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Curiosities of the Sky
Garrett Serviss
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Title: Curiosities of the Sky
Author: Garrett Serviss
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*** START OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY ***
Curiosities of the Sky
by Garrett Serviss
Curiosities of the Sky was first published in 1909 and the text is in
the public domain. The transcription was done by William McClain
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A printed version of this book is available from Sattre Press
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_________________________________________________________________
Preface
What Froude says of history is true also of astronomy: it is the most
impressive where it transcends explanation. It is not the mathematics
of astronomy, but the wonder and the mystery that seize upon the
imagination. The calculation of an eclipse owes all its prestige to
the sublimity of its data; the operation, in itself, requires no more
mental effort than the preparation of a railway time-table.
The dominion which astronomy has always held over the minds of men is
akin to that of poetry; when the former becomes merely instructive and
the latter purely didactic, both lose their power over the
imagination. Astronomy is known as the oldest of the sciences, and it
will be the longest-lived because it will always have arcana that have
not been penetrated.
Some of the things described in this book are little known to the
average reader, while others are well known; but all possess the
fascination of whatever is strange, marvelous, obscure, or mysterious
-- magnified, in this case, by the portentous scale of the phenomena.
The idea of the author is to tell about these things in plain
language, but with as much scientific accuracy as plain language will
permit, showing the wonder that is in them without getting away from
the facts. Most of them have hitherto been discussed only in technical
form, and in treatises that the general public seldom sees and never
reads.
Among the topics touched upon are:
* The strange unfixedness of the ``fixed stars,'' the vast
migrations of the suns and worlds constituting the universe.
* The slow passing out of existence of those collocations of stars
which for thousands of years have formed famous
``constellations,'' preserving the memory of mythological heroes
and heroines, and perhaps of otherwise unrecorded history.
* The tendency of stars to assemble in immense clouds, swarms, and
clusters.
* The existence in some of the richest regions of the universe of
absolutely black, starless gaps, deeps, or holes, as if one were
looking out of a window into the murkiest night.
* The marvelous phenomena of new, or temporary, stars, which appear
as suddenly as conflagrations, and often turn into something else
as eccentric as themselves.
* The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,''
and ``lace,'' or ``tress,'' nebulæ.
* The strange surroundings of the sun, only seen in particular
circumstances, but evidently playing a constant part in the daily
phenomena of the solar system.
* The mystery of the Zodiacal Light and the Gegenschein.
* The extraordinary transformations undergone by comets and their
tails.
* The prodigies of meteorites and masses of stone and metal fallen
ads:
from the sky.
* The cataclysms that have wrecked the moon.
* The problem of life and intelligence on the planet Mars.
* The problematical origin and fate of the asteroids.
* The strange phenomena of the auroral lights.
An attempt has been made to develop these topics in an orderly way,
showing their connection, so that the reader may obtain a broad
general view of the chief mysteries and problems of astronomy, and an
idea of the immense field of discovery which still lies, almost
unexplored, before it.
The Windows of Absolute Night
To most minds mystery is more fascinating than science. But when
science itself leads straight up to the borders of mystery and there
comes to a dead stop, saying, ``At present I can no longer see my
way,'' the force of the charm is redoubled. On the other hand, the
illimitable is no less potent in mystery than the invisible, whence
the dramatic effect of Keats' ``stout Cortez'' staring at the
boundless Pacific while all his men look at each other with a wild
surmise, ``silent upon a peak in Darien.'' It is with similar feelings
that the astronomer regards certain places where from the peaks of the
universe his vision seems to range out into endless empty space. He
sees there the shore of his little isthmus, and, beyond, unexplored
immensity.
The name, ``coal-sacks,'' given to these strange voids is hardly
descriptive. Rather they produce upon the mind the effect of blank
windows in a lonely house on a pitch-dark night, which, when looked at
from the brilliant interior, become appalling in their rayless murk.
Infinity seems to acquire a new meaning in the presence of these black
openings in the sky, for as one continues to gaze it loses its purely
metaphysical quality and becomes a kind of entity, like the ocean. The
observer is conscious that he can actually see the beginning of its
ebon depths, in which the visible universe appears to float like an
enchanted island, resplendent within with lights and life and gorgeous
spectacles, and encircled with screens of crowded stars, but with its
dazzling vistas ending at the fathomless sea of pure darkness which
encloses all.
The Galaxy, or Milky Way, surrounds the borders of our island in space
like a stellar garland, and when openings appear in it they are, by
contrast, far more impressive than the general darkness of the
interstellar expanse seen in other directions. Yet even that expanse
is not everywhere equally dark, for it contains gloomy deeps
discernable with careful watching. Here, too, contrast plays an
important part, though less striking than within the galactic region.
Some of Sir William Herschel's observations appear to indicate an
association between these tenebrious spots and neighboring star clouds
and nebulæ. It is an illuminating bit of astronomical history that
when he was sweeping the then virgin heavens with his great telescopes
he was accustomed to say to his sister who, note-book in hand, waited
at his side to take down his words, fresh with the inspiration of
discovery: ``Prepare to write; the nebulæ are coming; here space is
vacant.''
The most famous of the ``coal-sacks,'' and the first to be brought to
general attention before astronomers had awakened to the significance
of such things, lies adjacent to the ``Southern Cross,'' and is truly
an amazing phenomenon. It is not alone the conspicuousness of this
celestial vacancy, opening suddenly in the midst of one of the richest
parts of the Galaxy, that has given it its fame, but quite as much the
superstitious awe with which it was regarded by the early explorers of
the South Seas. To them, as well as to those who listened in rapt
wonder to their tales, the ``Coal-sack'' seemed to possess some occult
connection with the mystic ``Cross.'' In the eyes of the sailors it
was not a vacancy so much as a sable reality in the sky, and as,
shuddering, they stared at it, they piously crossed themselves. It was
another of the magical wonders of the unknown South, and as such it
formed the basis of many a ``wild surmise'' and many a sea-dog's yarn.
Scientific investigation has not diminished its prestige, and today no
traveler in the southern hemisphere is indifferent to its fascinating
strangeness, while some find it the most impressive spectacle of the
antarctic heavens.
All around, up to the very edge of the yawning gap, the sheen of the
Milky Way is surpassingly glorious; but there, as if in obedience to
an almighty edict, everything vanishes. A single faint star is visible
within the opening, producing a curious effect upon the sensitive
spectator, like the sight of a tiny islet in the midst of a black,
motionless, waveless tarn. The dimensions of the lagoon of darkness,
which is oval or pear-shaped, are eight degrees by five, so that it
occupies a space in the sky about one hundred and thirty times greater
than the area of the full moon. It attracts attention as soon as the
eye is directed toward the quarter where it exists, and by virtue of
the rarity of such phenomena it appears a far greater wonder than the
drifts of stars that are heaped around it. Now that observatories are
multiplying in the southern hemisphere, the great austral
``Coal-sack'' will, no doubt, receive attention proportioned to its
importance as one of the most significant features of the sky. Already
at the Sydney Observatory photographs have shown that the southern
portion of this Dead Sea of Space is not quite ``bottomless,''
although its northern part defies the longest sounding lines of the
astronomer.
There is a similar, but less perfect, ``coal-sack'' in the northern
hemisphere, in the constellation of ``The Swan,'' which, strange to
say, also contains a well-marked figure of a cross outlined by stars.
This gap lies near the top of the cross-shaped figure. It is best seen
by averted vision, which brings out the contrast with the Milky Way,
which is quite brilliant around it. It does not, however, exercise the
same weird attraction upon the eye as the southern ``Coal-sack,'' for
instead of looking like an absolute void in the sky, it rather appears
as if a canopy of dark gauze had been drawn over the stars. We shall
see the possible significance of this appearance later.
Just above the southern horizon of our northern middle latitudes, in
summer, where the Milky Way breaks up into vast sheets of nebulous
luminosity, lying over and between the constellations Scorpio and
Sagittarius, there is a remarkable assemblage of ``coal-sacks,''
though none is of great size. One of them, near a conspicuous
star-cluster in Scorpio, M80, is interesting for having been the first
of these strange objects noted by Herschel. Probably it was its
nearness to M80 which suggested to his mind the apparent connection of
such vacancies with star-clusters which we have already mentioned.
But the most marvelous of the ``coal-sacks'' are those that have been
found by photography in Sagittarius. One of Barnard's earliest and
most excellent photographs includes two of them, both in the
star-cluster M8. The larger, which is roughly rectangular in outline,
contains one little star, and its smaller neighbor is lune-shaped --
surely a most singular form for such an object. Both are associated
with curious dark lanes running through the clustered stars like
trails in the woods. Along the borders of these lanes the stars are
ranked in parallel rows, and what may be called the bottoms of the
lanes are not entirely dark, but pebbled with faint stellar points.
One of them which skirts the two dark gaps and traverses the cluster
along its greatest diameter is edged with lines of stars, recalling
the alignment of the trees bordering a French highway. This road of
stars cannot be less than many billions of miles in length!
All about the cluster the bed of the Galaxy is strangely disturbed,
and in places nearly denuded, as if its contents had been raked away
to form the immense stack and the smaller accumulations of stars
around it. The well-known ``Trifid Nebula'' is also included in the
field of the photograph, which covers a truly marvelous region, so
intricate in its mingling of nebulæ, star-clusters, star-swarms,
star-streams, and dark vacancies that no description can do it
justice. Yet, chaotic as it appears, there is an unmistakable
suggestion of unity about it, impressing the beholder with the idea
that all the different parts are in some way connected, and have not
been fortuitously thrown together. Miss Agnes M. Clerke made the
striking remark that the dusky lanes in M8 are exemplified on the
largest scale in the great rift dividing the Milky Way, from Cygnus in
the northern hemisphere all the way to the ``Cross'' in the southern.
Similar lanes are found in many other clusters, and they are generally
associated with flanking rows of stars, resembling in their
arrangement the thick-set houses and villas along the roadways that
traverse the approaches to a great city.
But to return to the black gaps. Are they really windows in the
star-walls of the universe? Some of them look rather as if they had
been made by a shell fired through a luminous target, allowing the eye
to range through the hole into the void space beyond. If science is
discretely silent about these things, what can the more venturesome
and less responsible imagination suggest? Would a huge ``runaway
sun,'' like Arcturus, for instance, make such an opening if it should
pass like a projectile through the Milky Way? It is at least a
stimulating inquiry. Being probably many thousands of times more
massive than the galactic stars, such a stellar missile would not be
stopped by them, though its direction of flight might be altered. It
would drag the small stars lying close to its course out of their
spheres, but the ultimate tendency of its attraction would be to sweep
them round in its wake, thus producing rather a star-swarm than a
vacancy. Those that were very close to it might be swept away in its
rush and become its satellites, careering away with it in its flight
into outer space; but those that were farther off, and they would, of
course, greatly outnumber the nearer ones, would tend inward from all
sides toward the line of flight, as dust and leaves collect behind a
speeding motor (though the forces operating would be different), and
would fill up the hole, if hole it were. A swarm thus collected should
be rounded in outline and bordered with a relatively barren ring from
which the stars had been ``sucked'' away. In a general sense the M8
cluster answers to this description, but even if we undertook to
account for its existence by a supposition like the above, the black
gaps would remain unexplained, unless one could make a further draft
on the imagination and suggest that the stars had been thrown into a
vast eddy, or system of eddies, whose vortices appear as dark holes.
Only a maelstrom-like motion could keep such a funnel open, for
without regard to the impulse derived from the projectile, the proper
motions of the stars themselves would tend to fill it. Perhaps some
other cause of the whirling motion may be found. As we shall see when
we come to the spiral nebulæ, gyratory movements are exceedingly
prevalent throughout the universe, and the structure of the Milky Way
is everywhere suggestive of them. But this is hazardous sport even for
the imagination -- to play with suns as if they were but thistle-down
in the wind or corks in a mill-race.
