’s light.
He had detected the star on January 24, but had only verified it as
a new one on the 31st. Harvard College Observatory then looked up
its archived plates. The plates showed that it had appeared sometime
between December 1 and 10. Its maximum had been attained on December
20, after which it declined, to record apparently another maximum on
February 3 of the 3.5 magnitude. From this time its light steadily
waned till on April 1 it was only of the 16th magnitude or ¹/₁₀₀₀₀₀ of
what it had been. In August it brightened again and then waned once
more.
Meanwhile its spectrum underwent equally strange fluctuations. At
first it exhibited the bright lines characteristic of the flaming red
solar prominences, the calcium, hydrogen, and helium lines flanked by
their dark correlatives upon a continuous background, showing that
both glowing and cooler gases were here concerned. The sodium lines,
too, appeared, like those that come out in comets as they approach the
furnace of the Sun. An outburst such as occurs in miniature in the
solar chromosphere or outermost gaseous layer of the Sun was here going
on upon a gigantic scale. A veritable spectral chaos next supervened,
staying until the star had practically faded away. Then, on its
reappearance, in August, Holden, Schaeberle, and Campbell discovered
to their surprise not what had been at all, but something utterly new:
the soberly bright lines only of a nebula. Finally, ten years later,
January, 1902, Campbell found its spectrum had become continuous, the
body having reverted to the condition of a star.
Now how are we to interpret these grandiose vicissitudes, visually and
spectrally revealed? That we witnessed some great catastrophe is clear.
The sudden increase of light of many thousand fold from invisibility to
prominence shows that a tremendous cataclysm occurred. The bright lines
in the spectrum confirm it and imply that vast upheavals like those
that shake the Sun were there in progress, but on so stupendous a scale
that, if for no other reason, we must dismiss the idea that explosions
alone can possibly be concerned. The dark correlatives of the bright
lines have been interpreted as indicating that two bodies were
concerned, each travelling at velocities of hundreds of miles a second.
But in Nova Aurigæ shiftings of the spectral lines implying six bodies
at least were recorded, if such be attributed to motion in the line of
sight, and Vogel was minded to throw in a few planets as well—as Miss
Clerke pithily puts it. There is not room for so many on the stage of
the cosmic drama. Other causes, as we now know, may also displace the
spectral lines. Great pressure has been shown to do it, thanks to the
labors of Humphreys and Mohler at Baltimore. “Anomalous refraction” may
do it, as Professor Julius of Utrecht has found out. Finally, changes
of density may produce it, as Michelson has discovered. To these
causes we may confidently ascribe most of the shiftings in the stellar
spectrum, for just such forces must be there at work.
Mr. Monck suggested the idea that new stars are the result of old dark
stars rushing through gaseous fields in space and rendered luminous
by the encounter. Seeliger revived and developed this idea, which in
certain cases is undoubtedly the truth. Probably this occurred to the
new star of 1885 which suddenly blazed out almost in the centre of the
great nebula in Andromeda. It behaved like a typical nova and in due
course faded to indistinguishability. Something like it happened, too,
in the nova of 1860, which suddenly flared up in the star cluster 80
Messier, outdoing in lustre the cluster itself, and then, too, faded
away.
But just as psychology teaches us that not only do we cry because we
are sorrowful, but that we are sorrowful because we cry, so while a
nova may be made by a nebula, no less may a nebula be made by a star.
Let us see how this might be brought about and what sign manuals it
would present. Suppose that the two bodies actually grazed. Then the
disruption would affect the star’s cuticle, first raising the outer
parts, consisting rather of carbon than of the metals, since that
substance is the lighter, to intense heat and the gases about it at
the same time. The glowing carbon would be intensely bright, and at
first its light would overpower that from the gases, and not till its
great glow had partially subsided would theirs be seen. Then the gases,
hydrogen, helium, and so forth, would make themselves evident. Finally
only the most tenuous ones, those peculiar to a nebula, would remain
visible. After which the more solid particles due to the disruption
would fall together and light up again by their individual collisions.
Much the same would result if without striking the stars passed close.
[Illustration: 1901 February 20th 1901 February 28th
Before appearance of Nova The Nova
NOVA PERSEI. Photographs by A. STANLEY WILLIAMS, Hove, Sussex.]
