sun, whose remoteness we have elsewhere tried to give an idea of, is
comparatively close at hand; quite at hand, one may say, for if his
distance, which we have found so enormous, be represented by that
of a man standing so close beside us that our hand may rest on his
shoulder, to obtain the proportionate distance of one of the _nearest_
stars, like Sirius, for instance, we should need to send the man over
a hundred miles away. It is probably impossible to give to any one an
adequate idea of the extent of the sidereal universe; but it certainly
is especially hard for the reader who has just realized with difficulty
the actual immensity of the distance of the sun, and who is next told
that this distance is literally a physical point as seen from the
nearest star. The jaded imagination can be spurred to no higher flight,
and the facts and the enormous numbers that convey them will not be
comprehended.
Look down at one of the nests of those smallest ants, which are made
in our paths. To these little people, we may suppose, the other side
of the gravel walk is the other side of the world, and the ant who
has been as far as the gate, a greater traveller than a man who comes
back from the Indies. It is very hard to think not only of ourselves
as relatively far smaller than such insects, but that, less than such
an ant-hill is to the whole landscape, is our solar system itself in
comparison with the new prospect before us; yet so it is.
All greatness and littleness are relative. When the traveller from the
great star Sirius (where, according to the author of “Micromegas,”
all the inhabitants are proportionately tall and proportionately
long-lived), discovered our own little solar system, and lighted on
what we call the majestic planet Saturn, he was naturally astonished at
the pettiness of everything compared with the world he had left. That
the Saturnian inhabitants were in his eyes a race of mere dwarfs (they
were only a mile high, instead of twenty-four miles like himself) did
not make them contemptible to his philosophic mind, for he reflected
that such little creatures might still think and reason; but when he
learned that these puny beings were also correspondingly short-lived,
and passed but fifteen thousand years between the cradle and the
grave, he could not but agree that this was like dying as soon as one
was born, that their life was but a span, and their globe an atom. Yet
it seems that when one of these very Saturnian dwarfs came afterward
with him to our own little ball, and by the aid of a microscope
discovered certain animalculæ on its surface, and even held converse
with two of them, he could not in turn make up his own mind that
intelligence could inhere in such invisible insects, till one of them
(it was an astronomer with his sextant) measured his height to an inch,
and the other, a divine, expounded to him the theology of some of these
mites, according to which all the heavenly host, including Saturn and
Sirius itself, were created for _them_.
Do not let us hold this parable as out of place here, for what use is
it to write down a long series of figures expressing the magnitude of
other worlds, if it leave us with the old sense of the importance to
creation of our own; and what use to describe their infinite number to
a human mite who reads, and remains of the opinion that _he_ is the
object they were all created for?
Above us are millions of suns like ours. The Milky Way (shown on page
225) spreads among them, vague and all-surrounding, as a type of the
infinities yet unexplored, and of the world of nebulæ of which we
still know so little. Let us say at once that it is impossible here to
undertake the description of the discoveries of the New Astronomy in
this region, for we can scarcely indicate the headings of the chapters
which would need to be written to describe what is most important.
[Illustration: FIG. 87.--THE MILKY WAY. (FROM A STUDY BY E. L.
TROUVELOT).]
The first of these chapters (if we treated our subjects in the order of
distance) would be one on space itself, and our changed ideas of the
void which separates us from the stars. Of this we will only say in
passing, that the old term “the temperature of space” has been nearly
abrogated; for while it used to be supposed that more than half of
the heat which warmed the earth came from this mysterious “space”
or from the stars, it is now recognized that the earth is principally
warmed only by the sun. Of the contents of the region between the
earth and the stars, we have, it must be admitted, still little but
conjecture; though perhaps that conjecture turns more than formerly to
the idea that the void is not a real void, but that it is occupied by
something which, if highly attenuated, is none the less matter, and
something other and more than the mere metaphysical conception of a
vehicle to transmit light to us.
Of the stars themselves, we should need another chapter to tell what
has been newly learned as to their color and light, even by the old
methods, that is, by the eye and the telescope alone; but if we
cannot dwell on this, we must at least refer, however inadequately,
to what American astronomers are doing in this department of the New
Astronomy, and first in the photometry of the stars, which has assumed
a new importance of late years, owing to the labors carried on in this
department at Cambridge.