Another question arises: What is the thickness of the hedge of stars
through which the holes penetrate? Is the depth of the openings
proportionate to their width? In other words, is the Milky Way round
in section like a rope, or flat and thin like a ribbon? The answer is
not obvious, for we have little or no information concerning the
relative distances of the faint galactic stars. It would be easier,
certainly, to conceive of openings in a thin belt than in a massive
ring, for in the first case they would resemble mere rifts and breaks,
while in the second they would be like wells or bore-holes. Then, too,
the fact that the Milky Way is not a continuous body but is made up of
stars whose actual distances apart is great, offers another quandary;
persistent and sharply bordered apertures in such an assemblage are a
priori as improbable, if not impossible, as straight, narrow holes
running through a swarm of bees.
The difficulty of these questions indicates one of the reasons why it
has been suggested that the seeming gaps, or many of them, are not
openings at all, but opaque screens cutting off the light from stars
behind them. That this is quite possible in some cases is shown by
Barnard's later photographs, particularly those of the singular region
around the star Rho Ophiuchi. Here are to be seen somber lanes and
patches, apparently forming a connected system which covers an immense
space, and which their discoverer thinks may constitute a ``dark
nebula.'' This seems at first a startling suggestion; but, after all,
why should their not be dark nebulæ as well as visible ones? In truth,
it has troubled some astronomers to explain the luminosity of the
bright nebulæ, since it is not to be supposed that matter in so
diffuse a state can be incandescent through heat, and phosphorescent
light is in itself a mystery. The supposition is also in accord with
what we know of the existence of dark solid bodies in space. Many
bright stars are accompanied by obscure companions, sometimes as
massive as themselves; the planets are non-luminous; the same is true
of meteors before they plunge into the atmosphere and become heated by
friction; and many plausible reasons have been found for believing
that space contains as many obscure as shining bodies of great size.
It is not so difficult, after all, then, to believe that there are
immense collections of shadowy gases and meteoric dust whose presence
is only manifested when they intercept the light coming from shining
bodies behind them.
This would account for the apparent extinguishment of light in open
space, which is indicated by the falling off in relative number of
telescopic stars below the tenth magnitude. Even as things are, the
amount of light coming to us from stars too faint to be seen with the
naked eye is so great that the statement of it generally surprises
persons who are unfamiliar with the inner facts of astronomy. It has
been calculated that on a clear night the total starlight from the
entire celestial sphere amounts to one-sixtieth of the light of the
full moon; but of this less than one-twenty-fifth is due to stars
separately distinguished by the eye. If there were no obscuring medium
in space, it is probable that the amount of starlight would be
noticeably and perhaps enormously increased.
But while it seems certain that some of the obscure spots in the Milky
Way are due to the presence of ``dark nebulæ,'' or concealing veils of
one kind or another, it is equally certain that there are many which
are true apertures, however they may have been formed, and by whatever
forces they may be maintained. These, then, are veritable windows of
the Galaxy, and when looking out of them one is face to face with the
great mystery of infinite space. There the known universe visibly
ends, but manifestly space itself does not end there. It is not within
the power of thought to conceive an end to space, for the instant we
think of a terminal point or line the mind leaps forward to the
beyond. There must be space outside as well as inside. Eternity of
time and infinity of space are ideas that the intellect cannot fully
grasp, but neither can it grasp the idea of a limitation to either
space or time. The metaphysical conceptions of hypergeometry, or
fourth-dimensional space, do not aid us.
Having, then, discovered that the universe is a thing contained in
something indefinitely greater than itself; having looked out of its
windows and found only the gloom of starless night outside -- what
conclusions are we to draw concerning the beyond? It seems as empty as
a vacuum, but is it really so? If it be, then our universe is a single
atom astray in the infinite; it is the only island in an ocean without
shores; it is the one oasis in an illimitable desert. Then the Milky
Way, with its wide-flung garland of stars, is afloat like a tiny
smoke-wreath amid a horror of immeasurable vacancy, or it is an
evanescent and solitary ring of sparkling froth cast up for a moment
on the viewless billows of immensity. From such conclusions the mind
instinctively shrinks. It prefers to think that there is something
beyond, though we cannot see it. Even the universe could not bear to
be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the
most elegant château, with its gardens, parks, and crowds of
attendants, would die of loneliness if they did not know that they
have neighbors, though not seen, and that a living world of indefinite
extent surrounds them, so we, when we perceive that the universe has
limits, wish to feel that it is not solitary; that beyond the hedges
and the hills there are other centers of life and activity. Could
anything be more terrible than the thought of an isolated universe?
The greater the being, the greater the aversion to seclusion. Only the
infinite satisfies; in that alone the mind finds rest.
We are driven, then, to believe that the universal night which
envelopes us is not tenantless; that as we stare out of the
star-framed windows of the Galaxy and see nothing but uniform
blackness, the fault is with our eyes or is due to an obscuring
medium. Since our universe is limited in extent, there must be other
universes beyond it on all sides. Perhaps if we could carry our
telescopes to the verge of the great ``Coal-sack'' near the ``Cross,''
being then on the frontier of our starry system, we could discern,
sparkling afar off in the vast night, some of the outer galaxies. They
may be grander than ours, just as many of the suns surrounding us are
immensely greater than ours. If we could take our stand somewhere in
the midst of immensity and, with vision of infinite reach, look about
us, we should perhaps see a countless number of stellar systems, amid
which ours would be unnoticeable, like a single star among the
multitude glittering in the terrestial sky on a clear night. Some
might be in the form of a wreath, like our own; some might be
globular, like the great star-clusters in Hercules and Centaurus; some
might be glittering circles, or disks, or rings within rings. If we
could enter them we should probably find a vast variety of
composition, including elements unknown to terrestrial chemistry; for
while the visible universe appears to contain few if any substances
not existing on the earth or in the sun, we have no warrant to assume
that others may not exist in infinite space.
And how as to gravitation? We do not know that gravitation acts beyond
the visible universe, but it is reasonable to suppose that it does. At
any rate, if we let go its sustaining hand we are lost, and can only
wander hopelessly in our speculations, like children astray. If the
empire of gravitation is infinite, then the various outer systems must
have some, though measuring by our standards an imperceptible,
attractive influence upon each other, for gravitation never lets go
its hold, however great the space over which it is required to act.
Just as the stars about us are all in motion, so the starry systems
beyond our sight may be in motion, and our system as a whole may be
moving in concert with them. If this be so, then after interminable
ages the aspect of the entire system of systems must change, its
various members assuming new positions with respect to one another. In
the course of time we may even suppose that our universe will approach
relatively close to one of the others; and then, if men are yet living
on the earth, they may glimpse through the openings which reveal
nothing to us now, the lights of another nearing star system, like the
signals of a strange squadron, bringing them the assurance (which can
be but an inference at present) that the ocean of space has other
argosies venturing on its limitless expanse.
There remains the question of the luminiferous ether by whose agency
the waves of light are borne through space. The ether is as mysterious
as gravitation. With regard to ether we only infer its existence from
the effects which we ascribe to it. Evidently the ether must extend as
far as the most distant visible stars. But does it continue on
indefinitely in outer space? If it does, then the invisibility of the
other systems must be due to their distance diminishing the quantity
of light that comes from them below the limit of perceptibility, or to
the interposition of absorbing media; if it does not, then the reason
why we cannot see them is owing to the absence of a means of
conveyance for the light waves, as the lack of an interplanetary
atmosphere prevents us from hearing the thunder of sun-spots. (It is
interesting to recall that Mr Edison was once credited with the
intention to construct a gigantic microphone which should render the
roar of sun-spots audible by transforming the electric vibrations into
sound-waves). On this supposition each starry system would be
enveloped in its own globule of ether, and no light could cross from
one to another. But the probability is that both the ether and
gravitation are ubiquitous, and that all the stellar systems are
immersed in the former like clouds of phosphorescent organisms in the
sea.
So astronomy carries the mind from height to greater height. Men were
long in accepting the proofs of the relative insignificance of the
earth; they were more quickly convinced of the comparative littleness
of the solar system; and now the evidence assails their reason that
what they had regarded as the universe is only one mote gleaming in
the sunbeams of Infinity.
Star-Clouds, Star-Clusters, and Star-Streams
In the preceding chapter we have seen something of the strangely
complicated structure of the Galaxy, or Milky Way. We now proceed to
study more comprehensively that garlanded ``Pathway of the Gods.''
Judged by the eye alone, the Milky Way is one of the most delicately
beautiful phenomena in the entire realm of nature -- a shimmer of
silvery gauze stretched across the sky; but studied in the light of
its revelations, it is the most stupendous object presented to human
ken. Let us consider, first, its appearance to ordinary vision. Its
apparent position in the sky shifts according to the season. On a
serene, cloudless summer evening, in the absence of the moon, whose
light obscures it, one sees the Galaxy spanning the heavens from north
to southeast of the zenith like a phosphorescent arch. In early spring
it forms a similar but, upon the whole, less brilliant arch west of
the zenith. Between spring and summer it lies like a long, faint,
twilight band along the northern horizon. At the beginning of winter
it again forms an arch, this time spanning the sky from east to west,
a little north of the zenith. These are its positions as viewed from
the mean latitude of the United States. Even the beginner in
star-gazing does not have to watch it throughout the year in order to
be convinced that it is, in reality, a great circle, extending
entirely around the celestial sphere. We appear to be situated near
its center, but its periphery is evidently far away in the depths of
space.
Although to the casual observer it seems but a delicate scarf of
light, brighter in some places than in others, but hazy and indefinite
at the best, such is not its appearance to those who study it with
care. They perceive that it is an organic whole, though marvelously
complex in detail. The telescope shows that it consists of stars too
faint and small through excess of distance to be separately visible.
Of the hundred million suns which some estimates have fixed as the
probable population of the starry universe, the vast majority (at
least thirty to one) are included in this strange belt of misty light.
But they are not uniformly distributed in it; on the contrary, they
are arrayed in clusters, knots, bunches, clouds, and streams. The
appearance is somewhat as if the Galaxy consisted of innumerable
swarms of silver-winged bees, more or less intermixed, some massed
together, some crossing the paths of others, but all governed by a
single purpose which leads them to encircle the region of space in
which we are situated.
From the beginning of the systematic study of the heavens, the fact
has been recognized that the form of the Milky Way denotes the scheme
of the sidereal system. At first it was thought that the shape of the
system was that of a vast round disk, flat like a cheese, and filled
with stars, our sun and his relatively few neighbors being placed near
the center. According to this view, the galactic belt was an effect of
perspective; for when looking in the direction of the plane of the
disk, the eye ranged through an immense extension of stars which
blended into a glimmering blur, surrounding us like a ring; while when
looking out from the sides of the disk we saw but few stars, and in
those directions the heavens appeared relatively blank. Finally it was
recognized that this theory did not correspond with the observed
appearances, and it became evident that the Milky Way was not a mere
effect of perspective, but an actual band of enormously distant stars,
forming a circle about the sphere, the central opening of the ring
(containing many scattered stars) being many times broader than the
width of the ring itself. Our sun is one of the scattered stars in the
central opening.
As already remarked, the ring of the Galaxy is very irregular, and in
places it is partly broken. With its sinuous outline, its pendant
sprays, its graceful and accordant curves, its bunching of masses, its
occasional interstices, and the manifest order of a general plan
governing the jumble of its details, it bears a remarkable resemblance
to a garland -- a fact which appears the more wonderful when we recall
its composition. That an elm-tree should trace the lines of beauty
with its leafy and pendulous branches does not surprise us; but we can
only gaze with growing amazement when we behold a hundred million suns
imitating the form of a chaplet! And then we have to remember that
this form furnishes the ground-plan of the universe.
