THE ASTRONOMICAL SIGNIFICANCE OF HEAT.
Heat and Astronomy--Distribution of Heat--The Presence of Heat in
the Earth--Heat in other Celestial Bodies--Varieties of
Temperature--The Law of Cooling--The Heat of the Sun--Can its
Temperature be Measured?--Radiation connected with the Sun's
Bulk--Can the Sun be Exhausting his Resources?--No marked Change
has occurred--Geological Evidence as to the Changes of the Sun's
Heat Doubtful--The Cooling of the Sun--The Sun cannot be merely an
Incandescent Solid Cooling--Combustion will not Explain the
Matter--Some Heat is obtained from Meteoric Matter, but this is not
Adequate to the Maintenance of the Sun's Heat--The Contraction of a
Heated Globe of Gas--An Apparent Paradox--The Doctrine of
Energy--The Nebular Theory--Evidence in Support of this
Theory--Sidereal Evidence of the Nebular Theory--Herschel's View of
Sidereal Aggregation--The Nebulae do not Exhibit Changes within the
Limits of our Observation.
That a portion of a work on astronomy should bear the title placed at
the head of this chapter will perhaps strike some of our readers as
unusual, if not actually inappropriate. Is not heat, it may be said, a
question merely of experimental physics? and how can it be legitimately
introduced into a treatise upon the heavenly bodies and their movements?
Whatever weight such objections might have once had need not now be
considered. The recent researches on heat have shown not only that heat
has important bearings on astronomy, but that it has really been one of
the chief agents by which the universe has been moulded into its actual
form. At the present time no work on astronomy could be complete without
some account of the remarkable connection between the laws of heat and
the astronomical consequences which follow from those laws.
In discussing the planetary motions and the laws of Kepler, or in
discussing the movements of the moon, the proper motions of the stars,
or the revolutions of the binary stars, we proceed on the supposition
that the bodies we are dealing with are rigid particles, and the
question as to whether these particles are hot or cold does not seem to
have any especial bearing. No doubt the ordinary periodic phenomena of
our system, such as the revolution of the planets in conformity with
Kepler's laws, will be observed for countless ages, whether the planets
be hot or cold, or whatever may be the heat of the sun. It must,
however, be admitted that the laws of heat introduce certain
modifications into the statement of these laws. The effects of heat may
not be immediately perceptible, but they exist--they are constantly
acting; and in the progress of time they are adequate to effecting the
mightiest changes throughout the universe.
Let us briefly recapitulate the circumstances of our system which give
to heat its potency. Look first at our earth, which at present seems--on
its surface, at all events--to be a body devoid of internal heat; a
closer examination will dispel this idea. Have we not the phenomena of
volcanoes, of geysers, and of hot springs, which show that in the
interior of the earth heat must exist in far greater intensity than we
find on the surface? These phenomena are found in widely different
regions of the earth. Their origin is, no doubt, involved in a good deal
of obscurity, but yet no one can deny that they indicate vast reservoirs
of heat. It would indeed seem that heat is to be found everywhere in the
deep inner regions of the earth. If we take a thermometer down a deep
mine, we find it records a temperature higher than at the surface. The
deeper we descend the higher is the temperature; and if the same rate of
progress should be maintained through those depths of the earth which we
are not able to penetrate, it can be demonstrated that at twenty or
thirty miles below the surface the temperature must be as great as that
of red-hot iron.
We find in the other celestial bodies abundant evidence of the present
or the past existence of heat. Our moon, as we have already mentioned,
affords a very striking instance of a body which must once have been
very highly heated. The extraordinary volcanoes on its surface place
this beyond any doubt. It is equally true that those volcanoes have
been silent for ages, so that, whatever may be the interior condition of
the moon, the surface has now cooled down. Extending our view further,
we see in the great planets Jupiter and Saturn evidence that they are
still endowed with a temperature far in excess of that which the earth
has retained; while, when we look at our sun, we see a body in a state
of brilliant incandescence, and glowing with a fervour to which we
cannot approximate in our mightiest furnaces. The various fixed stars
are bodies which glow with heat, like our sun; while we have in the
nebulae objects the existence of which is hardly intelligible to us,
unless we admit that they are possessed of heat.
