THE HISTORY OF THE SUN.
The Inconstant Sun—Representation of the Solar System at different
Epochs—Primæval Density of the Sun—Illustration of Gas in Extreme
Tenuity—Physical State of the Sun at that Period—The Sun was then a
Nebula.
WE pointed out in the last chapter how, in consequence of its perennial
loss of heat, the orb of day must be undergoing a gradual diminution in
size. In the present chapter we are to set down the remarkable
conclusions with respect to the early history of the sun to which we
have been conducted by pursuing to its legitimate consequences the
shrinkage which the sun had undergone in times past.
The outer circle in Fig. 19 represents the track in which our earth now
revolves around the sun, and we are to understand that the radius of
this circle is about ninety-three million miles. We must imagine that
the innermost of the four circles represents the position of the sun.
Along its track the earth revolves year after year; so it has revolved
for centuries, so it has revolved since the days of the first monarch
that ever held sway in Britain, so it has revolved during all the time
over which history extends, so it has doubtless revolved for illimitable
periods anterior to history. For an interval of time that no one
presumes to define with any accuracy the earth has revolved in the same
track round that sun in heaven which, during all those ages, has
dispensed its benefits of light and heat for the sustenance of life on
our globe.
[Illustration: Fig. 19.—TO ILLUSTRATE THE HISTORY OF THE SUN.
Present orbit of Earth.
Sun in times very much earlier still.
Sun in very early times.
Present Sun.]
The sun appears constant during those few years in which man is allowed
to strut his little hour. The size of the sun and the lustre of the sun
has not appreciably altered. But the sun does not always remain the
same. It has not always shone with the brightness and vigour with which
it shines now; it will not continue for ever to dispense its benefits
with the same liberality that it does at present. The sun is always in a
state of change. It would not indeed be correct to refer to these
changes as growths, in the same sense in which we speak of the growth in
a tree. Decade after decade the tree waxes greater; but the sun, as we
have already explained, does not increase with the time, for the change
indeed lies the other way. It may well be that in this present era the
sun is near its prime, in so far as its capacity to radiate warmth and
brightness is concerned. It is, however, certain that the sun is not now
so large as it was in ancient days. The diminution of the orb is still
in progress. In these present days of its glorious splendour the orb of
day is much larger than it will be in that gloomy old age which destiny
assigns to it.
We have already shown how to give numerical precision to our facts. We
have stated that the sun’s diameter is diminishing at the rate of one
mile every eleven years. We have dwelt upon the remarkable significance
of that shrinkage in accounting for the sustentation of the sun’s heat.
We have now to call on this perennial diminution of the sun’s diameter
to provide some information as to the early history of our luminary.
The innermost circle in our sketch is to suggest the sun as it is at
present. Millions of years ago the orb of day was as large as I have
indicated it by the circle with the words “sun in very early times.” It
will, of course, be understood that we do not make any claim to precise
representation of the magnitude of the orb. At a period much earlier
still, the sun must have been larger still, and we venture so to depict
it. We know the rate at which the sun is now contracting, and doubtless
this rate has continued sensibly unaltered during thousands of years,
and indeed we might say scores of thousands of years. But it would not
be at all safe to assume that the annual rate of change in the sun’s
radius has remained the same throughout excessively remote periods in
its evolutionary history. What we do affirm is, that in the course of
its evolution the sun must have been contracting continually, and we
have been able to learn the particular rate of contraction
characteristic of the present time. But though we are ignorant of the
rate of contraction at very early epochs, yet the sun ever looms larger
and larger in days earlier and still earlier. But in those early days
the sun was not heavier, was not, indeed, quite so heavy as it is at
present. For we remember that the sun is perennially adding thousands of
tons to its bulk by the influx of meteors. Perhaps we ought to add that
the gain of mass from the meteors may be to some extent compensated by
the loss of substance which the sun not infrequently experiences if, as
is sometimes supposed, it expels in some violent convulsion a mass of
material which takes the form of a comet (Fig. 21).
Let us now consider what the density of the sun must have been in those
primæval days, say, for example, when the luminary had ten times the
volume that it has at present. Even now, as already stated, it does not
weigh half as much again as a globe of water of the same size, so that
when it was ten times as big its density must have been only a small
fraction of that of water. But we may take a stage still earlier. Let us
think of a time—it was, perhaps, many scores of millions of years
ago—when the sun was a thousand times as big as it is at present. The
same quantity of matter which now constitutes the sun was then expanded
over a volume a thousand times greater. A remarkable conclusion follows
from this consideration. The air that we breathe has a density which is
about the seven-hundredth part of that of water. Hence we see that at
the time when the materials of the sun were expanded into a volume a
thousand times as great as it is at present the density of the luminary
must have been about equal to that of ordinary air. We refer, of course,
in such statements to the average density of the sun. It will be
remembered that the density of the sun cannot be uniform. The mutual
attractions and pressures of the particles in the interior must make the
density greater the nearer we approach to the centre.