As an indication of the extraordinary speculations to which the
mystery of the Milky Way has given rise, a theory recently (1909)
proposed by Prof. George C. Comstock may be mentioned. Starting with
the data (first) that the number of stars increases as the Milky Way
is approached, and reaches a maximum in its plane, while on the other
hand the number of nebulæ is greatest outside the Milky Way and
increases with distance from it, and (second) that the Milky Way,
although a complete ring, is broad and diffuse on one side through
one-half its course -- that half alone containing nebulæ -- and
relatively narrow and well defined on the opposite side, the author of
this singular speculation avers that these facts can best be explained
by supposing that the invisible universe consists of two
interpenetrating parts, one of which is a chaos of indefinite extent,
strewn with stars and nebulous dust, and the other a long, broad but
comparatively thin cluster of stars, including the sun as one of its
central members. This flat star-cluster is conceived to be moving
edgewise through the chaos, and, according to Professor Comstock, it
acts after the manner of a snow-plough sweeping away the cosmic dust
and piling it on either hand above and below the plane of the moving
cluster. It thus forms a transparent rift, through which we see
farther and command a view of more stars than through the intensified
dust-clouds on either hand. This rift is the Milky Way. The dust
thrown aside toward the poles of the Milky Way is the substance of the
nebulæ which abound there. Ahead, where the front of the star-plough
is clearing the way, the chaos is nearer at hand, and consequently
there the rift subtends a broader angle, and is filled with primordial
dust, which, having been annexed by the vanguard of the star-swarm,
forms the nebulæ seen only in that part of the Milky Way. But behind,
the rift appears narrow because there we look farther away between
dust-clouds produced ages ago by the front of the plough, and no
scattered dust remains in that part of the rift.
In quoting an outline of this strikingly original theory the present
writer should not be understood as assenting to it. That it appears
bizarre is not, in itself, a reason for rejecting it, when we are
dealing with so problematical and enigmatical a subject as the Milky
Way; but the serious objection is that the theory does not
sufficiently accord with the observed phenomena. There is too much
evidence that the Milky Way is an organic system, however fantastic
its form, to permit the belief that it can only be a rift in chaotic
clouds. As with every organism, we find that its parts are more or
less clearly repeated in its ensemble. Among all the strange things
that the Milky Way contains there is nothing so extraordinary as
itself. Every astronomer must many times have found himself marveling
at it in those comparatively rare nights when it shows all its beauty
and all its strangeness. In its great broken rifts, divisions, and
spirals are found the gigantic prototypes of similar forms in its
star-clouds and clusters. As we have said, it determines the general
shape of the whole sidereal system. Some of the brightest stars in the
sky appear to hang like jewels suspended at the ends of tassels
dropped from the Galaxy. Among these pendants are the Pleiades and the
Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of
light'' thrown out from it. The majority of the great first-magnitude
stars seem related to it, as if they formed an inner ring inclined at
an angle of some twenty degrees to its plane. Many of the long curves
that set off from it on both sides are accompanied by corresponding
curves of lucid stars. In a word, it offers every appearance of
structural connection with the entire starry system. That the universe
should have assumed the form of a wreath is certainly a matter for
astonishment; but it would have been still more astonishing if it had
been a cube, a rhomboid, or a dodecahedron, for then we should have
had to suppose that something resembling the forces that shape
crystals had acted upon the stars, and the difficulty of explaining
the universe by the laws of gravitation would have been increased.
From the Milky Way as a whole we pass to the vast clouds, swarms, and
clusters of stars of which it is made up. It may be, as some
astronomers hold, that most of the galactic stars are much smaller
than the sun, so that their faintness is not due entirely to the
effect of distance. Still, their intrinsic brilliance attests their
solar character, and considering their remoteness, which has been
estimated at not less than ten thousand to twenty thousand light-years
(a light-year is equal to nearly six thousand thousand million miles)
their actual masses cannot be extremely small. The minutest of them
are entitled to be regarded as real suns, and they vary enormously in
magnitude. The effects of their attractions upon one another can only
be inferred from their clustering, because their relative movements
are not apparent on account of the brevity of the observations that we
can make. But imagine a being for whom a million years would be but as
a flitting moment; to him the Milky Way would appear in a state of
ceaseless agitation -- swirling with ``a fury of whirlpool motion.''
The cloud-like aspect of large parts of the Galaxy must always have
attracted attention, even from naked-eye observers, but the true
star-clouds were first satisfactorily represented in Barnard's
photographs. The resemblance to actual clouds is often startling. Some
are close-packed and dense, like cumuli; some are wispy or mottled,
like cirri. The rifts and modulations, as well as the general
outlines, are the same as those of clouds of vapor or dust, and one
notices also the characteristic thinning out at the edges. But we must
beware of supposing that the component suns are thickly crowded as the
particles forming an ordinary cloud. They look, indeed, as if they
were matted together, because of the irradiation of light, but in
reality millions and billions of miles separate each star from its
neighbors. Nevertheless they form real assemblages, whose members are
far more closely related to one another than is our sun to the stars
around him, and if we were in the Milky Way the aspect of the
nocturnal sky would be marvelously different from its present
appearance.
Stellar clouds are characteristic of the Galaxy and are not found
beyond its borders, except in the ``Magellanic Clouds'' of the
southern hemisphere, which resemble detached portions of the Milky
Way. These singular objects form as striking a peculiarity of the
austral heavens as does the great ``Coal-sack'' described in Chapter
1. But it is their isolation that makes them so remarkable, for their
composition is essentially galactic, and if they were included within
its boundaries they would not appear more wonderful than many other
parts of the Milky Way. Placed where they are, they look like masses
fallen from the great stellar arch. They are full of nebulæ and
star-clusters, and show striking evidences of spiral movement.
Star-swarms, which are also characteristic features of the Galaxy,
differ from star-clouds very much in the way that their name would
imply -- i.e., their component stars are so arranged, even when they
are countless in number, that the idea of an exceedingly numerous
assemblage rather than that of a cloud is impressed on the observer's
mind. In a star-swarm the separate members are distinguishable because
they are either larger or nearer than the stars composing a ``cloud.''
A splendid example of a true star-swarm is furnished by Chi Persei, in
that part of the Milky Way which runs between the constellations
Perseus and Cassiopeia. This swarm is much coarser than many others,
and can be seen by the naked eye. In a small telescope it appears
double, as if the suns composing it had divided into two parties which
keep on their way side by side, with some commingling of their members
where the skirts of the two companies come in contact.
Smaller than either star-clouds or star-swarms, and differing from
both in their organization, are star-clusters. These, unlike the
others, are found outside as well as inside the Milky Way, although
they are more numerous inside its boundaries than elsewhere. The term
star-cluster is sometimes applied, though improperly, to assemblages
which are rather groups, such, for instance, as the Pleiades. In their
most characteristic aspect star-clusters are of a globular shape --
globes of suns! A famous example of a globular star-cluster, but one
not included in the Milky Way, is the ``Great Cluster in Hercules.''
This is barely visible to the naked eye, but a small telescope shows
its character, and in a large one it presents a marvelous spectacle.
Photographs of such clusters are, perhaps, less effective than those
of star-clouds, because the central condensation of stars in them is
so great that their light becomes blended in an indistinguishable
blur. The beautiful effect of the incessant play of infinitesimal rays
over the apparently compact surface of the cluster, as if it were a
globe of the finest frosted silver shining in an electric beam, is
also lost in a photograph. Still, even to the eye looking directly at
the cluster through a powerful telescope, the central part of the
wonderful congregation seems almost a solid mass in which the stars
are packed like the ice crystals in a snowball.
The same question rises to the lips of every observer: How can they
possibly have been brought into such a situation? The marvel does not
grow less when we know that, instead of being closely compacted, the
stars of the cluster are probably separated by millions of miles; for
we know that their distances apart are slight as compared with their
remoteness from the Earth. Sir William Herschel estimated their number
to be about fourteen thousand, but in fact they are uncountable. If we
could view them from a point just within the edge of the assemblage,
they would offer the appearance of a hollow hemisphere emblazoned with
stars of astonishing brilliancy; the near-by ones unparalleled in
splendor by any celestial object known to us, while the more distant
ones would resemble ordinary stars. An inhabitant of the cluster would
not know, except by a process of ratiocination, that he was dwelling
in a globular assemblage of suns; only from a point far outside would
their spherical arrangement become evident to the eye. Imagine
fourteen-thousand fire-balloons with an approach to regularity in a
spherical space -- say, ten miles in diameter; there would be an
average of less than thirty in every cubic mile, and it would be
necessary to go to a considerable distance in order to see them as a
globular aggregation; yet from a point sufficiently far away they
would blend into a glowing ball.
Photographs show even better than the best telescopic views that the
great cluster is surrounded with a multitude of dispersed stars,
suggestively arrayed in more or less curving lines, which radiate from
the principle mass, with which their connection is manifest. These
stars, situated outside the central sphere, look somewhat like vagrant
bees buzzing round a dense swarm where the queen bee is sitting. Yet
while there is so much to suggest the operation of central forces,
bringing and keeping the members of the cluster together, the
attentive observer is also impressed with the idea that the whole
wonderful phenomenon may be the result of explosion. As soon as this
thought seizes the mind, confirmation of it seems to be found in the
appearance of the outlying stars, which could be as readily explained
by the supposition that they have been blown apart as that they have
flocked together toward a center. The probable fact that the stars
constituting the cluster are very much smaller than our sun might be
regarded as favoring the hypothesis of an explosion. Of their real
size we know nothing, but, on the basis of an uncertain estimate of
their parallax, it has been calculated that they may average
forty-five thousand miles in diameter -- something more than half the
diameter of the planet Jupiter. Assuming the same mean density,
fourteen thousand such stars might have been formed by the explosion
of a body about twice the size of the sun. This recalls the theory of
Olbers, which has never been altogether abandoned or disproved, that
the Asteroids were formed by the explosion of a planet circulating
between the orbits of Mars and Jupiter. The Asteroids, whatever their
manner of origin, form a ring around the sun; but, of course, the
explosion of a great independent body, not originally revolving about
a superior center of gravitational force, would not result in the
formation of a ring of small bodies, but rather of a dispersed mass of
them. But back of any speculation of this kind lies the problem, at
present insoluble: How could the explosion be produced? (See the
question of explosions in Chapters 6 and 14).
Then, on the other hand, we have the observation of Herschel, since
abundantly confirmed, that space is unusually vacant in the immediate
neighborhood of condensed star-clusters and nebulæ, which, as far as
it goes, might be taken as an indication that the assembled stars had
been drawn together by their mutual attractions, and that the tendency
to aggregation is still bringing new members toward the cluster. But
in that case there must have been an original condensation of stars at
that point in space. This could probably have been produced by the
coagulation of a great nebula into stellar nuclei, a process which
seems now to be taking place in the Orion Nebula.
A yet more remarkable globular star-cluster exists in the southern
hemisphere, Omega Centauri. In this case the central condensation of
stars presents an almost uniform blaze of light. Like the Hercules
cluster, that in Centaurus is surrounded with stars scattered over a
broad field and showing an appearance of radial arrangement. In fact,
except for its greater richness, Omega Centauri is an exact duplicate
of its northern rival. Each appears to an imaginative spectator as a
veritable ``city of suns.'' Mathematics shrinks from the task of
disentangling the maze of motions in such an assemblage. It would seem
that the chance of collisions is not to be neglected, and this idea
finds a certain degree of confirmation in the appearance of
``temporary stars'' which have more than once blazed out in, or close
by, globular star-clusters.
This leads up to the notable fact, first established by Professor
Bailey a few years ago, that such clusters are populous with variable
stars. Omega Centauri and the Hercules cluster are especially
remarkable in this respect. The variables found in them are all of
short period and the changes of light show a noteworthy tendency to
uniformity. The first thought is that these phenomena must be due to
collisions among the crowded stars, but, if so, the encounters cannot
be between the stars themselves, but probably between stars and meteor
swarms revolving around them. Such periodic collisions might go on for
ages without the meteors being exhausted by incorporation with the
stars. This explanation appears all the more probable because one
would naturally expect that flocks of meteors would abound in a close
aggregation of stars. It is also consistent with Perrine's discovery
-- that the globular star clusters are powdered with minute stars
strewn thickly among the brighter ones.