From this rapid survey of the different bodies in our universe one
conclusion is obvious. We may have great doubts as to the actual
temperature of any individual body of the system; but it cannot be
doubted that there is a wide range of temperature among the different
bodies. Some are hotter than others. The stars and suns are perhaps the
hottest of all; but it is not improbable that they may be immeasurably
outnumbered by the cold and dark bodies of the universe, which are to us
invisible, and only manifest their existence in an indirect and casual
manner.
The law of cooling tells us that every body radiates heat, and that the
quantity of heat which it radiates increases when the temperature of the
body increases relatively to the surrounding medium. This law appears to
be universal. It is obeyed on the earth, and it would seem that it must
be equally obeyed by every other body in space. We thus see that each of
the planets and each of the stars is continuously pouring forth in all
directions a never-ceasing stream of heat. This radiation of heat is
productive of very momentous consequences. Let us study them, for
instance, in the case of the sun.
Our great luminary emits an incessant flood of radiant heat in all
directions. A minute fraction of that heat is intercepted by our earth,
and is, directly or indirectly, the source of all life, and of nearly
all movement, on our earth. To pour forth heat as the sun does, it is
necessary that his temperature be enormously high. And there are some
facts which permit us to form an estimate of what that temperature must
actually be.
It is difficult to form any numerical statement of the actual
temperature of the sun. The intensity of that temperature vastly
transcends the greatest artificial heat, and any attempt to clothe such
estimates in figures is necessarily very precarious. But assuming the
greatest artificial temperature to be about 4,000 deg. Fahr., we shall
probably be well within the truth if we state the effective temperature
of the sun to be about 14,000 deg. Fahr. This is the result of a recent
investigation by Messrs. Wilson and Gray, which seems to be entitled to
considerable weight.
The copious outflow of heat from the sun corresponds with its enormous
temperature. We can express the amount of heat in various ways, but it
must be remembered that considerable uncertainty still attaches to such
measurements. The old method of measuring heat by the quantity of ice
melted may be used as an illustration. It is computed that a shell of
ice 43-1/2 feet thick surrounding the whole sun would in one minute be
melted by the sun's heat underneath. A somewhat more elegant
illustration was also given by Sir John Herschel, who showed that if a
cylindrical glacier 45 miles in diameter were to be continually flowing
into the sun with the velocity of light, the end of that glacier would
be melted as quickly as it advanced. From each square foot in the
surface of the sun emerges a quantity of heat as great as could be
produced by the daily combustion of sixteen tons of coal. This is,
indeed, an amount of heat which, properly transformed into work, would
keep an engine of many hundreds of horse-power running from one year's
end to the other. The heat radiated from a few acres on the sun would be
adequate to drive all the steam engines in the world. When we reflect on
the vast intensity of the radiation from each square foot of the sun's
surface, and when we combine with this the stupendous dimensions of the
sun, imagination fails to realise how vast must be the actual
expenditure of heat.
In presence of the prodigal expenditure of the sun's heat, we are
tempted to ask a question which has the most vital interest for the
earth and its inhabitants. We live from hour to hour by the sun's
splendid generosity; and, therefore, it is important for us to know what
security we possess for the continuance of his favours. When we witness
the terrific disbursement of the sun's heat each hour, we are compelled
to ask whether our great luminary may not be exhausting its resources;
and if so, what are the prospects of the future? This question we can
partly answer. The whole subject is indeed of surpassing interest, and
redolent with the spirit of modern scientific thought.
Our first attempt to examine this question must lie in an appeal to the
facts which are attainable. We want to know whether the sun is showing
any symptoms of decay. Are the days as warm and as bright now as they
were last year, ten years ago, one hundred years ago? We can find no
evidence of any change since the beginning of authentic records. If the
sun's heat had perceptibly changed within the last two thousand years,
we should expect to find corresponding changes in the distribution of
plants and of animals; but no such changes have been detected. There is
no reason to think that the climate of ancient Greece or of ancient Rome
was appreciably different from the climates of the Greece and the Rome
that we know at this day. The vine and the olive grow now where they
grew two thousand years ago.