[Illustration: SPECTRUM OF NOVA PERSEI. (F. Ellerman, 40 in. Yerkes.)]
Now to put this theory to the proof. In the early morning of the 22d of
February, 1901, Dr. Anderson, the discoverer of Nova Aurigæ, perceived
that Algol had a neighbor, a star as bright as itself, which had never
been there before. Within twenty-four hours of its detection the
newcomer rivalled Capella, and shortly after took rank as the premier
star of the northern hemisphere. Its spectrum on the 22d was found at
Harvard College Observatory to be like that of Rigel, a continuous one
crossed by some thirty faint dark lines. On the 24th, however, _so soon
as it began to wane_, the bright lines of hydrogen were conspicuous
with their dark correlatives, just as they had been with Nova Aurigæ
and other novæ. At the same time each particular spectral line proved a
law unto itself, some shifted more than others, thus negativing motion
as their only cause and indicating change of pressure or density as
concerned concomitants of the affair. Blue emissions like those of
Wolf-Rayet stars next made their appearance; then a band, found by
Wright at the Lick to characterize nebulæ, shone out, and finally in
July the change to a nebular spectrum stood complete.
[Illustration: THE MOVING NEBULA SURROUNDING NOVA PERSEI.
1901, September 20th. 1901, November 13th.
Drawn by G. W. RITCHEY, from Photographs taken with the 24-in.
Reflector, YERKES OBSERVATORY.]
Then came what is the most suggestive feature in the whole event. On
August 22 and 23 Dr. Wolf at Königstahl took with his then new Bruce
objective some long exposure plates of the nova, and on them found, to
his surprise, wisps of nebulous matter to the southeast of the star. On
September 20 Ritchey, with a two-foot mirror of his own constructing
exposed for four hours, brought the whole formation to light. It turned
out to be a spiral nebula encircling and apparently emanating from
the star. Its connection with the nova was patent. But there was more
to come. Later plates taken at the Lick on November 7 disclosed the
startling fact that the nebula was visibly expanding, uncoiling outward
from the star. A plate by Ritchey on November 13 confirmed this, and
still later plates by him in December, January, and February showed the
motion to be progressive. At the same time the star showed no parallax,
and the speed of the motion seemed thus to be indicated as enormous.
Kapteyn suggested to account for it that appearance, not reality, was
here concerned; that the nebula had always existed, and was only shown
up by the light from the conflagration travelling outward from the nova
at the rate of one hundred and eighty-six thousand miles a second. This
would make the catastrophe to have occurred as far back as the time of
James I, of which the news more truthful but less timely than that of
the morning papers had only just reached us.
[Illustration: December 14, 1901.]
[Illustration: January 7 and 9, 1902.]
[Illustration: 1902. February 8, 1902.
THE MOVING NEBULA SURROUNDING NOVA PERSEI—AFTER RITCHEY.]
But a little of that simple reasoning by which Zadig recovered the
lost horses of the Sultan, and which from its unaccustomedness in the
affairs of men got him suspected of having stolen them and very nearly
caused his death, will show the untenableness of this idea and help us
to a solution. In the first place we note that the star holds the very
centre of the nebular stage, a remarkable prominence if the star has
no creative right to the position. Then the same knots and patches of
the nebulous configuration are visible in all the photographs, in the
same relative positions, turned through corresponding angles as one
will see for himself, all having moved symmetrically from one date to
another. At the truly marvellous mimicry implied if different objects
were concerned common sense instinctively shies, and very properly,
as the chances against it are millions to one. Clearly it was not a
mere matter of ethereal motion, but a very material motion of matter,
which was here concerned. Something corpuscular emanating from the nova
spread outward into space.
Clinching this conclusion is the result of a search by Perrine for
traces of the nebula on earlier plates. For on one taken by him on
March 29 (1901) he found the process already started in two close
coils, its conception thus clearly dating from the time of the star’s
outburst. In Nova Persei, then, we actually witnessed a spiral nebula
evolved from a disrupted star.
What was this ejectum and what drove it forth? Professor Very regarded
it as composed of corpuscles such as give rise to cathode rays
discharged from the star under the stress of light pressure or electric
repulsion. But I think we may see in it something simpler still; to
wit, gaseous molecules driven off by light pressure alone—the smoke,
as one may say, of the catastrophe—akin exactly to the constituents of
comet’s tails. The mere light of the conflagration pushed the hydrogen
molecules away. This would explain their presence and their exceeding
hurry at the same time. They were started on their travels by domestic
jars and kept going by the vivid after-effects of that infelicity.