That one star differs from another star in glory we have long heard,
but our knowledge of physical things depends largely on our ability
to answer the question, “how much?” and the value of this new work
lies in the accuracy and fulness of its measures; for in this case the
whole heavens visible from Cambridge to near the southern horizon have
been surveyed, and the brightness of every naked-eye star repeatedly
measured, so that all future changes can be noted. This great work has
taxed the resources of a great observatory, and its results are only to
be adequately valued by other astronomers; but Professor Pickering’s
own investigations on variable stars have a more popular interest. It
is surely an amazing fact that suns as large or larger than our own
should seem to dwindle almost to extinction, and regain their light
within a few days or even hours; yet the fact has long been known,
while the cause has remained a mystery. A mystery, in most cases, it
remains still; but in some we have begun to get knowledge, as in the
well-known instance of Algol, the star in the head of Medusa. Here it
has always been thought probable that the change was due to something
coming between us and the star; but it is on this very account that
the new investigation is more interesting, as showing how much can be
done on an old subject by fresh reasoning alone, and how much valuable
ore may lie in material which has already been sifted. The discussion
of the subject by Professor Pickering, apart from its elevated aim,
has if, in its acute analysis only, the interest belonging to a story
where the reader first sees a number of possible clews to some mystery,
and then the gradual setting aside, one by one, of those which are
only loose ends, and the recognition of the real ones which lead to
the successful solution. The skill of the novelist, however, is more
apparent than real, since the riddle he solves for us is one he has
himself constructed, while here the enigma is of Nature’s propounding;
and if the solution alone were given us, the means by which it is
reached would indeed seem to be inexplicable.
This is especially so when we remember what a point there is to work
on, for the whole system reasoned about, though it may be larger than
our own, is at such a distance that it appears, literally and exactly,
far smaller to the eye than the point of the finest sewing-needle;
and it is a course of accurate reasoning, and reasoning alone, on
the character of the observed changing brightness of this point,
which has not only shown the existence of some great dark satellite,
but indicated its size, its distance from its sun, its time of
revolution, the inclination of its orbit, and still more. The existence
of dark invisible bodies in space, then, is in one case at least
demonstrated, and in this instance the dark body is of enormous size;
for, to illustrate by our own solar system, we should probably have
to represent it in imagination by a planet or swarm of planetoids
hundreds of times the size of Jupiter, and (it may be added) whirling
around the sun at less than a tenth the distance of Mercury.
Of a wholly different class of variables are those which have till
lately only been known at intervals of centuries, like that new star
Tycho saw in 1572. I infer from numerous inquiries that there is such a
prevalent popular notion that the “Star of Bethlehem” may be expected
to show itself again at about the present time, that perhaps I may be
excused for answering these questions in the present connection.
In the first place, the idea is not a new, but a very old one, going
back to the time of Tycho himself, who disputed the alleged identity
of his star with that which appeared to the shepherds at the Nativity.
The evidence relied on is, that bright stars are said to have appeared
in this constellation repeatedly at intervals of from three hundred
and eight to three hundred and nineteen years (though even this is
uncertain); and as the mean of these numbers is about three hundred and
fourteen, which again is about one-fifth of 1572 (the then number of
years from the birth of Christ), it has been suggested, in support of
the old notion, that the Star of Bethlehem might have been a variable,
shining out every three hundred and fourteen or three hundred and
fifteen years, whose fifth return would fall in with the appearance
that Tycho saw, and whose _sixth_ return would come in 1886 or 1887.
This is all there is about it, and there is nothing like evidence,
either that this was the star seen by the Wise Men, or that it is to
be seen again by us. On the other hand, nothing in our knowledge, or
rather in our ignorance, authorizes us to say positively it cannot
come again; and it may be stated for the benefit of those who like to
believe in its speedy return, that if it does come, it will make its
appearance some night in the northern constellation of Cassiopeia’s
chair, the position originally determined by Tycho at its last
appearance, being twenty-eight degrees and thirteen minutes from the
pole, and twenty-six minutes in right ascension.
We were speaking of these new stars as having till lately only appeared
at intervals of centuries; but it is not to be inferred that if they
now appear oftener it is because there are more of them. The reason
is, that there are more persons looking for them; and the fact is
recognized that, if we have observers enough and look closely enough,
the appearance of “new stars” is not so very rare a phenomenon. Every
one at all interested in such matters remembers that in 1866 a new
star broke out in the Northern Crown so suddenly that it was shining
as bright as the Polar Star, where six hours before there had been
nothing visible to the eve. Now all stars are not as large as our sun,
though some are much larger; but there are circumstances which make
it improbable that this was a small or near object, and it is well
remembered how the spectroscope showed the presence of abnormal amounts
of incandescent hydrogen, the material which is perhaps the most
widely diffused in the universe (and which is plentiful, too, in our
own bodies), so that there was some countenance to the popular notion
that this was a world in flames. We were, at any rate, witnessing a
catastrophe which no earthly experience can give us a notion of, in a
field of action so remote that the flash of light which brought the
news was unknown years on the way, so that all this--strange but now
familiar thought--occurred long before we _saw_ it happen. The star
faded in a few days to invisibility to the naked eye, though not to the
telescope; and, in fact, all these phenomena at present appear rather
to be enormous and sudden enlargements of the light of existing bodies
than the creation of absolutely new ones; while of these “new stars”
the examples may almost be said to be now growing numerous, two having
appeared in the last two years.