We must not, however, lay too much stress on this argument; for the
effects of slight changes in the sun's heat may have been neutralised by
corresponding adaptations in the pliable organisms of cultivated plants.
All we can certainly conclude is that no marked change has taken place
in the heat of the sun during historical time. But when we come to look
back into much earlier ages, we find copious evidence that the earth has
undergone great changes in climate. Geological records can on this
question hardly be misinterpreted. Yet it is curious to note that these
changes are hardly such as could arise from the gradual exhaustion of
the sun's radiation. No doubt, in very early times we have evidence
that the earth's climate must have been much warmer than at present. We
had the great carboniferous period, when the temperature must almost
have been tropical in Arctic latitudes. Yet it is hardly possible to
cite this as evidence that the sun was then much more powerful; for we
are immediately reminded of the glacial period, when our temperate zones
were overlaid by sheets of solid ice, as Northern Greenland is at
present. If we suppose the sun to have been hotter than it is at present
to account for the vegetation which produced coal, then we ought to
assume the sun to be colder than it is now to account for the glacial
period. It is not reasonable to attribute such phenomena to fluctuations
in the radiation from the sun. The glacial periods prove that we cannot
appeal to geology in aid of the doctrine that a secular cooling of the
sun is now in progress. The geological variations of climate may have
been caused by changes in the earth itself, or by changes in its actual
orbit; but however they have been caused, they hardly tell us much with
regard to the past history of our sun.
The heat of the sun has lasted countless ages; yet we cannot credit the
sun with the power of actually creating heat. We must apply to the
tremendous mass of the sun the same laws which we have found by our
experiments on the earth. We must ask, whence comes the heat sufficient
to supply this lavish outgoing? Let us briefly recount the various
suppositions that have been made.
Place two red-hot spheres of iron side by side, a large one and a small
one. They have been taken from the same fire; they were both equally
hot; they are both cooling, but the small sphere cools more rapidly. It
speedily becomes dark, while the large sphere is still glowing, and
would continue to do so for some minutes. The larger the sphere, the
longer it will take to cool; and hence it has been supposed that a
mighty sphere of the prodigious dimensions of our sun would, if once
heated, cool gradually, but the duration of the cooling would be so long
that for thousands and for millions of years it could continue to be a
source of light and heat to the revolving system of planets. This
suggestion will not bear the test of arithmetic. If the sun had no
source of heat beyond that indicated by its high temperature, we can
show that radiation would cool the sun a few degrees every year. Two
thousand years would then witness a very great decrease in the sun's
heat. We are certain that no such decrease can have taken place. The
source of the sun's radiation cannot be found in the mere cooling of an
incandescent mass.
Can the fires in the sun be maintained by combustion, analogous to that
which goes on in our furnaces? Here we would seem to have a source of
gigantic heat; but arithmetic also disposes of this supposition. We know
that if the sun were made of even solid coal itself, and if that coal
were burning in pure oxygen, the heat that could be produced would only
suffice for 6,000 years. If the sun which shone upon the builders of the
great Pyramid had been solid coal from surface to centre, it must by
this time have been in great part burned away in the attempt to maintain
its present rate of expenditure. We are thus forced to look to other
sources for the supply of the sun's heat, since neither the heat of
incandescence nor the heat of combustion will suffice.