In speaking of Professor Comstock's extraordinary theory of the Milky
Way, the fact was mentioned that, broadly speaking, the nebulæ are
less numerous in the galactic belt than in the comparatively open
spaces on either side of it, but that they are, nevertheless, abundant
in the broader half of the Milky Way which he designates as the front
of the gigantic ``plough'' supposed to be forcing its way through the
enveloping chaos. In and around the Sagittarius region the
intermingling of nebulæ and galactic star clouds and clusters is
particularly remarkable. That there is a causal connection no
thoughtful person can doubt. We are unable to get away from the
evidence that a nebula is like a seed-ground from which stars spring
forth; or we may say that nebulæ resemble clouds in whose bosom
raindrops are forming. The wonderful aspect of the admixtures of
nebulæ and star-clusters in Sagittarius has been described in Chapter
1. We now come to a still more extraordinary phenomenon of this kind
-- the Pleiades nebulæ.
The group of the Pleiades, although lying outside the main course of
the Galaxy, is connected with it by a faint loop, and is the scene of
the most remarkable association of stars and nebulous matter known in
the visible universe. The naked eye is unaware of the existence of
nebulæ in the Pleiades, or, at the best, merely suspects that there is
something of the kind there; and even the most powerful telescopes are
far from revealing the full wonder of the spectacle; but in
photographs which have been exposed for many hours consecutively, in
order to accumulate the impression of the actinic rays, the revelation
is stunning. The principle stars are seen surrounded by, and, as it
were, drowned in, dense nebulous clouds of an unparalleled kind. The
forms assumed by these clouds seem at first sight inexplicable. They
look like fleeces, or perhaps more like splashes and daubs of luminous
paint dashed carelessly from a brush. But closer inspection shows that
they are, to a large extent, woven out of innumerable threads of filmy
texture, and there are many indications of spiral tendencies. Each of
the bright stars of the group -- Alcyone, Merope, Maia, Electra,
Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
remember, alike to the naked eye and to the telescope), and these
particular stars are veiled from sight behind the strange mists.
Running in all directions across the relatively open spaces are
nebulous wisps and streaks of the most curious forms. On some of the
nebular lines, which are either straight throughout, or if they change
direction do so at an angle, little stars are strung like beads. In
one case seven or eight stars are thus aligned, and, as if to
emphasize their dependence upon the chain which connects them, when it
makes a slight bend the file of stars turns the same way. Many other
star rows in the group suggest by their arrangement that they, too,
were once strung upon similar threads which have now disappeared,
leaving the stars spaced along their ancient tracks. We seem forced to
the conclusion that there was a time when the Pleiades were embedded
in a vast nebula resembling that of Orion, and that the cloud has now
become so rare by gradual condensation into stars that the merest
trace of it remains, and this would probably have escaped detection
but for the remarkable actinic power of the radiant matter of which it
consists. The richness of many of these faint nebulous masses in
ultra-violet radiations, which are those that specifically affect the
photographic plate, is the cause of the marvelous revelatory power of
celestial photography. So the veritable unseen universe, as
distinguished from the ``unseen universe'' of metaphysical
speculation, is shown to us.
A different kind of association between stars and nebulæ is shown in
some surprising photographic objects in the constellation Cygnus,
where long, wispy nebulæ, billions of miles in length, some of them
looking like tresses streaming in a breeze, lie amid fields of stars
which seem related to them. But the relation is of a most singular
kind, for notwithstanding the delicate structure of the long nebulæ
they appear to act as barriers, causing the stars to heap themselves
on one side. The stars are two, three, or four times as numerous on
one side of the nebulæ as on the other. These nebulæ, as far as
appearance goes, might be likened to rail fences, or thin hedges,
against which the wind is driving drifts of powdery snow, which, while
scattered plentifully all around, tends to bank itself on the leeward
side of the obstruction. The imagination is at a loss to account for
these extraordinary phenomena; yet there they are, faithfully giving
us their images whenever the photographic plate is exposed to their
radiations.
Thus the more we see of the universe with improved methods of
observation, and the more we invent aids to human senses, each
enabling us to penetrate a little deeper into the unseen, the greater
becomes the mystery. The telescope carried us far, photography is
carrying us still farther; but what as yet unimagined instrument will
take us to the bottom, the top, and the end? And then, what hitherto
untried power of thought will enable us to comprehend the meaning of
it all?
Stellar Migrations
To the untrained eye the stars and the planets are not
distinguishable. It is customary to call them all alike ``stars.'' But
since the planets more or less rapidly change their places in the sky,
in consequence of their revolution about the sun, while the stars
proper seem to remain always in the same relative positions, the
latter are spoken of as ``fixed stars.'' In the beginnings of
astronomy it was not known that the ``fixed stars'' had any motion
independent of their apparent annual revolution with the whole sky
about the earth as a seeming center. Now, however, we know that the
term ``fixed stars'' is paradoxical, for there is not a single really
fixed object in the whole celestial sphere. The apparent fixity in the
positions of the stars is due to their immense distance, combined with
the shortness of the time during which we are able to observe them. It
is like viewing the plume of smoke issuing from a steamer, hull down,
at sea: if one does not continue to watch it for a long time it
appears to be motionless, although in reality it may be traveling at
great speed across the line of sight. Even the planets seem fixed in
position if one watches them for a single night only, and the more
distant ones do not sensibly change their places, except after many
nights of observation. Neptune, for instance, moves but little more
than two degrees in the course of an entire year, and in a month its
change of place is only about one-third of the diameter of the full
moon.
Yet, fixed as they seem, the stars are actually moving with a speed in
comparison with which, in some cases, the planets might almost be said
to stand fast in their tracks. Jupiter's speed in his orbit is about
eight miles per second, Neptune's is less than three and one-half
miles, and the earth's is about eighteen and one-half miles; while
there are ``fixed stars'' which move two hundred or three hundred
miles per second. They do not all, however, move with so great a
velocity, for some appear to travel no faster than the planets. But in
all cases, notwithstanding their real speed, long-continued and
exceedingly careful observations are required to demonstrate that they
are moving at all. No more overwhelming impression of the frightful
depths of space in which the stars are buried can be obtained than by
reflecting upon the fact that a star whose actual motion across the
line of sight amounts to two hundred miles per second does not change
its apparent place in the sky, in the course of a thousand years,
sufficiently to be noticed by the casual observer of the heavens!
There is one vast difference between the motions of the stars and
those of the planets to which attention should be at once called: the
planets, being under the control of a central force emanating from
their immediate master, the sun, all move in the same direction and in
orbits concentric about the sun; the stars, on the other hand, move in
every conceivable direction and have no apparent center of motion, for
all efforts to discover such a center have failed. At one time, when
theology had finally to accept the facts of science, a grandiose
conception arose in some pious minds, according to which the Throne of
God was situated at the exact center of His Creation, and, seated
there, He watched the magnificent spectacle of the starry systems
obediently revolving around Him. Astronomical discoveries and
speculations seemed for a time to afford some warrant for this view,
which was, moreover, an acceptable substitute for the abandoned
geocentric theory in minds that could only conceive of God as a
superhuman artificer, constantly admiring his own work. No longer ago
than the middle of the nineteenth century a German astronomer,
Maedler, believed that he had actually found the location of the
center about which the stellar universe revolved. He placed it in the
group of the Pleiades, and upon his authority an extraordinary
imaginative picture was sometimes drawn of the star Alcyone, the
brightest of the Pleiades, as the very seat of the Almighty. This idea
even seemed to gain a kind of traditional support from the mystic
significance, without known historical origin, which has for many
ages, and among widely separated peoples, been attached to the
remarkable group of which Alcyone is the chief. But since Maedler's
time it has been demonstrated that the Pleiades cannot be the center
of revolution of the universe, and, as already remarked, all attempts
to find or fix such a center have proved abortive. Yet so powerful was
the hold that the theory took upon the popular imagination, that even
today astronomers are often asked if Alcyone is not the probable site
of ``Jerusalem the Golden.''
If there were a discoverable center of predominant gravitative power,
to which the motions of all the stars could be referred, those motions
would appear less mysterious, and we should then be able to conclude
that the universe was, as a whole, a prototype of the subsidiary
systems of which it is composed. We should look simply to the law of
gravitation for an explanation, and, naturally, the center would be
placed within the opening enclosed by the Milky Way. If it were there
the Milky Way itself should exhibit signs of revolution about it, like
a wheel turning upon its hub. No theory of the star motions as a whole
could stand which failed to take account of the Milky Way as the basis
of all. But the very form of that divided wreath of stars forbids the
assumption of its revolution about a center. Even if it could be
conceived as a wheel having no material center it would not have the
form which it actually presents. As was shown in Chapter 2, there is
abundant evidence of motion in the Milky Way; but it is not motion of
the system as a whole, but motion affecting its separate parts.
Instead of all moving one way, the galactic stars, as far as their
movements can be inferred, are governed by local influences and
conditions. They appear to travel crosswise and in contrary
directions, and perhaps they eddy around foci where great numbers have
assembled; but of a universal revolution involving the entire mass we
have no evidence.
Most of our knowledge of star motions, called ``proper motions,''
relates to individual stars and to a few groups which happen to be so
near that the effects of their movements are measurable. In some cases
the motion is so rapid (not in appearance, but in reality) that the
chief difficulty is to imagine how it can have been imparted, and what
will eventually become of the ``runaways.'' Without a collision, or a
series of very close approaches to great gravitational centers, a star
traveling through space at the rate of two hundred or three hundred
miles per second could not be arrested or turned into an orbit which
would keep it forever flying within the limits of the visible
universe. A famous example of these speeding stars is ``1830
Groombridge,'' a star of only the sixth magnitude, and consequently
just visible to the naked eye, whose motion across the line of sight
is so rapid that it moves upon the face of the sky a distance equal to
the apparent diameter of the moon every 280 years. The distance of
this star is at least 200,000,000,000,000 miles, and may be two or
three times greater, so that its actual speed cannot be less than two
hundred, and may be as much as four hundred, miles per second. It
could be turned into a new course by a close approach to a great sun,
but it could only be stopped by collision, head-on, with a body of
enormous mass. Barring such accidents it must, as far as we can see,
keep on until it has traversed our stellar system, whence in may
escape and pass out into space beyond, to join, perhaps, one of those
other universes of which we have spoken. Arcturus, one of the greatest
suns in the universe, is also a runaway, whose speed of flight has
been estimated all the way from fifty to two hundred miles per second.
Arcturus, we have every reason to believe, possesses hundreds of times
the mass of our sun -- think, then, of the prodigious momentum that
its motion implies! Sirius moves more moderately, its motion across
the line of sight amounting to only ten miles per second, but it is at
the same time approaching the sun at about the same speed, its actual
velocity in space being the resultant of the two displacements.
What has been said about the motion of Sirius brings us to another
aspect of this subject. The fact is, that in every case of stellar
motion the displacement that we observe represents only a part of the
actual movement of the star concerned. There are stars whose motion
carries them straight toward or straight away from the earth, and such
stars, of course, show no cross motion. But the vast majority are
traveling in paths inclined from a perpendicular to our line of sight.
Taken as a whole, the stars may be said to be flying about like the
molecules in a mass of gas. The discovery of the radial component in
the movements of the stars is due to the spectroscope. If a star is
approaching, its spectral lines are shifted toward the violet end of
the spectrum by an amount depending upon the velocity of approach; if
it is receding, the lines are correspondingly shifted toward the red
end. Spectroscopic observation, then, combined with micrometric
measurements of the cross motion, enables us to detect the real
movement of the star in space. Sometimes it happens that a star's
radial movement is periodically reversed; first it approaches, and
then it recedes. This indicates that it is revolving around a near-by
companion, which is often invisible, and superposed upon this motion
is that of the two stars concerned, which together may be approaching
or receding or traveling across the line of sight. Thus the
complications involved in the stellar motions are often exceedingly
great and puzzling.