Not to enlarge, then, on this chapter of photometry, let us add, in
reference to another department of stellar astronomical work, that the
recognized master in the study of double stars the world over is not an
astronomer by profession, at the head of some national observatory in
Berlin or Paris, but a stenographer in the Chicago law-courts, Mr. W.
S. Burnham, who, after his day’s duties, by nightly labor, prolonged
for years with the small means at an amateur’s command, has perhaps
added more to our knowledge of his special subject in ten years than
all other living astronomers.
[Illustration: FIG. 88.--SPECTRA OF STARS IN PLEIADES.]
We have here only alluded to the spectroscope in its application to
stellar research, and we cannot now do more than to note the mere
headlines of the chapters that should be written on it.
First, there is the memorable fact that, after reaching across the
immeasurable distances, we find that the stars are like _us_,--like in
their ultimate elements to those found in our own sun, our own earth,
our own bodies. Any fuller view of the subject than that which we here
only indicate, would begin with the evidence of this truth, which is
perhaps on the whole the most momentous our science has brought us, and
with which no familiarity should lessen our wonder, or our sense of its
deep and permanent significance.
Next, perhaps, we should understand that, invading the province of the
Old Astronomy, the spectroscope now tells us of the motions of these
stars, which we cannot see move,--motions in what we have always called
the “fixed” stars, to signify a state of fixity to the human eye, which
is such, that to it at the close of the nineteenth century they remain
in the same relative positions that they occupied when that eye first
looked on them, in some period long before the count of centuries began.
In perhaps the earliest and most enduring work of man’s hands, the
great pyramid of Egypt, is a long straight shaft, cut slopingly through
the solid stone, and pointing, like a telescope, to the heavens near
the pole. If we look through it now we see--nothing; but when it was
set up it pointed to a particular star which is no longer there. That
pyramid was built when the savages of Britain saw the Southern Cross
at night; and the same slow change in the direction of the earth’s
axis, that in thousands of years has borne that constellation to
southern skies, has carried the stone tube away from the star that it
once pointed at. The actual motion of the star itself, relatively to
our system, is slower yet,--so inconceivably slow that we can hardly
realize it by comparison with the duration of the longest periods
of human history. The stone tube was pointed at the star by the old
Egyptians, but “Egypt itself is now become the land of obliviousness,
and doteth. Her ancient civility is gone, and her glory hath vanished
as a phantasma. She poreth not upon the heavens, astronomy is dead unto
her, and knowledge maketh other cycles. Canopus is afar off, Memnon
resoundeth not to the Sun, and Nilus heareth strange voices.” In all
this lapse of ages, the star’s own motion could not have so much as
carried it across the mouth of the narrow tube. Yet a motion to or
from us of this degree, so slow that the unaided eve could not see it
in thousands of years of watching, the spectroscope, first efficiently
in the hands of the English astronomer, Dr. Huggins, and later in
those of Professor Young of Princeton, not only reveals at a look, but
tells us the amount and direction of it, in a way that is as strange
and unexpected, in the view of our knowledge a generation ago, as its
revelation of the essential composition of the bodies themselves.
[Illustration: FIG. 89.--SPECTRUM OF ALDEBARAN.]
[Illustration: FIG. 90.--SPECTRUM OF VEGA.]
Again, in showing us this composition, it has also shown us more, for
it has enabled us to form a conjecture as to the relative ages of the
stars and suns; and this work of classifying them, not only according
to their brightness, but each after his kind, we may observe was
begun by a countryman of our own, Mr. Rutherfurd, who seems to have
been among the first after Fraunhofer to apply the newly-invented
instrument to the stars, and quite the first to recognize that these
were, broadly speaking, divisible into a few leading types, depending
not on their size but on their essential nature. After him Secchi
(to whom the first conception is often wrongly attributed) developed
it, and gave four main classes into which the stars are in this way
divisible, a classification which has been much extended by others;
while the first carefully delineated spectra were those of Dr. Huggins,
who has done so much for all departments of our science that in a
fuller account his name would reappear in every chapter of this New
Astronomy, and than whom there is no more eminent living example of
its study. Owing to their feeble light, years were needed when he
began his work to depict completely so full a single spectrum as that
he gives of Aldebaran, though he has lived to see stellar spectrum
photography, whose use he first made familiar, producing in its
newest development, which we give here, the same result in almost as
many minutes. Before we present this latest achievement of celestial
photography, let us employ the old method of an engraving made from
eye-drawings, once more, to illustrate on page 222 the distinct
character of these spectra, and their meaning. In the telespectroscope,
the star is drawn out into a band of colored light, but here we note
only in black and white the lines which are seen crossing it, the
red end in these drawings being at the left, and the violet at the
right; and we may observe of this illustration, that though it may
be criticised by the professional student, and though it lack to the
general reader the attraction of color, or of beautiful form, it is
yet full of interest to any one who wishes to learn the meaning of the
message the star’s light can be made to yield through the spectroscope,
and to know how significant the differences are it indicates between
one star and another, where all look so alike to the eye. First is
the spectrum of a typical white or blue-white star, Sirius,--the very
brightest star in the sky, and which we all know. The brighter part
of the spectrum is a nearly continuous ribbon of color, crossed by
conspicuous, broad, dark lines, exactly corresponding in place to
narrower ones in our sun, and due principally to hydrogen. Iron and
magnesium are also indicated in this class, but by too fine lines to be
here shown.