There is probably--indeed, we may say certainly--one external source
from which the heat of the sun is recruited. It will be necessary for us
to consider this source with some care, though I think we shall find it
to be merely an auxiliary of comparatively trifling moment. According to
this view, the solar heat receives occasional accessions from the fall
upon the sun's surface of masses of meteoric matter. There can be hardly
a doubt that such masses do fall upon the sun; there is certainly no
doubt that if they do, the sun must gain some heat thereby. We have
experience on the earth of a very interesting kind, which illustrates
the development of heat by meteoric matter. There lies a world of
philosophy in a shooting star. Some of these myriad objects rush into
our atmosphere and are lost; others, no doubt, rush into the sun with
the same result. We also admit that the descent of a shooting star into
the atmosphere of the sun must be attended with a flash of light and of
heat. The heat acquired by the earth from the flashing of the shooting
stars through our air is quite insensible. It has been supposed,
however, that the heat accruing to the sun from the same cause may be
quite sensible--nay, it has been even supposed that the sun may be
re-invigorated from this source.
Here, again, we must apply the cold principles of weights and measures
to estimate the plausibility of this suggestion. We first calculate the
actual weight of meteoric indraught to the sun which would be adequate
to sustain the fires of the sun at their present vigour. The mass of
matter that would be required is so enormous that we cannot usefully
express it by imperial weights; we must deal with masses of imposing
magnitude. It fortunately happens that the weight of our moon is a
convenient unit. Conceive that our moon--a huge globe, 2,000 miles in
diameter--were crushed into a myriad of fragments, and that these
fragments were allowed to rain in on the sun; there can be no doubt that
this tremendous meteoric shower would contribute to the sun rather more
heat than would be required to supply his radiation for a whole year. If
we take our earth itself, conceive it comminuted into dust, and allow
that dust to fall on the sun as a mighty shower, each fragment would
instantly give out a quantity of heat, and the whole would add to the
sun a supply of heat adequate to sustain the present rate of radiation
for nearly one hundred years. The mighty mass of Jupiter treated in the
same way would generate a meteoric display greater in the ratio in which
the mass of Jupiter exceeds the mass of earth. Were Jupiter to fall into
the sun, enough heat would be thereby produced to scorch the whole solar
system; while all the planets together would be capable of producing
heat which, if properly economised, would supply the radiation of the
sun for 45,000 years.
It must be remembered that though the moon could supply one year's heat,
and Jupiter 30,000 years' heat, yet the practical question is not
whether the solar system could supply the sun's heat, but whether it
does. Is it likely that meteors equal in mass to the moon fall into the
sun every year? This is the real question, and I think we are bound to
reply to it in the negative. It can be shown that the quantity of
meteors which could be caught by the sun in any one year can be only an
excessively minute fraction of the total amount. If, therefore, a
moon-weight of meteors were caught every year, there must be an
incredible mass of meteoric matter roaming at large through the system.
There must be so many meteors that the earth would be incessantly pelted
with them, and heated to such a degree as to be rendered uninhabitable.
There are also other reasons which preclude the supposition that a
stupendous quantity of meteoric matter exists in the vicinity of the
sun. Such matter would produce an appreciable effect on the movement of
the planet Mercury. There are, no doubt, some irregularities in the
movements of Mercury not yet fully explained, but these irregularities
are very much less than would be the case if meteoric matter existed in
quantity adequate to the sustentation of the sun. Astronomers, then,
believe that though meteors may provide a rate in aid of the sun's
current expenditure, yet that the greater portion of that expenditure
must be defrayed from other resources.
It is one of the achievements of modern science to have effected the
solution of the problem--to have shown how it is that, notwithstanding
the stupendous radiation, the sun still maintains its temperature. The
question is not free from difficulty in its exposition, but the matter
is one of such very great importance that we are compelled to make the
attempt.
Let us imagine a vast globe of heated gas in space. This is not an
entirely gratuitous supposition, inasmuch as there are globes apparently
of this character; they have been already alluded to as planetary
nebulae. This globe will radiate heat, and we shall suppose that it emits
more heat than it receives from the radiation of other bodies. The globe
will accordingly lose heat, or what is equivalent thereto, but it will
be incorrect to assume that the globe will necessarily fall in
temperature. That the contrary is, indeed, the case is a result almost
paradoxical at the first glance; but yet it can be shown to be a
necessary consequence of the laws of heat and of gases.
