conditions, may have an intimate relation with yet another of the
mysteries of the universe. It is a fundamental law of the universe that
when a body emits light or heat, or anything capable of being
transformed into light or heat, it can do so only by the expenditure of
force, limited in supply. The sun and stars are continually sending out
a flood of heat. They are exhausting the internal supply of something
which must be limited in extent. Whence comes the supply? How is the
heat of the sun kept up? If it were a hot body cooling off, a very few
years would suffice for it to cool off so far that its surface would
become solid and very soon cold. In recent years, the theory
universally accepted has been that the supply of heat is kept up by the
continual contraction of the sun, by mutual gravitation of its parts as
it cools off. This theory has the advantage of enabling us to
calculate, with some approximation to exactness, at what rate the sun
must be contracting in order to keep up the supply of heat which it
radiates. On this theory, it must, ten millions of years ago, have had
twice its present diameter, while less than twenty millions of years
ago it could not have existed except as an immense nebula filling the
whole solar system. We must bear in mind that this theory is the only
one which accounts for the supply of heat, even through human history.
If it be true, then the sun, earth, and solar system must be less than
twenty million years old.
Here the geologists step in and tell us that this conclusion is wholly
inadmissible. The study of the strata of the earth and of many other
geological phenomena, they assure us, makes it certain that the earth
must have existed much in its present condition for hundreds of
millions of years. During all that time there can have been no great
diminution in the supply of heat radiated by the sun.
The astronomer, in considering this argument, has to admit that he
finds a similar difficulty in connection with the stars and nebulas. It
is an impossibility to regard these objects as new; they must be as old
as the universe itself. They radiate heat and light year after year. In
all probability, they must have been doing so for millions of years.
Whence comes the supply? The geologist may well claim that until the
astronomer explains this mystery in his own domain, he cannot declare
the conclusions of geology as to the age of the earth to be wholly
inadmissible.
Now, the scientific experiments of the last two years have brought this
mystery of the celestial spaces right down into our earthly
laboratories. M. and Madame Curie have discovered the singular metal
radium, which seems to send out light, heat, and other rays
incessantly, without, so far as has yet been determined, drawing the
required energy from any outward source. As we have already pointed
out, such an emanation must come from some storehouse of energy. Is the
storehouse, then, in the medium itself, or does the latter draw it from
surrounding objects? If it does, it must abstract heat from these
objects. This question has been settled by Professor Dewar, at the
Royal Institution, London, by placing the radium in a medium next to
the coldest that art has yet produced--liquid air. The latter is
surrounded by the only yet colder medium, liquid hydrogen, so that no
heat can reach it. Under these circumstances, the radium still gives
out heat, boiling away the liquid air until the latter has entirely
disappeared. Instead of the radiation diminishing with time, it rather
seems to increase.
Called on to explain all this, science can only say that a molecular
change must be going on in the radium, to correspond to the heat it
gives out. What that change may be is still a complete mystery. It is a
mystery which we find alike in those minute specimens of the rarest of
substances under our microscopes, in the sun, and in the vast nebulous
masses in the midst of which our whole solar system would be but a
speck. The unravelling of this mystery must be the great work of
science of the twentieth century. What results shall follow for mankind
one cannot say, any more than he could have said two hundred years ago
what modern science would bring forth. Perhaps, before future
developments, all the boasted achievements of the nineteenth century
may take the modest place which we now assign to the science of the
eighteenth century--that of the infant which is to grow into a man.
III
THE STRUCTURE OF THE UNIVERSE
The questions of the extent of the universe in space and of its
duration in time, especially of its possible infinity in either space
or time, are of the highest interest both in philosophy and science.
The traditional philosophy had no means of attacking these questions
except considerations suggested by pure reason, analogy, and that
general fitness of things which was supposed to mark the order of
nature. With modern science the questions belong to the realm of fact,
and can be decided only by the results of observation and a study of
the laws to which these results may lead.
From the philosophic stand-point, a discussion of this subject which is
of such weight that in the history of thought it must be assigned a
place above all others, is that of Kant in his "Kritik." Here we find
two opposing propositions--the thesis that the universe occupies only a
finite space and is of finite duration; the antithesis that it is
infinite both as regards extent in space and duration in time. Both of
these opposing propositions are shown to admit of demonstration with
equal force, not directly, but by the methods of reductio ad absurdum.
The difficulty, discussed by Kant, was more tersely expressed by
Hamilton in pointing out that we could neither conceive of infinite
space nor of space as bounded. The methods and conclusions of modern
astronomy are, however, in no way at variance with Kant's reasoning, so
far as it extends. The fact is that the problem with which the
philosopher of Konigsberg vainly grappled is one which our science
cannot solve any more than could his logic. We may hope to gain
complete information as to everything which lies within the range of
the telescope, and to trace to its beginning every process which we can
now see going on in space. But before questions of the absolute
beginning of things, or of the boundary beyond which nothing exists,
our means of inquiry are quite powerless.