Yet another source of complication exists in the movement of our own
star, the sun. There is no more difficult problem in astronomy than
that of disentangling the effects of the solar motion from those of
the motions of the other stars. But the problem, difficult as it is,
has been solved, and upon its solution depends our knowledge of the
speed and direction of the movement of the solar system through space,
for of course the sun carries its planets with it. One element of the
solution is found in the fact that, as a result of perspective, the
stars toward which we are going appear to move apart toward all points
of the compass, while those behind appear to close up together. Then
the spectroscopic principle already mentioned is invoked for studying
the shift of the lines, which is toward the violet in the stars ahead
of us and toward the red in those that we are leaving behind. Of
course the effects of the independent motions of the stars must be
carefully excluded. The result of the studies devoted to this subject
is to show that we are traveling at a speed of twelve to fifteen miles
per second in a northerly direction, toward the border of the
constellations Hercules and Lyra. A curious fact is that the more
recent estimates show that the direction is not very much out of a
straight line drawn from the sun to the star Vega, one of the most
magnificent suns in the heavens. But it should not be inferred from
this that Vega is drawing us on; it is too distant for its gravitation
to have such an effect.
Many unaccustomed thoughts are suggested by this mighty voyage of the
solar system. Whence have we come, and whither do we go? Every year of
our lives we advance at least 375,000,000 miles. Since the traditional
time of Adam the sun has led his planets through the wastes of space
no less than 225,000,000,000 miles, or more than 2400 times the
distance that separates him from the earth. Go back in imagination to
the geologic ages, and try to comprehend the distance over which the
earth has flown. Where was our little planet when it emerged out of
the clouds of chaos? Where was the sun when his ``thunder march''
began? What strange constellations shone down upon our globe when its
masters of life were the monstrous beasts of the ``Age of Reptiles''?
A million years is not much of a span of time in geologic reckoning,
yet a million years ago the earth was farther from its present place
in space than any of the stars with a measurable parallax are now. It
was more than seven times as far as Sirius, nearly fourteen times as
far as Alpha Centauri, three times as far as Vega, and twice as far as
Arcturus. But some geologists demand two hundred, three hundred, even
one thousand million years to enable them to account for the
evolutionary development of the earth and its inhabitants. In a
thousand million years the earth would have traveled farther than from
the remotest conceivable depths of the Milky Way!
Other curious reflections arise when we think of the form of the
earth's track as it follows the lead of the sun, in a journey which
has neither known beginning nor conceivable end. There are probably
many minds which have found a kind of consolation in the thought that
every year the globe returns to the same place, on the same side of
the sun. This idea may have an occult connection with our traditional
regard for anniversaries. When that period of the year returns at
which any great event in our lives has occurred we have the feeling
that the earth, in its annual round, has, in a manner, brought us back
to the scene of that event. We think of the earth's orbit as a
well-worn path which we traverse many times in the course of a
lifetime. It seems familiar to us, and we grow to have a sort of
attachment to it. The sun we are accustomed to regard as a fixed
center in space, like the mill or pump around which the harnessed
patient mule makes his endless circuits. But the real fact is that the
earth never returns to the place in space where it has once quitted.
In consequence of the motion of the sun carrying the earth and the
other planets along, the track pursued by our globe is a vast spiral
in space continually developing and never returning upon its course.
It is probable that the tracks of the sun and the others stars are
also irregular, and possibly spiral, although, as far as can be at
present determined, they appear to be practically straight. Every
star, wherever it may be situated, is attracted by its fellow-stars
from many sides at once, and although the force is minimized by
distance, yet in the course of many ages its effects must become
manifest.
Looked at from another side, is there not something immensely
stimulating and pleasing to the imagination in the idea of so
stupendous a journey, which makes all of us the greatest of travelers?
In the course of a long life a man is transported through space thirty
thousand million miles; Halley's Comet does not travel one-quarter as
far in making one of its immense circuits. And there are adventures on
this voyage of which we are just beginning to learn to take account.
Space is full of strange things, and the earth must encounter some of
them as it advances through the unknown. Many singular speculations
have been indulged in by astronomers concerning the possible effects
upon the earth of the varying state of the space that it traverses.
Even the alternation of hot and glacial periods has sometimes been
ascribed to this source. When tropical life flourished around the
poles, as the remains in the rocks assure us, the needed high
temperature may, it has been thought, have been derived from the
presence of the earth in a warm region of space. Then, too, there is a
certain interest for us in the thought of what our familiar planet has
passed through. We cannot but admire it for its long journeying as we
admire the traveler who comes to us from remote and unexplored lands,
or as we gaze with a glow of interest upon the first locomotive that
has crossed a continent, or a ship that has visited the Arctic or
Antarctic regions. If we may trust the indications of the present
course, the earth, piloted by the sun, has come from the Milky Way in
the far south and may eventually rejoin that mighty band of stars in
the far north.
While the stars in general appear to travel independently of one
another, except when they are combined in binary or trinary systems,
there are notable exceptions to this rule. In some quarters of the sky
we behold veritable migrations of entire groups of stars whose members
are too widely separated to show any indications of revolution about a
common center of gravity. This leads us back again to the wonderful
group of the Pleiades. All of the principle stars composing that group
are traveling in virtually parallel lines. Whatever force set them
going evidently acted upon all alike. This might be explained by the
assumption that when the original projective force acted upon them
they were more closely united than they are at present, and that in
drifting apart they have not lost the impulse of the primal motion. Or
it may be supposed that they are carried along by some current in
space, although it would be exceedingly difficult, in the present
state of our knowledge, to explain the nature of such a current. Yet
the theory of a current has been proposed. As to an attractive center
around which they might revolve, none has been found. Another instance
of similar ``star-drift'' is furnished by five of the seven stars
constituting the figure of the ``Great Dipper.'' In this case the
stars concerned are separated very widely, the two extreme ones by not
less than fifteen degrees, so that the idea of a common motion would
never have been suggested by their aspect in the sky; and the case
becomes the more remarkable from the fact that among and between them
there are other stars, some of the same magnitude, which do not share
their motion, but are traveling in other directions. Still other
examples of the same phenomenon are found in other parts of the sky.
Of course, in the case of compact star-clusters, it is assumed that
all the members share a like motion of translation through space, and
the same is probably true of dense star-swarms and star-clouds.
The whole question of star-drift has lately assumed a new phase, in
consequence of the investigations of Kapteyn, Dyson, and Eddington on
the ``systematic motions of the stars.'' This research will, it is
hoped, lead to an understanding of the general law governing the
movements of the whole body of stars constituting the visible
universe. Taking about eleven hundred stars whose proper motions have
been ascertained with an approach to certainty, and which are
distributed in all parts of the sky, it has been shown that there
exists an apparent double drift, in two independent streams, moving in
different and nearly opposed directions. The apex of the motion of
what is called ``Stream I'' is situated, according to Professor
Kapteyn, in right ascension 85°, declination south 11°, which places
it just south of the constellation Orion; while the apex of ``Stream
II'' is in right ascension 260°, declination south 48°, placing it in
the constellation Ara, south of Scorpio. The two apices differ very
nearly 180° in right ascension and about 120° in declination. The
discovery of these vast star-streams, if they really exist, is one of
the most extraordinary in modern astronomy. It offers the correlation
of stellar movements needed as the basis of a theory of those
movements, but it seems far from revealing a physical cause for them.
As projected against the celestial sphere the stars forming the two
opposite streams appear intermingled, some obeying one tendency and
some the other. As Professor Dyson has said, the hypothesis of this
double movement is of a revolutionary character, and calls for further
investigation. Indeed, it seems at first glance not less surprising
than would be the observation that in a snow-storm the flakes over our
heads were divided into two parties and driving across each other's
course in nearly opposite directions, as if urged by interpenetrating
winds.
But whatever explanation may eventually be found for the motions of
the stars, the knowledge of the existence of those motions must always
afford a new charm to the contemplative observer of the heavens, for
they impart a sense of life to the starry system that would otherwise
be lacking. A stagnant universe, with every star fixed immovably in
its place, would not content the imagination or satisfy our longing
for ceaseless activity. The majestic grandeur of the evolutions of the
celestial hosts, the inconceivable vastness of the fields of space in
which they are executed, the countless numbers, the immeasurable
distances, the involved convolutions, the flocking and the scattering,
the interpenetrating marches and countermarches, the strange community
of impulsion affecting stars that are wide apart in space and causing
them to traverse the general movement about them like aides and
despatch-bearers on a battle-field -- all these arouse an intensity of
interest which is heightened by the mystery behind them.
The Passing of the Constellations
From a historical and picturesque point of view, one of the most
striking results of the motions of the stars described in the last
chapter is their effect upon the forms of the constellations, which
have been watched and admired by mankind from a period so early that
the date of their invention is now unknown. The constellations are
formed by chance combinations of conspicuous stars, like figures in a
kaleidoscope, and if our lives were commensurate with the æons of
cosmic existence we should perceive that the kaleidoscope of the
heavens was ceaselessly turning and throwing the stars into new
symmetries. Even if the stars stood fast, the motion of the solar
system would gradually alter the configurations, as the elements of a
landscape dissolve and recombine in fresh groupings with the
traveler's progress amid them. But with the stars themselves all in
motion at various speeds and in many directions, the changes occur
more rapidly. Of course, ``rapid'' is here understood in a relative
sense; the wheel of human history to an eye accustomed to the majestic
progression of the universe would appear to revolve with the velocity
of a whirling dynamo. Only the deliberation of geological movements
can be contrasted with the evolution and devolution of the
constellations.
And yet this secular fluctuation of the constellation figures is not
without keen interest for the meditative observer. It is another
reminder of the swift mutability of terrestial affairs. To the passing
glance, which is all that we can bestow upon these figures, they
appear so immutable that they have been called into service to form
the most lasting records of ancient thought and imagination that we
possess. In the forms of the constellations, the most beautiful, and,
in imaginative quality, the finest, mythology that the world has ever
known has been perpetuated. Yet, in a broad sense, this scroll of
human thought imprinted on the heavens is as evanescent as the summer
clouds. Although more enduring than parchment, tombs, pyramids, and
temples, it is as far as they from truly eternizing the memory of what
man has fancied and done.
Before studying the effects that the motions of the stars have had and
will have upon the constellations, it is worth while to consider a
little further the importance of the stellar pictures as archives of
history. To emphasize the importance of these effects it is only
necessary to recall that the constellations register the oldest
traditions of our race. In the history of primeval religions they are
the most valuable of documents. Leaving out of account for the moment
the more familiar mythology of the Greeks, based on something older
yet, we may refer for illustration to that of the mysterious Maya race
of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
of ruins perched, after the Mexican and Central-American plan, on the
summits of pyramidal mounds which mark the site of an ancient
theogonic center of the Mayas. Here the temples all evidently refer to
a cult based upon the constellations as symbols. The figures and the
names, of course, were not the same as those that we have derived from
our Aryan ancestors, but the star groups were the same or nearly so.
For instance, the loftiest of the temples at Izamal was connected with
the sign of the constellation known to us as Cancer, marking the place
of the sun at the summer solstice, at which period the sun was
supposed to descend at noon like a great bird of fire and consume the
offerings left upon the altar. Our Scorpio was known to the Mayas as a
sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the
idea of a divine weighing out of justice has always been connected,
seems to be identical with the Mayan constellation Teoyaotlatohua,
with which was associated a temple where dwelt the priests whose
special business it was to administer justice and to foretell the
future by means of information obtained from the spirits of the dead.
Orion, the ``Hunter'' of our celestial mythology, was among the Mayas
a ``Warrior,'' while Sagittarius and others of our constellations were
known to them (under different names, of course), and all were endowed
with a religious symbolism. And the same star figures, having the same
significance, were familiar to the Peruvians, as shown by the temples
at Cuzco. Thus the imagination of ancient America sought in the
constellations symbols of the unchanging gods.