Let us fix our attention on a portion of the gas lying on the surface
of the globe. This is, of course, attracted by all the rest of the
globe, and thus tends in towards the centre of the globe. If equilibrium
subsists, this tendency must be neutralised by the pressure of the gas
beneath; so that the greater the gravitation, the greater is the
pressure. When the globe of gas loses heat by radiation, let us suppose
that it grows colder--that its temperature accordingly falls; then,
since the pressure of a gas decreases when the temperature falls, the
pressure beneath the superficial layer of the gas will decrease, while
the gravitation is unaltered. The consequence will inevitably be that
the gravitation will now conquer the pressure, and the globe of gas will
accordingly contract. There is, however, another way in which we can
look at the matter. We know that heat is equivalent to energy, so that
when the globe radiates forth heat, it must expend energy. A part of the
energy of the globe will be due to its temperature; but another, and in
some respects a more important, part is that due to the separation of
its particles. If we allow the particles to come closer together we
shall diminish the energy due to separation, and the energy thus set
free can take the form of heat. But this drawing in of the particles
necessarily involves a shrinking of the globe.
And now for the remarkable consequence, which seems to have a very
important application in astronomy. As the globe contracts, a part of
its energy of separation is changed into heat; that heat is partly
radiated away, but not so rapidly as it is produced by the contraction.
The consequence is, that although the globe is really losing heat and
really contracting, yet that its temperature is actually rising.[43] A
simple case will suffice to demonstrate this result, paradoxical as it
may at first seem. Let us suppose that by contraction of the sphere it
had diminished to one-half its diameter; and let us fix our attention on
a cubic inch of the gaseous matter in any point of the mass. After the
contraction has taken place each edge of the cube would be reduced to
half an inch, and the volume would therefore be reduced to one-eighth
part of its original amount. The law of gases tells us that if the
temperature be unaltered the pressure varies inversely as the volume,
and consequently the internal pressure in the cube would in that case be
increased eightfold. As, however, in the case before us, the distance
between every two particles is reduced to one-half, it will follow that
the gravitation between every two particles is increased fourfold, and
as the area is also reduced to one-fourth, it will follow that the
pressure inside the reduced cube is increased sixteenfold; but we have
already seen that with a constant temperature it only increases
eightfold, and hence the temperature cannot be constant, but must rise
with the contraction.
We thus have the somewhat astonishing result that a gaseous globe in
space radiating heat, and thereby growing smaller, is all the time
actually increasing in temperature. But, it may be said, surely this
cannot go on for ever. Are we to suppose that the gaseous mass will go
on contracting and contracting with a temperature ever fiercer and
fiercer, and actually radiating out more and more heat the more it
loses? Where lies the limit to such a prospect? As the body contracts,
its density must increase, until it either becomes a liquid, or a solid,
or, at any rate, until it ceases to obey the laws of a purely gaseous
body which we have supposed. Once these laws cease to be observed the
argument disappears; the loss of heat may then really be attended with a
loss of temperature, until in the course of time the body has sunk to
the temperature of space itself.
It is not assumed that this reasoning can be applied in all its
completeness to the present state of the sun. The sun's density is now
so great that the laws of gases cannot be there strictly followed. There
is, however, good reason to believe that the sun was once more gaseous
than at present; possibly at one time he may have been quite gaseous
enough to admit of this reasoning in all its fulness. At present the sun
appears to be in some intermediate stage of its progress from the
gaseous condition to the solid condition. We cannot, therefore, say
that the temperature of the sun is now increasing in correspondence with
the process of contraction. This may be true or it may not be true; we
have no means of deciding the point. We may, however, feel certain that
the sun is still sufficiently gaseous to experience in some degree the
rise of temperature associated with the contraction. That rise in
temperature may be partly or wholly obscured by the fall in temperature
which would be the more obvious consequence of the radiation of heat
from the partially solid body. It will, however, be manifest that the
cooling of the sun may be enormously protracted if the fall of
temperature from the one cause be nearly compensated by the rise of
temperature from the other. It can hardly be doubted that in this we
find the real explanation of the fact that we have no historical
evidence of any appreciable alteration in the radiation of heat from the
sun.