Another example of the ancient method is found in the great work of
Copernicus. It is remarkable how completely the first expounder of the
system of the world was dominated by the philosophy of his time, which
he had inherited from his predecessors. This is seen not only in the
general course of thought through the opening chapters of his work, but
among his introductory propositions. The first of these is that the
universe--mundus--as well as the earth, is spherical in form. His
arguments for the sphericity of the earth, as derived from observation,
are little more than a repetition of those of Ptolemy, and therefore
not of special interest. His proposition that the universe is spherical
is, however, not based on observation, but on considerations of the
perfection of the spherical form, the general tendency of bodies--a
drop of water, for example--to assume this form, and the sphericity of
the sun and moon. The idea retained its place in his mind, although the
fundamental conception of his system did away with the idea of the
universe having any well-defined form.
The question as attacked by modern astronomy is this: we see scattered
through space in every direction many millions of stars of various
orders of brightness and at distances so great as to defy exact
measurement, except in the case of a few of the nearest. Has this
collection of stars any well-defined boundary, or is what we see merely
that part of an infinite mass which chances to lie within the range of
our telescopes? If we were transported to the most distant star of
which we have knowledge, should we there find ourselves still
surrounded by stars on all sides, or would the space beyond be void?
Granting that, in any or every direction, there is a limit to the
universe, and that the space beyond is therefore void, what is the form
of the whole system and the distance of its boundaries? Preliminary in
some sort to these questions are the more approachable ones: Of what
sort of matter is the universe formed? and into what sort of bodies is
this matter collected?
To the ancients the celestial sphere was a reality, instead of a mere
effect of perspective, as we regard it. The stars were set on its
surface, or at least at no great distance within its crystalline mass.
Outside of it imagination placed the empyrean. When and how these
conceptions vanished from the mind of man, it would be as hard to say
as when and how Santa Claus gets transformed in the mind of the child.
They are not treated as realities by any astronomical writer from
Ptolemy down; yet, the impressions and forms of thought to which they
gave rise are well marked in Copernicus and faintly evident in Kepler.
The latter was perhaps the first to suggest that the sun might be one
of the stars; yet, from defective knowledge of the relative brightness
of the latter, he was led to the conclusion that their distances from
each other were less than the distance which separated them from the
sun. The latter he supposed to stand in the centre of a vast vacant
region within the system of stars.
For us the great collection of millions of stars which are made known
to us by the telescope, together with all the invisible bodies which
may be contained within the limits of the system, form the universe.
Here the term "universe" is perhaps objectionable because there may be
other systems than the one with which we are acquainted. The term
stellar system is, therefore, a better one by which to designate the
collection of stars in question.
It is remarkable that the first known propounder of that theory of the
form and arrangement of the system which has been most generally
accepted seems to have been a writer otherwise unknown in
science--Thomas Wright, of Durham, England. He is said to have
published a book on the theory of the universe, about 1750. It does not
appear that this work was of a very scientific character, and it was,
perhaps, too much in the nature of a speculation to excite notice in
scientific circles. One of the curious features of the history is that
it was Kant who first cited Wright's theory, pointed out its accordance
with the appearance of the Milky Way, and showed its general
reasonableness. But, at the time in question, the work of the
philosopher of Konigsberg seems to have excited no more notice among
his scientific contemporaries than that of Wright.
Kant's fame as a speculative philosopher has so eclipsed his scientific
work that the latter has but recently been appraised at its true value.
He was the originator of views which, though defective in detail,
embodied a remarkable number of the results of recent research on the
structure and form of the universe, and the changes taking place in it.
The most curious illustration of the way in which he arrived at a
correct conclusion by defective reasoning is found in his anticipation
of the modern theory of a constant retardation of the velocity with
which the earth revolves on its axis. He conceived that this effect
must result from the force exerted by the tidal wave, as moving towards
the west it strikes the eastern coasts of Asia and America. An opposite
conclusion was reached by Laplace, who showed that the effect of this
force was neutralized by forces producing the wave and acting in the
opposite direction. And yet, nearly a century later, it was shown that
while Laplace was quite correct as regards the general principles
involved, the friction of the moving water must prevent the complete
neutralization of the two opposing forces, and leave a small residual
force acting towards the west and retarding the rotation. Kant's
conclusion was established, but by an action different from that which
he supposed.
