I
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.
[Illustration: The Milky Way. Region near M.S. Photographed by
Professor Barnard]
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.
II
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.
[Illustration: Star-cluster in Hercules
(Photographed with a two-foot reflector)]
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.
[Illustration: Great southern star-cluster, Omega Centauri]
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.
[Illustration: The Pleiades]
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?
III
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.
IV
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 (α), Merak (β), Phaed (γ), Megrez (δ),
Alioth (ε), Mizar (ζ), and Benetnasch (η). 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.
[Illustration: The “Great Dipper”]
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.
[Illustration: Cassiopeia]
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 (β),
Alpha (α), Gamma (γ), Delta (δ), and Epsilon (ε). 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.
[Illustration: The “Northern Crown”]
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 sixdegrees—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.
[Illustration: The “Southern 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.
V
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.
[Illustration: Chart showing location of Tycho’s star, 1572, and Nova
Persei of 1901]
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!
[Illustration: Nova Persei, with its nebular rings]