But, in fact, there is no nation and no people that has not recognized
the constellations, and at one period or another in its history
employed them in some symbolic or representative capacity. As handled
by the Greeks from prehistoric times, the constellation myths became
the very soul of poetry. The imagination of that wonderful race
idealized the principal star groups so effectively that the figures
and traditions thus attached to them have, for civilized mankind,
displaced all others, just as Greek art in its highest forms stands
without parallel and eclipses every rival. The Romans translated no
heroes and heroines of the mythical period of their history to the
sky, and the deified Cæsars never entered that lofty company, but the
heavens are filled with the early myths of the Greeks. Herakles
nightly resumes his mighty labors in the stars; Zeus, in the form of
the white ``Bull,'' Taurus, bears the fair Europa on his back through
the celestial waves; Andromeda stretches forth her shackled arms in
the star-gemmed ether, beseeching aid; and Perseus, in a blaze of
diamond armor, revives his heroic deeds amid sparkling clouds of
stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair,
while the Great King, Cepheus, towers gigantic over the pole.
Professor Young has significantly remarked that a great number of the
constellations are connected in some way or other with the Argonautic
Expedition -- that strangely fascinating legend of earliest Greek
story which has never lost its charm for mankind. In view of all this,
we may well congratulate ourselves that the constellations will
outlast our time and the time of countless generations to follow us;
and yet they are very far from being eternal. Let us now study some of
the effects of the stellar motions upon them.
We begin with the familiar figure of the ``Great Dipper.'' He who has
not drunk inspiration from its celestial bowl is not yet admitted to
the circle of Olympus. This figure is made up of seven conspicuous
stars in the constellation Ursa Major, the ``Greater Bear.'' The
handle of the ``Dipper'' corresponds to the tail of the imaginary
``Bear,'' and the bowl lies upon his flank. In fact, the figure of a
dipper is so evident and that of a bear so unevident, that to most
persons the ``Great Dipper'' is the only part of the constellation
that is recognizable. Of the seven stars mentioned, six are of nearly
equal brightness, ranking as of the second magnitude, while the
seventh is of only the third magnitude. The difference is very
striking, since every increase of one magnitude involves an increase
of two-and-a-half times in brightness. There appears to be little
doubt that the faint star, which is situated at the junction of the
bowl and the handle, is a variable of long period, since three hundred
years ago it was as bright as its companions. But however that may be,
its relative faintness at the present time interferes but little with
the perfection of the ``Dipper's'' figure. In order the more readily
to understand the changes which are taking place, it will be well to
mention both the names and the Greek letters which are attached to the
seven stars. Beginning at the star in the upper outer edge of the rim
of the bowl and running in regular order round the bottom and then out
to the end of the handle, the names and letters are as follows: Dubhe
({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}),
Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
is the faint star already mentioned at the junction of the bowl and
handle, and Mizar, in the middle of the handle, has a close, naked-eye
companion which is named Alcor. The Arabs called this singular pair of
stars ``The Horse and Rider.'' Merak and Duhbe are called ``The
Pointers,'' because an imaginary line drawn northward through them
indicates the Pole Star.
Now it has been found that five of these stars -- viz., Merak, Phaed,
Megrez, Alioth, and Mizar (with its comrade) -- are moving with
practically the same speed in an easterly direction, while the other
two, Dubhe and Benetnasch, are simultaneously moving westward, the
motions of Benetnasch being apparently more rapid. The consequence of
these opposed motions is, of course, that the figure of the ``Dipper''
cannot always have existed and will not continue to exist. In the
accompanying diagrams it has been thought interesting to show the
relative positions of these seven stars, as seen from the point which
the earth now occupies, both in the past and in the future. Arrows
attached to the stars in the figure representing the present
appearance of the ``Dipper'' indicate the directions of the motions
and the distances over which they will carry the stars in a period of
about five hundred centuries. The time, no doubt, seems long, but
remember the vast stretch of ages through which the earth has passed,
and then reflect that no reason is apparent why our globe should not
continue to be a scene of animation for ten thousand centuries yet to
come. The fact that the little star Alcor placed so close to Mizar
should accompany the latter in its flight is not surprising, but that
two of the principal stars of the group should be found moving in a
direction directly opposed to that pursued by the other five is
surprising in the highest degree; and it recalls the strange theory of
a double drift affecting all the stars, to which attention was called
in the preceding chapter. It would appear that Benetnasch and Dubhe
belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar
to the other. As far as is known, the motion of the seven stars are
not shared by the smaller stars scattered about them, but on the
theory of currents there should be such a community of motion, and
further investigation may reveal it.
From the ``Great Dipper'' we turn to a constellation hardly less
conspicuous and situated at an equal distance from the pole on the
other side -- Cassiopeia. This famous star-group commemorating the
romantic Queen of Ethiopia whose vain boasting of her beauty was
punished by the exposure of her daughter Andromeda to the ``Sea
Monster,'' is well-marked by five stars which form an irregular letter
``W'' with its open side toward the pole. Three of these stars are
usually ranked as of the second magnitude, and two of the third; but
to ordinary observation they appear of nearly equal brightness, and
present a very striking picture. They mark out the chair and a part of
the figure of the beautiful queen. Beginning at the right-hand, or
western, end of the ``W,'' their Greek letter designations are: Beta
({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and
Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon
are traveling eastwardly at various speeds, while the fifth, Gamma,
moves in a westerly direction. The motion of Beta is more rapid than
that of any of the others. It should be said, however, that no little
uncertainty attaches to the estimates of the rate of motion of stars
which are not going very rapidly, and different observers often vary
considerably in their results.
In the beautiful ``Northern Crown,'' one of the most perfect and
charming of all the figures to be found in the stars, the alternate
combining and scattering effects of the stellar motions are shown by
comparing the appearance which the constellation must have had five
hundred centuries ago with that which it has at present and that which
it will have in the future. The seven principle stars of the asterism,
forming a surprisingly perfect coronet, have movements in three
directions at right angles to one another. That in these circumstances
they should ever have arrived at positions giving them so striking an
appearance of definite association is certainly surprising; from its
aspect one would have expected to find a community of movement
governing the brilliants of the ``Crown,'' but instead of that we find
evidence that they will inevitably drift apart and the beautiful
figure will dissolve.
A similar fate awaits such asterisms as the ``Northern Cross'' in
Cygnus; the ``Crow'' (Corvus), which stands on the back of the great
``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin''
(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);
the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades
in Taurus. In the case of the Hyades, two controlling movements are
manifest: one, affecting five of the stars which form the well-known
figure of a letter ``V,'' is directed northerly; the other, which
controls the direction of two stars, has an easterly trend. The chief
star of the group, Aldebaran, one of the finest of all stars both for
its brilliance and its color, is the most affected by the easterly
motion. In time it will drift entirely out of connection with its
present neighbors. Although the Hyades do not form so compact a group
as the Pleiades in the same constellation, yet their appearance of
relationship is sufficient to awaken a feeling of surprise over the
fact that, as with the stars of the ``Dipper,'' their association is
only temporary or apparent.
The great figure of Orion appears to be more lasting, not because its
stars are physically connected, but because of their great distance,
which renders their movements too deliberate to be exactly
ascertained. Two of the greatest of its stars, Betelgeuse and Rigel,
possess, as far as has been ascertained, no perceptible motion across
the line of sight, but there is a little movement perceptible in the
``Belt.'' At the present time this consists of an almost perfect
straight line, a row of second-magnitude stars about equally spaced
and of the most striking beauty. In the course of time, however, the
two right-hand stars, Mintaka and Alnilam (how fine are these Arabic
star names!) will approach each other and form a naked-eye double, but
the third, Alnita, will drift away eastward, so that the ``Belt'' will
no longer exist.
For one more example, let us go to the southern hemisphere, whose most
celebrated constellation, the ``Southern Cross,'' has found a place in
all modern literatures, although it has no claim to consideration on
account of association with ancient legends. This most attractive
asterism, which has never ceased to fascinate the imagination of
Christendom since it was first devoutly described by the early
explorers of the South, is but a passing collocation of brilliant
stars. Yet even in its transfigurations it has been for hundreds of
centuries, and will continue to be for hundreds of centuries to come,
a most striking object in the sky. Our figures show its appearance in
three successive phases: first, as it was fifty thousand years ago
(viewed from the earth's present location); second, as it is in our
day; and, third, as it will be an equal time in the future. The
nearness of these bright stars to one another -- the length of the
longer beam of the ``Cross'' is only six degrees -- makes this group
very noticeable, whatever the arrangement of its components may be.
The largest star, at the base of the ``Cross,'' is of the first
magnitude, two of the others are of the second magnitude, and the
fourth is of the third. Other stars, not represented in the figures,
increase the effect of a celestial blazonry, although they do not help
the resemblance to a cross.
But since the motion of the solar system itself will, in the course of
so long a period as fifty thousand years, produce a great change in
the perspective of the heavens as seen from the earth, by carrying us
nearly nineteen trillion miles from our present place, why, it may be
asked, seek to represent future appearances of the constellations
which we could not hope to see, even if we could survive so long? The
answer is: Because these things aid the mind to form a picture of the
effects of the mobility of the starry universe. Only by showing the
changes from some definite point of view can we arrive at a due
comprehension of them. The constellations are more or less familiar to
everybody, so that impending changes of their forms must at once
strike the eye and the imagination, and make clearer the significance
of the movements of the stars. If the future history of mankind is to
resemble its past and if our race is destined to survive yet a million
years, then our remote descendents will see a ``new heavens'' if not a
``new earth,'' and will have to invent novel constellations to
perpetuate their legends and mythologies.
If our knowledge of the relative distances of the stars were more
complete, it would be an interesting exercise in celestial geometry to
project the constellations probably visible to the inhabitants of
worlds revolving around some of the other suns of space. Our sun is
too insignificant for us to think that he can make a conspicuous
appearance among them, except, perhaps, in a few cases. As seen, for
instance, from the nearest known star, Alpha Centauri, the sun would
appear of the average first magnitude, and consequently from that
standpoint he might be the gem of some little constellation which had
no Sirius, or Arcturus, or Vega to eclipse him with its superior
splendor. But from the distance of the vast majority of the stars the
sun would probably be invisible to the naked eye, and as seen from
nearer systems could only rank as a fifth or sixth magnitude star,
unnoticed and unknown except by the star-charting astronomer.
Conflagrations in the Heavens
Suppose it were possible for the world to take fire and burn up -- as
some pessimists think that it will do when the Divine wrath shall have
sufficiently accumulated against it -- nobody out of our own little
corner of space would ever be aware of the catastrophe! With all their
telescopes, the astronomers living in the golden light of Arcturus or
the diamond blaze of Canopus would be unable to detect the least
glimmer of the conflagration that had destroyed the seat of Adam and
his descendents, just as now they are totally ignorant of its
existence.
But at least fifteen times in the course of recorded history men
looking out from the earth have beheld in the remote depths of space
great outbursts of fiery light, some of them more splendidly luminous
than anything else in the firmament except the sun! If they were
conflagrations, how many million worlds like ours were required to
feed their blaze?
It is probable that ``temporary'' or ``new'' stars, as these wonderful
apparitions are called, really are conflagrations; not in the sense of
a bonfire or a burning house or city, but in that of a sudden eruption
of inconceivable heat and light, such as would result from the
stripping off the shell of an encrusted sun or the crashing together
of two mighty orbs flying through space with a hundred times the
velocity of the swiftest cannon-shot.
Temporary stars are the rarest and most erratic of astronomical
phenomena. The earliest records relating to them are not very clear,
and we cannot in every instance be certain that it was one of these
appearances that the ignorant and superstitious old chroniclers are
trying to describe. The first temporary star that we are absolutely
sure of appeared in 1572, and is known as ``Tycho's Star,'' because
the celebrated Danish astronomer (whose remains, with his
gold-and-silver artificial nose -- made necessary by a duel -- still
intact, were disinterred and reburied in 1901) was the first to
perceive it in the sky, and the most assiduous and successful in his
studies of it. As the first fully accredited representative of its
class, this new star made its entry upon the scene with becoming
éclat. It is characteristic of these phenomena that they burst into
view with amazing suddenness, and, of course, entirely unexpectedly.