The theory of Wright and Kant, which was still further developed by
Herschel, was that our stellar system has somewhat the form of a
flattened cylinder, or perhaps that which the earth would assume if, in
consequence of more rapid rotation, the bulging out at its equator and
the flattening at its poles were carried to an extreme limit. This form
has been correctly though satirically compared to that of a grindstone.
It rests to a certain extent, but not entirely, on the idea that the
stars are scattered through space with equal thickness in every
direction, and that the appearance of the Milky Way is due to the fact
that we, situated in the centre of this flattened system, see more
stars in the direction of the circumference of the system than in that
of its poles. The argument on which the view in question rests may be
made clear in the following way.
Let us chose for our observations that hour of the night at which the
Milky Way skirts our horizon. This is nearly the case in the evenings
of May and June, though the coincidence with the horizon can never be
exact except to observers stationed near the tropics. Using the figure
of the grindstone, we at its centre will then have its circumference
around our horizon, while the axis will be nearly vertical. The points
in which the latter intersects the celestial sphere are called the
galactic poles. There will be two of these poles, the one at the hour
in question near the zenith, the other in our nadir, and therefore
invisible to us, though seen by our antipodes. Our horizon corresponds,
as it were, to the central circle of the Milky Way, which now surrounds
us on all sides in a horizontal direction, while the galactic poles are
90 degrees distant from every part of it, as every point of the horizon
is 90 degrees from the zenith.
Let us next count the number of stars visible in a powerful telescope
in the region of the heavens around the galactic pole, now our zenith,
and find the average number per square degree. This will be the
richness of the region in stars. Then we take regions nearer the
horizontal Milky Way--say that contained between 10 degrees and 20
degrees from the zenith--and, by a similar count, find its richness in
stars. We do the same for other regions, nearer and nearer to the
horizon, till we reach the galaxy itself. The result of all the counts
will be that the richness of the sky in stars is least around the
galactic pole, and increases in every direction towards the Milky Way.
Without such counts of the stars we might imagine our stellar system to
be a globular collection of stars around which the object in question
passed as a girdle; and we might take a globe with a chain passing
around it as representative of the possible figure of the stellar
system. But the actual increase in star-thickness which we have pointed
out shows us that this view is incorrect. The nature and validity of
the conclusions to be drawn can be best appreciated by a statement of
some features of this tendency of the stars to crowd towards the
galactic circle.
Most remarkable is the fact that the tendency is seen even among the
brighter stars. Without either telescope or technical knowledge, the
careful observer of the stars will notice that the most brilliant
constellations show this tendency. The glorious Orion, Canis Major
containing the brightest star in the heavens, Cassiopeia, Perseus,
Cygnus, and Lyra with its bright-blue Vega, not to mention such
constellations as the Southern Cross, all lie in or near the Milky Way.
Schiaparelli has extended the investigation to all the stars visible to
the naked eye. He laid down on planispheres the number of such stars in
each region of the heavens of 5 degrees square. Each region was then
shaded with a tint that was darker as the region was richer in stars.
The very existence of the Milky Way was ignored in this work, though
his most darkly shaded regions lie along the course of this belt. By
drawing a band around the sky so as to follow or cover his darkest
regions, we shall rediscover the course of the Milky Way without any
reference to the actual object. It is hardly necessary to add that this
result would be reached with yet greater precision if we included the
telescopic stars to any degree of magnitude--plotting them on a chart
and shading the chart in the same way. What we learn from this is that
the stellar system is not an irregular chaos; and that notwithstanding
all its minor irregularities, it may be considered as built up with
special reference to the Milky Way as a foundation.
Another feature of the tendency in question is that it is more and more
marked as we include fainter stars in our count. The galactic region is
perhaps twice as rich in stars visible to the naked eye as the rest of
the heavens. In telescopic stars to the ninth magnitude it is three or
four times as rich. In the stars found on the photographs of the sky
made at the Harvard and other observatories, and in the stargauges of
the Herschels, it is from five to ten times as rich.
Another feature showing the unity of the system is the symmetry of the
heavens on the two sides of the galactic belt Let us return to our
supposition of such a position of the celestial sphere, with respect to
the horizon, that the latter coincides with the central line of this
belt, one galactic pole being near our zenith. The celestial hemisphere
which, being above our horizon, is visible to us, is the one to which
we have hitherto directed our attention in describing the distribution
of the stars. But below our horizon is another hemisphere, that of our
antipodes, which is the counterpart of ours. The stars which it
contains are in a different part of the universe from those which we
see, and, without unity of plan, would not be subject to the same law.
But the most accurate counts of stars that have been made fail to show
any difference in their general arrangement in the two hemispheres.