Sirius, as will be presently seen, belongs to the division of stars
whose spectrum indicates a very high temperature, and in this case, as
in what follows, we may remark (to use in part Mr. Lockyer’s words)
that one of the most important distinctions between the stars in the
heavens is one not depending upon their mass or upon anything of that
kind, but upon conditions which make their spectra differ, just in the
way that in our laboratories the spectrum of one and the same body will
differ at different temperatures.
What these absolutely are in the case of the stars, we may not
know; but placing them in their most probable relative order, we
have taken as an instance of the second class, or lower-temperature
stage, our own sun. The impossibility of giving a just notion of its
real complexity may be understood, when we state that in the recent
magnificent photographs by Professor Rowland, a part alone of this
spectrum occupies something like fifty times the space here given to
the whole, so that, crowded with lines as this appears, scarcely one
in fifty of those actually visible can be given in it. Without trying
to understand all these now, let us notice only the identity of two
or three of its principal elements with those found in other stars,
as shown by the corresponding identity of some leading lines. Thus, C
and F (with others) are known to be caused by hydrogen; D, by sodium;
_b_, by magnesium; while fainter lines are given by iron and by other
substances. These elements can be traced by their lines in most of
the different star-spectra on this plate, and all those named are
constituents of our own frames.
The hydrogen lines are not quite accurately shown in the plate from
which our engraving is made, those in Sirius, for instance, being
really wider by comparison than they are here given; and we may observe
in this connection, that by the particular appearance such lines wear
in the spectrum itself we can obtain some notion of the _mass_ of a
star, as well as of its chemical constitution. We can compare the
essential characteristics of such bodies, then, without reference
to their apparent size, or as though they were all equally remote;
and it is a striking thought, that when we thus rise to an impartial
contemplation of the whole stellar universe, our sun, whose least ray
makes the whole host of stars disappear, is found to be not only
itself a star, but by comparison a small one,--one at least which is
more probably below than above the average individual of its class,
while some, such as Sirius, are not impossibly hundreds of times its
size.
Then comes a third class, such as is shown in the spectrum of the
brightest star in Orion, looking still a little like that of our sun;
but yet more distinctively in that of the brightest star in Hercules,
looking like a columnar or fluted structure, and concerning which the
observations of Lockyer and others create the strong presumption, not
to say certainty, that we have here a lower temperature still. Antares
and other reddish stars belong to this division, which in the very
red stars passes into the fourth type, and there are more classes and
subclasses without end; but we invite here attention particularly to
the first three, much as we might present a child, an adult, and an
old man, as types of the stages of human existence, without meaning to
deny that there are any number of ages between. We can even say that
this may be something more than a mere figure of speech, and that a
succession in age is not improbably pointed at in these types.
[Illustration: FIG. 91.--GREAT NEBULA IN ORION. (FROM A PHOTOGRAPH BY
A. A. COMMON, F. R. S.)]
We may have considered--perhaps not without a sort of awe at the
vastness of the retrospect--the past life of the worlds of our own
system, from our own globe of fluid fire as we see it by analogy in
the past, through the stages of planetary life to the actual condition
of our present green earth, and on to the stillness of the moon. Yet
the life history of our sun, we can hardly but admit, is indefinitely
longer than this. We feel, rather than comprehend, the vastness of
the period that separates our civilization from the early life of the
world; but what is this to the age of the sun, which has looked on and
seen its planetary children grow? Yet if we admit this temperature
classification of the stars, we are not far from admitting that the
spectroscope is now pointing out the stages in the life of suns
themselves; suns just beginning their life of almost infinite years;
suns in the middle of their course; suns which are growing old and
casting feebler beams,--all these and many more it brings before us.
Another division of our subject would, with more space, include a
fuller account of that strange and most interesting development of
photography which is going on even while we write; and this is so new
and so important, that we must try to give some hint of it even in
this brief summary, for even since the first numbers of this series
were written, great advances have taken place in its application to
celestial objects.