Tycho's star appeared in the constellation Cassiopeia, near a now
well-known and much-watched little star named Kappa, on the evening of
November 11, 1572. The story has often been repeated, but it never
loses interest, how Tycho, going home that evening, saw people in the
street pointing and staring at the sky directly over their heads, and
following the direction of their hands and eyes he was astonished to
see, near the zenith, an unknown star of surpassing brilliance. It
outshone the planet Jupiter, and was therefore far brighter than the
first magnitude. There was not another star in the heavens that could
be compared with it in splendor. Tycho was not in all respects free
from the superstitions of his time -- and who is? -- but he had the
true scientific instinct, and immediately he began to study the
stranger, and to record with the greatest care every change in its
aspect. First he determined as well as he could with the imperfect
instruments of his day, many of which he himself had invented, the
precise location of the phenomena in the sky. Then he followed the
changes that it underwent. At first it brightened until its light
equaled or exceeded that of the planet Venus at her brightest, a
statement which will be appreciated at its full value by anyone who
has ever watched Venus when she plays her dazzling rôle of ``Evening
Star,'' flaring like an arc light in the sunset sky. It even became so
brilliant as to be visible in full daylight, since, its position being
circumpolar, it never set in the latitude of Northern Europe. Finally
it began to fade, turning red as it did so, and in March, 1574, it
disappeared from Tycho's searching gaze, and has never been seen again
from that day to this. None of the astronomers of the time could make
anything of it. They had not yet as many bases of speculation as we
possess today.
Tycho's star has achieved a romantic reputation by being fancifully
identified with the ``Star of Bethlehem,'' said to have led the
wondering Magi from their eastern deserts to the cradle-manger of the
Savior in Palestine. Many attempts have been made to connect this
traditional ``star'' with some known phenomenon of the heavens, and
none seems more idle than this. Yet it persistently survives, and no
astronomer is free from eager questions about it addressed by people
whose imagination has been excited by the legend. It is only necessary
to say that the supposition of a connection between the phenomenon of
the Magi and Tycho's star is without any scientific foundation. It was
originally based on an unwarranted assumption that the star of Tycho
was a variable of long period, appearing once every three hundred and
fifteen years, or thereabout. If that were true there would have been
an apparition somewhere near the traditional date of the birth of
Christ, a date which is itself uncertain. But even the data on which
the assumption was based are inconsistent with the theory. Certain
monkish records speak of something wonderful appearing in the sky in
the years 1264 and 945, and these were taken to have been outbursts of
Tycho's star. Investigation shows that the records more probably refer
to comets, but even if the objects seen were temporary stars, their
dates do not suit the hypothesis; from 945 to 1264 there is a gap of
319 years, and from 1264 to 1572 one of only 308 years; moreover 337
years have now (1909) elapsed since Tycho saw the last glimmer of his
star. Upon a variability so irregular and uncertain as that, even if
we felt sure that it existed, no conclusion could be found concerning
an apparition occurring 2000 years ago.
In the year 1600 (the year in which Giordano Bruno was burned at the
stake for teaching that there is more than one physical world), a
temporary star of the third magnitude broke out in the constellation
Cygnus, and curiously enough, considering the rarity of such
phenomena, only four years later another surprisingly brilliant one
appeared in the constellation Ophiuchus. This is often called
``Kepler's star,'' because the great German astronomer devoted to it
the same attention that Tycho had given to the earlier phenomenon. It,
too, like Tycho's, was at first the brightest object in the stellar
heavens, although it seems never to have quite equaled its famous
predecessor in splendor. It disappeared after a year, also turning of
a red color as it became more faint. We shall see the significance of
this as we go on. Some of Kepler's contemporaries suggested that the
outburst of this star was due to a meeting of atoms in space, and idea
bearing a striking resemblance to the modern theory of ``astronomical
collisions.''
In 1670, 1848, and 1860 temporary stars made their appearance, but
none of them was of great brilliance. In 1866 one of the second
magnitude broke forth in the ``Northern Crown'' and awoke much
interest, because by that time the spectroscope had begun to be
employed in studying the composition of the stars, and Huggins
demonstrated that the new star consisted largely of incandescent
hydrogen. But this star, apparently unlike the others mentioned, was
not absolutely new. Before its outburst it had shown as a star of the
ninth magnitude (entirely invisible, of course, to the naked eye), and
after about six weeks it faded to its original condition in which it
has ever since remained. In 1876 a temporary star appeared in the
constellation Cygnus, and attained at one time the brightness of the
second magnitude. Its spectrum and its behavior resembled those of its
immediate predecessor. In 1885, astronomers were surprised to see a
sixth-magnitude star glimmering in the midst of the hazy cloud of the
great Andromeda Nebula. It soon absolutely disappeared. Its spectrum
was remarkable for being ``continuous,'' like that of the nebula
itself. A continuous spectrum is supposed to represent a body, or a
mass, which is either solid or liquid, or composed of gas under great
pressure. In January, 1892, a new star was suddenly seen in the
constellation Auriga. It never rose much above the fourth magnitude,
but it showed a peculiar spectrum containing both bright and dark
lines of hydrogen.
But a bewildering surprise was now in store; the world was to behold
at the opening of the twentieth century such a celestial spectacle as
had not been on view since the times of Tycho and Kepler. Before
daylight on the morning of February 22, 1901, the Rev. Doctor
Anderson, of Edinburgh, an amateur astronomer, who had also been the
first to see the new star in Auriga, beheld a strange object in the
constellation Perseus not far from the celebrated variable star Algol.
He recognized its character at once, and immediately telegraphed the
news, which awoke the startled attention of astronomers all over the
world. When first seen the new star was no brighter than Algol (less
than the second magnitude), but within twenty-four hours it was
ablaze, outshining even the brilliant Capella, and far surpassing the
first magnitude. At the spot in the sky where it appeared nothing
whatever was visible on the night before its coming. This is known
with certainty because a photograph had been made of that very region
on February 21, and this photograph showed everything down to the
twelfth magnitude, but not a trace of the stranger which burst into
view between the 21st and the 22nd like the explosion of a rocket.
Upon one who knew the stars the apparition of this intruder in a
well-known constellation had the effect of a sudden invasion. The new
star was not far west of the zenith in the early evening, and in that
position showed to the best advantage. To see Capella, the hitherto
unchallenged ruler of that quarter of the sky, abased by comparison
with this stranger of alien aspect, for there was always an unfamiliar
look about the ``nova,'' was decidedly disconcerting. It seemed to
portend the beginning of a revolution in the heavens. One could
understand what the effect of such an apparition must have been in the
superstitious times of Tycho. The star of Tycho had burst forth on the
northern border of the Milky Way; this one was on its southern border,
some forty-five degrees farther east.
Astronomers were well-prepared this time for the scientific study of
the new star, both astronomical photography and spectroscopy having
been perfected, and the results of their investigations were
calculated to increase the wonder with which the phenomenon was
regarded. The star remained at its brightest only a few days; then,
like a veritable conflagration, it began to languish; and, like the
reflection of a dying fire, as it sank it began to glow with the red
color of embers. But its changes were spasmodic; once about every
three days it flared up only to die away again. During these
fluctuations its light varied alternately in the ratio of one to six.
Finally it took a permanent downward course, and after a few months
the naked eye could no longer perceive it; but it remained visible
with telescopes, gradually fading until it had sunk to the ninth
magnitude. Then another astonishing change happened: in August
photographs taken at the Yerkes Observatory and at Heidelberg showed
that the ``nova'' was surrounded by a spiral nebula! The nebula had
not been there before, and no one could doubt that it represented a
phase of the same catastrophe that had produced the outburst of the
new star. At one time the star seemed virtually to have disappeared,
as if all its substance had been expanded into the nebulous cloud, but
always there remained a stellar nucleus about which the misty spiral
spread wider and ever wider, like a wave expanding around a center of
disturbance. The nebula too showed a variability of brightness, and
four condensations which formed in it seemed to have a motion of
revolution about the star. As time went on the nebula continued to
expand at a rate which was computed to be not less than twenty
thousand miles per second! And now the star itself, showing
indications of having turned into a nebula, behaved in a most erratic
manner, giving rise to the suspicion that it was about to burst out
again. But this did not occur, and at length it sunk into a state of
lethargy from which it has to the present time not recovered. But the
nebulous spiral has disappeared, and the entire phenomena as it now
(1909) exists consists of a faint nebulous star of less than the ninth
magnitude.
The wonderful transformations just described had been forecast in
advance of the discovery of the nebulous spiral encircling the star by
the spectroscopic study of the latter. At first there was no
suggestion of a nebular constitution, but within a month or two
characteristic nebular lines began to appear, and in less than six
months the whole spectrum had been transformed to the nebular type. In
the mean time the shifting of the spectral lines indicated a
complication of rapid motions in several directions simultaneously.
These motions were estimated to amount to from one hundred to five
hundred miles per second.
The human mind is so constituted that it feels forced to seek an
explanation of so marvelous a phenomenon as this, even in the absence
of the data needed for a sound conclusion. The most natural
hypothesis, perhaps, is that of a collision. Such a catastrophe could
certainly happen. It has been shown, for instance, that in infinity of
time the earth is sure to be hit by a comet; in the same way it may be
asserted that, if no time limit is fixed, the sun is certain to run
against some obstacle in space, either another star, or a dense meteor
swarm, or one of the dark bodies which there is every reason to
believe abound around us. The consequences of such a collision are
easy to foretell, provided that we know the masses and the velocities
of the colliding bodies. In a preceding chapter we have discussed the
motions of the sun and stars, and have seen that they are so swift
that an encounter between any two of them could not but be disastrous.
But this is not all; for as soon as two stars approached within a few
million miles their speed would be enormously increased by their
reciprocal attractions and, if their motion was directed radially with
respect to their centers, they would come together with a crash that
would reduce them both to nebulous clouds. It is true that the chances
of such a ``head-on'' collision are relatively very small; two stars
approaching each other would most probably fall into closed orbits
around their common center of gravity. If there were a collision it
would most likely be a grazing one instead of a direct front-to-front
encounter. But even a close approach, without any actual collision,
would probably prove disastrous, owing to the tidal influence of each
of the bodies on the other. Suns, in consequence of their enormous
masses and dimensions and the peculiarities of their constitution, are
exceedingly dangerous to one another at close quarters. Propinquity
awakes in them a mutually destructive tendency. Consisting of matter
in the gaseous, or perhaps, in some cases, liquid, state, their tidal
pull upon each other if brought close together might burst them
asunder, and the photospheric envelope being destroyed the internal
incandescent mass would gush out, bringing fiery death to any planets
that were revolving near. Without regard to the resulting disturbance
of the earth's orbit, the close approach of a great star to the sun
would be in the highest degree perilous to us. But this is a danger
which may properly be regarded as indefinitely remote, since, at our
present location in space, we are certainly far from every star except
the sun, and we may feel confident that no great invisible body is
near, for if there were one we should be aware of its presence from
the effects of its attraction. As to dark nebulæ which may possibly
lie in the track that the solar system is pursuing at the rate of
375,000,000 miles per year, that is another question -- and they, too,
could be dangerous!