The fairly steady rate of regression from the nova observed may be
explained by the observed decrease in the light of the repellent
source. Such combined with the retarding effect of gravity might make
the regression equable. This is the more explanatory as the speed was
certainly much less than that of light, though greatly exceeding any
possible from the direct disruption. At the same time both the bright
and the dark lines of hydrogen seen in the spectrum stand accounted
for; the colliding molecules, at their starting on their travels
from the star, shining through their sparser fellows farther out. An
interesting biograph of the levity of light!
Nova Persei thus introduces us at its birth to one of a class of most
interesting objects comparatively recently discovered and of most
pregnant import,—the spiral nebulæ.
[Illustration: GREAT NEBULA IN ORION—AFTER RITCHEY.]
[Illustration: GREAT NEBULA IN ANDROMEDA—AFTER RITCHEY.]
[Illustration: NEBULA M. 100 COMÆ—AFTER ROBERTS.]
In 1843 when Lord Rosse’s giant speculum, six feet across, was turned
upon the sky, a nebula was brought to light which was unlike any ever
before seen. It was neither irregular like the great nebula in Orion
nor round like the so called planetary nebulæ,—the two great classes
at that time known,—but exhibited a striking spiral structure. It
proved the forerunner of a remarkable revelation. For the specimen
thus disclosed has turned out to typify not only the most interesting
form of those heavenly wreaths of light, but by far the commonest as
well. As telescopic and especially photographic means improved, the
number of such objects detected steadily increased until about thirteen
years ago Keeler by his systematic discoveries of them came to the
conclusion that a spiral structure pervaded the great majority of all
the nebulæ visible. Their relative universality was outdone only by
the invariability of their form. For they all represent spirals of one
type: two coiled arms radiating diametrically from a central nucleus
and dilating outward. Even nebulæ not originally supposed spiral have
disclosed on better revelation the dominant form. Thus the great nebula
in Andromeda formerly thought lens-shaped proves to be a huge spiral
coiled in a plane not many degrees inclined to the plane of sight.
[Illustration: NEBULA ♅ I. 226 URSÆ MAJORIS—AFTER ROBERTS.]
As should happen if the spirals are unrelated, left-handed and
right-handed ones are about equally common. In Dr. Roberts’ great
collection of those in which the structure is distinctly discernible,
nine are right-handed, ten left-handed, showing that they partake of
the ambidextrous impartiality of space.
[Illustration: NEBULA ♅ V. 24 COMÆ—AFTER ROBERTS.
Showing globular structure.]
Lastly the spirals are evidently thicker near the centre, thinning out
at the edge, and when the central nucleus is pronounced, it seems to
have a certain globularity not shared by the arms, and more or less
detached from them. This appears in those cases where they are shown us
edgewise, and it has been thought perceptible in the great nebula of
Andromeda. The difficulty in establishing the phenomenon comes from the
impossibility of both features showing at their best together. For the
globularity to come out well, the spiral must be presented to us nearly
in the plane of sight; for the spirality, in a plane at right angles to
it.
Much may be learnt by pondering on these peculiarities. The widespread
character of the phenomenon points to some universal law. We are here
clearly confronted by the embodiment of a great cosmic principle,
causing the helices it is for us to uncoil. It is a problem in
mechanics.
In the first place, a spiral structure denotes action on the face
of it. It implies a rotation combined with motion out or in. We are
familiar with the fact in the sparks of pin-wheel pyrotechnics. Any
rotating fluid urged by an outward or an inward impulse must take the
spiral form. A common example occurs in the water let out of a basin
through a hole in the centre when we draw out the plug. Here the
force is inward, and because the bowl and orifice are not perfectly
symmetric, a rotation is set up in the water trying to escape, and the
two combine to give us a beautiful conchoidal swirl. In this case the
particles seek the centre, but the same general shape is assumed when
they seek to leave it.