They are just as thick around the south galactic poles as around the
north one. They show the same tendency to crowd towards the Milky Way
in the hemisphere invisible to us as in the hemisphere which we see.
Slight differences and irregularities, are, indeed, found in the
enumeration, but they are no greater than must necessarily arise from
the difficulty of stopping our count at a perfectly fixed magnitude.
The aim of star-counts is not to estimate the total number of stars,
for this is beyond our power, but the number visible with a given
telescope. In such work different observers have explored different
parts of the sky, and in a count of the same region by two observers we
shall find that, although they attempt to stop at the same magnitude,
each will include a great number of stars which the other omits. There
is, therefore, room for considerable difference in the numbers of stars
recorded, without there being any actual inequality between the two
hemispheres.
A corresponding similarity is found in the physical constitution of the
stars as brought out by the spectroscope. The Milky Way is extremely
rich in bluish stars, which make up a considerable majority of the
cloudlike masses there seen. But when we recede from the galaxy on one
side, we find the blue stars becoming thinner, while those having a
yellow tinge become relatively more numerous. This difference of color
also is the same on the two sides of the galactic plane. Nor can any
systematic difference be detected between the proper motions of the
stars in these two hemispheres. If the largest known proper motion is
found in the one, the second largest is in the other. Counting all the
known stars that have proper motions exceeding a given limit, we find
about as many in one hemisphere as in the other. In this respect, also,
the universe appears to be alike through its whole extent. It is the
uniformity thus prevailing through the visible universe, as far as we
can see, in two opposite directions, which inspires us with confidence
in the possibility of ultimately reaching some well-founded conclusion
as to the extent and structure of the system.
All these facts concur in supporting the view of Wright, Kant, and
Herschel as to the form of the universe. The farther out the stars
extend in any direction, the more stars we may see in that direction.
In the direction of the axis of the cylinder, the distances of the
boundary are least, so that we see fewer stars. The farther we direct
our attention towards the equatorial regions of the system, the greater
the distance from us to the boundary, and hence the more stars we see.
The fact that the increase in the number of stars seen towards the
equatorial region of the system is greater, the smaller the stars, is
the natural consequence of the fact that distant stars come within our
view in greater numbers towards the equatorial than towards the polar
regions.
Objections have been raised to the Herschelian view on the ground that
it assumes an approximately uniform distribution of the stars in space.
It has been claimed that the fact of our seeing more stars in one
direction than in another may not arise merely from our looking through
a deeper stratum, as Herschel supposed, but may as well be due to the
stars being more thinly scattered in the direction of the axis of the
system than in that of its equatorial region. The great inequalities in
the richness of neighboring regions in the Milky Way show that the
hypothesis of uniform distribution does not apply to the equatorial
region. The claim has therefore been made that there is no proof of the
system extending out any farther in the equatorial than in the polar
direction.
The consideration of this objection requires a closer inquiry as to
what we are to understand by the form of our system. We have already
pointed out the impossibility of assigning any boundary beyond which we
can say that nothing exists. And even as regards a boundary of our
stellar system, it is impossible for us to assign any exact limit
beyond which no star is visible to us. The analogy of collections of
stars seen in various parts of the heavens leads us to suppose that
there may be no well-defined form to our system, but that, as we go out
farther and farther, we shall see occasional scattered stars to,
possibly, an indefinite distance. The truth probably is that, as in
ascending a mountain, we find the trees, which may be very dense at its
base, thin out gradually as we approach the summit, where there may be
few or none, so we might find the stars to thin out could we fly to the
distant regions of space. The practical question is whether, in such a
flight, we should find this sooner by going in the direction of the
axis of our system than by directing our course towards the Milky Way.
If a point is at length reached beyond which there are but few
scattered stars, such a point would, for us, mark the boundary of our
system. From this point of view the answer does not seem to admit of
doubt. If, going in every direction, we mark the point, if any, at
which the great mass of the stars are seen behind us, the totality of
all these points will lie on a surface of the general form that
Herschel supposed.
There is still another direct indication of the finitude of our stellar
system upon which we have not touched. If this system extended out
without limit in any direction whatever, it is shown by a geometric
process which it is not necessary to explain in the present connection,
but which is of the character of mathematical demonstration, that the
heavens would, in every direction where this was true, blaze with the
light of the noonday sun. This would be very different from the
blue-black sky which we actually see on a clear night, and which, with
a reservation that we shall consider hereafter, shows that, how far
so-ever our stellar system may extend, it is not infinite. Beyond this
negative conclusion the fact does not teach us much. Vast, indeed, is
the distance to which the system might extend without the sky appearing
much brighter than it is, and we must have recourse to other
considerations in seeking for indications of a boundary, or even of a
well-marked thinning out, of stars.