This brings us directly back to ``Nova Persei,'' for among the many
suggestions offered to explain its outburst, as well as those of other
temporary stars, one of the most fruitful is that of a collision
between a star and a vast invisible nebula. Professor Seeliger, of
Munich, first proposed this theory, but it afterward underwent some
modifications from others. Stated in a general form, the idea is that
a huge dark body, perhaps an extinguished sun, encountered in its
progress through space a widespread flock of small meteors forming a
dark nebula. As it plunged into the swarm the friction of the
innumerable collisions with the meteors heated its surface to
incandescence, and being of vast size it then became visible to us as
a new star. Meanwhile the motion of the body through the nebula, and
its rotation upon itself, set up a gyration in the blazing atmosphere
formed around it by the vaporized meteors; and as this atmosphere
spread wider, under the laws of gyratory motion a rotation in the
opposite direction began in the inflamed meteoric cloud outside the
central part of the vortex. Thus the spectral lines were caused to
show motion in opposite directions, a part of the incandescent mass
approaching the earth simultaneously with the retreat of another part.
So the curious spectroscopic observations before mentioned were
explained. This theory might also account for the appearance of the
nebulous spiral first seen some six months after the original
outburst. The sequent changes in the spectrum of the ``nova'' are
accounted for by this theory on the assumption, reasonable enough in
itself, that at first the invading body would be enveloped in a
vaporized atmosphere of relatively slight depth, producing by its
absorption the fine dark lines first observed; but that as time went
on and the incessant collisions continued, the blazing atmosphere
would become very deep and extensive, whereupon the appearance of the
spectral lines would change, and bright lines due to the light of the
incandescent meteors surrounding the nucleus at a great distance would
take the place of the original dark ones. The vortex of meteors once
formed would protect the flying body within from further immediate
collisions, the latter now occurring mainly among the meteors
themselves, and then the central blaze would die down, and the
original splendor of the phenomenon would fade.
But the theories about Nova Persei have been almost as numerous as the
astronomers who have speculated about it. One of the most startling of
them assumed that the outburst was caused by the running amuck of a
dark star which had encountered another star surrounded with planets,
the renewed outbreaks of light after the principal one had faded being
due to the successive running down of the unfortunate planets! Yet
another hypothesis is based on what we have already said of the tidal
influence that two close approaching suns would have upon each other.
Supposing two such bodies which had become encrusted, but remained
incandescent and fluid within, to approach within almost striking
distance; they would whirl each other about their common center of
gravity, and at the same time their shells would burst under the tidal
strain, and their glowing nuclei being disclosed would produce a great
outburst of light. Applying this theory to a ``nova,'' like that of
1866 in the ``Northern Crown,'' which had been visible as a small star
before the outbreak, and which afterward resumed its former aspect, we
should have to assume that a yet shining sun had been approached by a
dark body whose attraction temporarily burst open its photosphere. It
might be supposed that in this case the dark body was too far advanced
in cooling to suffer the same fate from the tidal pull of its victim.
But a close approach of that kind would be expected to result in the
formation of a binary system, with orbits of great eccentricity,
perhaps, and after the lapse of a certain time the outburst should be
renewed by another approximation of the two bodies. A temporary star
of that kind would rather be ranked as a variable.
The celebrated French astronomer, Janssen, had a different theory of
Nova Persei, and of temporary stars in general. According to his idea,
such phenomena might be the result of chemical changes taking place in
a sun without interference by, or collision with, another body.
Janssen was engaged for many years in trying to discover evidence of
the existence of oxygen in the sun, and he constructed his observatory
on the summit of Mount Blanc specially to pursue that research. He
believed that oxygen must surely exist in the sun since we find so
many other familiar elements included in the constitution of the solar
globe, and as he was unable to discover satisfactory evidence of its
presence he assumed that it existed in a form unknown on the earth. If
it were normally in the sun's chromosphere, or coronal atmosphere, he
said, it would combine with the hydrogen which we know is there and
form an obscuring envelope of water vapor. It exists, then, in a
special state, uncombined with hydrogen; but let the temperature of
the sun sink to a critical point and the oxygen will assume its normal
properties and combine with the hydrogen, producing a mighty outburst
of light and heat. This, Janssen thought, might explain the phenomena
of the temporary stars. It would also, he suggested, account for their
brief career, because the combination of the elements would be quickly
accomplished, and then the resulting water vapor would form an
atmosphere cutting off the radiation from the star within.
This theory may be said to have a livelier human interest than some of
the others, since, according to it, the sun may carry in its very
constitution a menace to mankind; one does not like to think of it
being suddenly transformed into a gigantic laboratory for the
explosive combination of oxygen and hydrogen! But while Janssen's
theory might do for some temporary stars, it is inadequate to explain
all the phenomena of Nova Persei, and particularly the appearance of
the great spiral nebula that seemed to exhale from the heart of the
star. Upon the whole, the theory of an encounter between a star and a
dark nebula seems best to fit the observations. By that hypothesis the
expanding billow of light surrounding the core of the conflagration is
very well accounted for, and the spectroscopic peculiarities are also
explained.
Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
temporary stars are the result of atomic explosion; but we shall touch
upon this more fully in Chapter 14.
Twice in the course of this discussion we have called attention to the
change of color invariably undergone by temporary stars in the later
stages of their career. This was conspicuous with Nova Persei which
glowed more and more redly as it faded, until the nebulous light began
to overpower that of the stellar nucleus. Nothing could be more
suggestive of the dying out of a great fire. Moreover, change of color
from white to red is characteristic of all variable stars of long
period, such as ``Mira'' in Cetus. It is also characteristic of stars
believed to be in the later stages of evolution, and consequently
approaching extinction, like Antares and Betelgeuse, and still more
notably certain small stars which ``gleam like rubies in the field of
the telescope.'' These last appear to be suns in the closing period of
existence as self-luminous bodies. Between the white stars, such as
Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
Herculis, there is a progressive series of colors from golden yellow
through orange to deep red. The change is believed to be due to the
increase of absorbing vapors in the stellar atmosphere as the body
cools down. In the case of ordinary stars these changes no doubt
occupy many millions of years, which represent the average duration of
solar life; but the temporary stars run through similar changes in a
few months: they resemble ephemeral insects -- born in the morning and
doomed to perish with the going down of the sun.
Explosive and Whirling Nebulæ
One of the most surprising triumphs of celestial photography was
Professor Keeler's discovery, in 1899, that the great majority of the
nebulæ have a distinctly spiral form. This form, previously known in
Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be
exceptional; now the photographs, far excelling telescopic views in
the revelation of nebular forms, showed the spiral to be the typical
shape. Indeed, it is a question whether all nebulæ are not to some
extent spiral. The extreme importance of this discovery is shown in
the effect that it has had upon hitherto prevailing views of solar and
planetary evolution. For more than three-quarters of a century
Laplace's celebrated hypothesis of the manner of origin of the solar
system from a rotating and contracting nebula surrounding the sun had
guided speculation on that subject, and had been tentatively extended
to cover the evolution of systems in general. The apparent forms of
some of the nebulæ which the telescope had revealed were regarded, and
by some are still regarded, as giving visual evidence in favor of this
theory. There is a ``ring nebula'' in Lyra with a central star, and a
``planetary nebula'' in Gemini bearing no little resemblance to the
planet Saturn with its rings, both of which appear to be practical
realizations of Laplace's idea, and the elliptical rings surrounding
the central condensation of the Andromeda Nebula may be cited for the
same kind of proof.
But since Keeler's discovery there has been a decided turning away of
speculation another way. The form of the spiral nebulæ seems to be
entirely inconsistent with the theory of an originally globular or
disk-shaped nebula condensing around a sun and throwing or leaving off
rings, to be subsequently shaped into planets. Some astronomers,
indeed, now reject Laplace's hypothesis in toto, preferring to think
that even our solar system originated from a spiral nebula. Since the
spiral type prevails among the existing nebulæ, we must make any
mechanical theory of the development of stars and planetary systems
from them accord with the requirements which that form imposes. A
glance at the extraordinary variations upon the spiral which Professor
Keeler's photographs reveal is sufficient to convince one of the
difficulty of the task of basing a general theory upon them. In truth,
it is much easier to criticize Laplace's hypothesis than to invent a
satisfactory substitute for it. If the spiral nebulæ seem to oppose it
there are other nebulæ which appear to support it, and it may be that
no one fixed theory can account for all the forms of stellar evolution
in the universe. Our particular planetary system may have originated
very much as the great French mathematician supposed, while others
have undergone, or are now undergoing, a different process of
development. There is always a too strong tendency to regard an
important new discovery and the theories and speculations based upon
it as revolutionizing knowledge, and displacing or overthrowing
everything that went before. Upon the plea that ``Laplace only made a
guess'' more recent guesses have been driven to extremes and treated
by injudicious exponents as ``the solid facts at last.''
Before considering more recent theories than Laplace's, let us see
what the nature of the photographic revelations is. The vast celestial
maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
taken as the leading type of the spiral nebulæ, although there are
less conspicuous objects of the kind which, perhaps, better illustrate
some of their peculiarities. Lord Rosse's nebula appears far more
wonderful in the photographs than in his drawings made with the aid of
his giant reflecting telescope at Parsonstown, for the photographic
plate records details that no telescope is capable of showing. Suppose
we look at the photograph of this object as any person of common sense
would look at any great and strange natural phenomenon. What is the
first thing that strikes the mind? It is certainly the appearance of
violent whirling motion. One would say that the whole glowing mass had
been spun about with tremendous velocity, or that it had been set
rotating so rapidly that it had become the victim of ``centrifugal
force,'' one huge fragment having broken loose and started to gyrate
off into space. Closer inspection shows that in addition to the
principal focus there are various smaller condensations scattered
through the mass. These are conspicuous in the spirals. Some of them
are stellar points, and but for the significance of their location we
might suppose them to be stars which happen to lie in a line between
us and the nebula. But when we observe how many of them follow most
faithfully the curves of the spirals we cannot but conclude that they
form an essential part of the phenomenon; it is not possible to
believe that their presence in such situations is merely fortuitous.
One of the outer spirals has at least a dozen of these star-like
points strung upon it; some of them sharp, small, and distinct, others
more blurred and nebulous, suggesting different stages of
condensation. Even the part which seems to have been flung loose from
the main mass has, in addition to its central condensation, at least
one stellar point gleaming in the half-vanished spire attached to it.
Some of the more distant stars scattered around the ``whirlpool'' look
as if they too had been shot out of the mighty vortex, afterward
condensing into unmistakable solar bodies. There are at least two
curved rows of minute stars a little beyond the periphery of the
luminous whirl which clearly follow lines concentric with those of the
nebulous spirals. Such facts are simply dumbfounding for anyone who
will bestow sufficient thought upon them, for these are suns, though
they may be small ones; and what a birth is that for a sun!
Look now again at the glowing spirals. We observe that hardly have
they left the central mass before they begin to coagulate. In some
places they have a ``ropy'' aspect; or they are like peascods filled
with growing seeds, which eventually will become stars. The great
focus itself shows a similar tendency, especially around its
circumference. The sense that it imparts of a tremendous shattering
force at work is overwhelming. There is probably more matter in that
whirling and bursting nebula than would suffice to make a hundred
solar systems! It must be confessed at once that there is no
confirmation of the Laplacean hypothesis here; but what hypothesis
will fit the facts? There is one which it has been claimed does so,
but we shall come to that later. In the meanwhile, as a preparation,
fix in the memory the appearance of that second spiral mass spinning
beside its master which seems to have spurned it away.
For a second example of the spiral nebulæ look at the one in the
constellation Triangulum. God, how hath the imagination of puny man
failed to comprehend Thee! Here is creation through destruction with a
vengeance! The spiral form of the nebula is unmistakable, but it is
half obliterated amid the turmoil of flying masses hurled away on all
sides with tornadic fury. The focus itself is splitting asunder under
the intolerable strain, and in a little while, as time is reckoned in
the Cosmos, it will be gyrating into stars. And then look at the
cyclonic rain of already finished stars whirling round the outskirts
of the storm. Observe how scores of them are yet involved in the
fading streams of the nebulous spirals; see how they have been thrown
into vast loops and curves, of a beauty that half redeems the terror
of the spectacle enclosed within their lines -- like iridescent cirri
hovering about the edges of a hurricane. And so again are suns born!
Let us turn to the exquisite spiral in Ursa Major; how different its
aspect f
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