Another point to be noticed is that a spiral nebula could not develop
of itself and subsist. To continue it must have outside help. For if
it were due to internal explosive action in the pristine body, each
ejectum must return to the point it started from, or else depart
forever into space, for the orbit it would describe must either be
closed or unclosed. If the former, it would revisit its starting-point;
if the latter, it would never return. Explosion, therefore, of itself
could not have produced the forms we see, unless they be ephemeral
apparitions, a supposition their presence throughout the heavens seems
effectually to exclude.
[Illustration: NEBULA M. 101 URSÆ MAJORIS—AFTER RITCHEY.]
The form of the spiral nebulæ proclaims their motion, but one of its
particular features discloses more. For it implies the past cause which
set this motion going. A distinctive detail of these spirals, which so
far as we know is shared by all of them, are the two arms which leave
the centre from diametrically opposite sides. This indicates that the
outward driving force acted only in two places, the one the antipodes
of the other. Now what kind of force is capable of this peculiar
effect? If we think of the matter, we shall realize that tidal action
would produce just this result. We see it daily in the case of the
Moon; when it is high tide in the open ocean hereabouts, it is high
tide also at the opposite end of the Earth. The reason is that the
tideraising body pulls the fluid nearest it more strongly than it pulls
the Earth as a whole, and pulls the Earth as a whole more than it pulls
the fluid at the opposite extremity.
Suppose, now, a stranger to approach a body in space near enough; it
will inevitably raise tides in the other’s mass, and if the approach be
very close, the tides will be so great as to tear the body in pieces
along the line due to their action; that is, parts of the body will
be separated from the main mass in two antipodal directions. This is
precisely what we see in the spiral nebula. Nor is there any other
action that we know of which would thus handle the body. If it were
to disintegrate under increased speed of rotation due to contraction
upon itself, parts of its periphery should be shed continually and
a pin-wheel of matter, not a two-armed spiral, be thrown off. If
explosion were the disintegrating cause, disruption would occur
unsymmetrically in one or more directions, not symmetrically as here.
[Illustration: REPRESENTATIVE STELLAR SPECTRA
_Photographed, in 1907 and 1908, by_ V. M. SLIPHER, _at_ LOWELL
OBSERVATORY _Flagstaff, Arizona, with prism spectrograph._]
As the stranger passed on, his effect would diminish until his
attraction no longer overbalanced that of the body for its disrupted
portions. These might then be controlled and forced to move in elliptic
orbits about the mass of which they had originally made part. Thence
would come into being a solar system, the knots in the nebula going to
form the planets that were to be.
Before proceeding to what proof we have that it actually did occur in
this way we may pause to consider some consequences of what we have
already learned. Thus what brought about the beginning of the system
may also compass its end. If one random encounter took place in the
past, a second is as likely to occur in the future. Another celestial
body may any day run into the Sun, and it is to a dark body that we
must look for such destruction, because they are so much more numerous
in space.
That any of the lucent stars, the stars commonly so called, could
collide with the Sun, or come near enough to amount to the same thing,
is demonstrably impossible for æons of years. But this is far from the
case for a dark star. Such a body might well be within a hundredth of
the distance of the nearest of our known neighbors, Alpha Centauri, at
the present moment without our being aware of it at all. Our senses
could only be cognizant of its proximity by the borrowed light it
reflected from our own Sun. Dark in itself, our own head-lights alone
would show it up when close upon us. It would loom out of the void thus
suddenly before the crash.
We can calculate how much warning we should have of the coming
catastrophe. The Sun with its retinue is speeding through space at
the rate of eleven miles a second toward a point near the bright star
Vega. Since the tramp would probably also be in motion with a speed
comparable with our own, it might hit us coming from any point in
space, the likelihood depending upon the direction and amount of its
own speed. So that at the present moment such a body may be in any
part of the sky. But the chances are greatest if it be coming from the
direction toward which the sun is travelling, since it would then be
approaching us head on. If it were travelling itself as fast as the
Sun, its relative speed of approach would be twenty-two miles a second.
The previousness of the warning would depend upon the stranger’s size.