We must push our argument further still. We have ascertained that the
primæval sun could not have been a dense solid body like a ball of
metal. It must have been more nearly represented by a ball of gas. There
was a time when that collection of matter which now constitutes the sun
was so big that a balloon of equal size, filled at ordinary pressure
with the lightest of known gases, would contain within it a heavier
weight than the sun. At this early period the sun must have been as
light as an equal volume of hydrogen. The reasoning which has conducted
us to this point remains still unimpaired. From that early period we may
therefore look back to periods earlier still. We see that the sun must
have been ever larger and larger, for the same quantity of material must
have been ever more and more diffused. There was a time when the mean
density of the sun must have been far less than that of the gas in any
balloon.
We must not pause to consider intermediate stages. We shall look back at
once to an excessively early period when the sun—or perhaps we ought
rather to say the matter which in a more condensed form now constitutes
the sun—was expanded throughout the volume of a globe whose radius was
as great as the present distance from the sun to the earth. Have we not
here truly an astonishing result, deduced as a necessary consequence
from the fundamental laws of heat?
[Illustration: Fig. 20.—THE SOLAR CORONA (January 1st, 1899).
(_Photographed during Eclipse by Professor W. H. Pickering._)]
I need hardly say that the sun at that early date did not at all
resemble the glorious orb to which we owe our very existence. The
primæval sun must have been a totally different object, as we can easily
imagine if we try to think that the sun’s materials then filled a volume
twelve million times as great as they occupy at present. Instead of
comparing such an object with the gases in our ordinary atmosphere, it
should rather be likened to the residue left in an exhausted receiver
after the resources of chemistry have been taxed to make as near an
approach as possible to a perfect vacuum.
We can give a familiar illustration of gas in a state of extreme
tenuity. Look at the beautiful incandescent light with which in these
days our buildings are illuminated. How brilliantly those little globes
shine! The globe has to be most carefully sealed against the outside
air. If there were the smallest opportunity for access, the air from
outside would rush in and the lamp would be destroyed. In the
preparation of such a lamp elaborate precautions have to be taken to
secure that the exhaustion of the air from the little globe shall be as
nearly perfect as possible. Of course it is impossible to remove all the
air. No known processes can produce a perfect vacuum. Some traces of gas
would remain after the air-pump had been applied even for hours.
We must now imagine a globe, not merely two inches in diameter like one
of these little lamps, but a globe 186,000,000 miles in diameter, a
globe so large that the earth’s orbit would just form a girdle round it.
Even if this globe had been exhausted, so that its density was only the
twelve-thousandth part of the ordinary atmospheric density, it would
still contain more material than is found in the sun in heaven. Thus our
reasoning has conducted us to the notion of an epoch when the sun—or
rather I should say the matter composing the sun—formed something
totally different from the orb which we know so well. The matter in that
very diffuse state would not dispense light and heat as a sun in the
sense in which we understand the word. However vast might be the store
of energy which it contained—a store indeed thousands of times greater
than our present sun possesses—yet it would hardly be possessed of the
power of effective radiation. It would assuredly not be able to warm and
light a world associated with it, in the same way as the sun now
provides so gloriously for our wants and comfort.
[Illustration: Fig. 21.—THE GREAT COMET OF 1882.
(_Photographed on November 7th, 1882, by Sir David Gill, K.C.B._)]
But it is certain that in those early days there was no earth to be
warmed and lighted. Our globe, even if it can be said to have existed at
all, was truly “without form and void.” At the time when the sun was
swollen into a great globe of gas or rarefied matter, the elementary
substances which were to form the future earth were in a condition
utterly different from that of our present globe. The history of this
earth itself involves another chapter of the argument. Let it suffice to
notice, for the present, that our reasoning has led us to a time when
the sun consisted only of a rarefied gaseous material, and let us give
to the matter in this condition the name which astronomers apply to any
object of a similar character wherever they may meet with it in the
universe. Suppose that we could observe through our telescopes at the
present moment an object in remote space which was like what the sun
must have been at that early stage of its existence which we have been
considering, I do not think that the object would be unfamiliar to
astronomers. There is, indeed, no doubt that there are many objects
visible at this moment, and nightly studied in our observatories, which
are formed of matter just in the same state as the sun was in those
early times. Examined with a good telescope, the object would seem like
a small stain of light on the black background of the sky. The observer
would at once call it a nebula. In these modern days he would probably
apply the spectroscope to it, and this instrument would assure him that
the object he was looking at was a mass of incandescent gas. Such an
object would in all probability not greatly differ from many nebulæ now
known to us.
This being so, why should we withhold from the sun of primitive days the
designation to which it seems to be so fully entitled? Why should we not
speak of it as a nebula? The application of the laws of heat has shown
that the great orb of day was once one of those numerous objects which
astronomers know as nebulæ, and perhaps it may not be too fanciful to
suppose that a trace of the primæval nebula still survives in what we
call the Solar Corona (Fig. 20).
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