The warning would be long according as the stranger was large. Let
us assume it the mass of the Sun, a most probable supposition. Being
dark, it must have cooled to a solid, and its density therefore be much
greater than the Sun’s, probably something like eight times as great,
giving it a diameter about half his or four hundred and thirty thousand
miles. Its apparent brightness would depend both upon its distance and
upon its intrinsic brightness or albedo, and this last would itself
vary according to its distance from the Sun. While it was still in the
depths of space and its atmosphere lay inert, owing to the cold there,
its intrinsic brightness might be that of the Moon or Mercury. As its
own rotation would greatly affect the speed with which its sunward side
was warmed, we can form no exact idea of the law of its increase in
light. That the augmentation would be great we see from the behavior of
comets as they approach the great hearth of our solar system. But we
are not called upon to evaluate the question to that nicety. We shall
assume, therefore, that its brilliancy would be only that of the Moon,
remembering that the last stages of its fateful journey would be much
more resplendently set off.
With these data we can find how long it would be visible before
the collision occurred. As a very small telescopic star it would
undoubtedly escape detection. It is not likely that the stranger
would be noticed simply from its appearance until it had attained
the eleventh magnitude. It would then be one hundred and forty-nine
astronomical units from the Sun or at five times the distance of
Neptune. But its detection would come about not through the eye of
the body, but through the eye of the mind. Long before it could have
attracted man’s attention to itself directly its effects would have
betrayed it. Previous, indeed, to its possible showing in any telescope
the behavior of the outer planets of the system would have revealed
its presence. The far plummet of man’s analysis would have sounded
the cause of their disturbance and pointed out the point from which
that disturbance came. Celestial mechanics would have foretold, as
once the discovery of another planet, so now the end of the world.
Unexplained perturbations in the motions of the planets, the far
tremors of its coming, would have spoken to astronomers as the first
heralding of the stranger and of the destruction it was about to bring.
Neptune and Uranus would begin to deviate from their prescribed paths
in a manner not to be accounted for except by the action of some new
force. Their perturbations would resemble those caused by an unknown
exterior planet, but with this difference that the period of the
disturbance would be exactly that of the disturbed planet’s own period
of revolution round the Sun.
Our exterior sentinels might fail thus to give us warning of the
foreign body because of being at the time in the opposite parts of
their orbits. We should then be first apprised of its coming by Saturn,
which would give us less prefatory notice.
It would be some twenty-seven years from the time it entered the range
of vision of our present telescopes before it rose to that of the
unarmed eye. It would then have reached forty-nine astronomical units’
distance, or two-thirds as far again as Neptune. From here, however,
its approach would be more rapid. Humanity by this time would have been
made acquainted with its sinister intent from astronomic calculation,
and would watch its slow gaining in conspicuousness with ever growing
alarm. During the next three years it would have ominously increased
to a first magnitude star, and two years and three months more have
reached the distance of Jupiter and surpassed by far in lustre Venus at
her brightest.
Meanwhile the disturbance occasioned not simply in the outer planets
but in our own Earth would have become very alarming indeed. The
seasons would have been already greatly changed, and the year itself
lengthened, and all these changes fraught with danger to everything
upon the Earth’s face would momentarily grow worse. In one hundred and
forty-five days from the time it passed the distance of Jupiter it
would reach the distance of the Earth. Coming from Vega, it would not
hit the Earth or any of the outer planets, as the Sun’s way is inclined
to the planetary planes by some sixty degrees, but the effects would be
none the less marked for that. Day and night alone of our astronomic
relations would remain. It would be like going mad and yet remaining
conscious of the fact. Instead of following the Sun we should now in
whole or part, according to the direction of its approach, obey the
stranger. For nineteen more days this frightful chaos would continue;
as like some comet glorified a thousand fold the tramp dropped silently
upon the Sun. Toward the close of the nineteenth day the catastrophe
would occur, and almost in merciful deliverance from the already
chaotic cataclysm and the yet greater horror of its contemplation, we
should know no more.
Unless the universe is otherwise articulated than we have reason to
suppose, such a catastrophe sometime seems certain. But we may bear
ourselves with equanimity in its prospect for two mitigating details.
One is that there is no sign whatever at the moment that any such
stranger is near. The unaccounted-for errors in the planetary theories
are not such as point to the advent of any tramp. Another is, that
judged by any scale of time we know, the chance of such occurrence
is immeasurably remote. Not only may each of us rest content in the
thought that he will die from causes of his own choosing or neglect,
but the Earth herself will cease to be a possible abode of life, and
even the Sun will have become cold and dark and dead so long before
that day arrives that when the final shock shall come, it will be quite
ready for another resurrection.