This question is one of such interest that it may be worth while to look
at it from a slightly different point of view. The sun contains a
certain store of energy, part of which is continually disappearing in
the form of radiant heat. The energy remaining in the sun is partly
transformed in character; some of it is transformed into heat, which
goes wholly or partly to supply the loss by radiation. The total energy
of the sun must, however, be decreasing; and hence it would seem the sun
must at some time or other have its energy exhausted, and cease to be a
source of light and of heat. It is true that the rate at which the sun
contracts is very slow. We are, indeed, not able to measure with
certainty the decrease in the sun's bulk. It is a quantity so minute,
that the contraction since the birth of accurate astronomy is not large
enough to be perceptible in our telescopes. It is, however, possible to
compute what the contraction of the sun's bulk must be, on the
supposition that the energy lost by that contraction just suffices to
supply the daily radiation of heat. The change is very small when we
consider the present size of the sun. At the present time the sun's
diameter is about 860,000 miles. If each year this diameter decreases by
about 300 feet, sufficient energy will be yielded to account for the
entire radiation. This gradual decrease is always in progress.
These considerations are of considerable interest when we apply them
retrospectively. If it be true that the sun is at this moment shrinking,
then in past times his globe must have been greater than it is at
present. Assuming the figures already given, it follows that one hundred
years ago the diameter of the sun must have been nearly six miles
greater than it is now; one thousand years ago the diameter was
fifty-seven miles greater; ten thousand years ago the diameter of the
sun was five hundred and seventy miles greater than it is to-day. When
man first trod this earth it would seem that the sun must have been many
hundreds, perhaps many thousands, of miles greater than it is at this
time.
We must not, however, over-estimate the significance of this statement.
The diameter of the sun is so great, that a diminution of 10,000 miles
would be but little more than the hundredth part of its diameter. If it
were suddenly to shrink to the extent of 10,000 miles, the change would
not be appreciable to ordinary observation, though a much smaller change
would not elude delicate astronomical measurement. It does not
necessarily follow that the climates on our earth in these early times
must have been very different from those which we find at this day, for
the question of climate depends upon other matters besides sunbeams.
Yet we need not abruptly stop our retrospect at any epoch, however
remote. We may go back earlier and earlier, through the long ages which
geologists claim for the deposition of the stratified rocks; and back
again still further, to those very earliest epochs when life began to
dawn on the earth. Still we can find no reason to suppose that the law
of the sun's decreasing heat is not maintained; and thus we would seem
bound by our present knowledge to suppose that the sun grows larger and
larger the further our retrospect extends. We cannot assume that the
rate of that growth is always the same. No such assumption is required;
it is sufficient for our purpose that we find the sun growing larger
and larger the further we peer back into the remote abyss of time past.
If the present order of things in our universe has lasted long enough,
then it would seem that there was a time when the sun must have been
twice as large as it is at present; it must once have been ten times as
large. How long ago that was no one can venture to say. But we cannot
stop at the stage when the sun was even ten times as large as it is at
present; the arguments will still apply in earlier ages. We see the sun
swelling and swelling, with a corresponding decrease in its density,
until at length we find, instead of our sun as we know it, a mighty
nebula filling a gigantic region of space.
Such is, in fact, the doctrine of the origin of our system which has
been advanced in that celebrated speculation known as the nebular theory
of Laplace. Nor can it be ever more than a speculation; it cannot be
established by observation, nor can it be proved by calculation. It is
merely a conjecture, more or less plausible, but perhaps in some degree
necessarily true, if our present laws of heat, as we understand them,
admit of the extreme application here required, and if also the present
order of things has reigned for sufficient time without the intervention
of any influence at present unknown to us. This nebular theory is not
confined to the history of our sun. Precisely similar reasoning may be
extended to the individual planets: the farther we look back, the hotter
and the hotter does the whole system become. It has been thought that if
we could look far enough back, we should see the earth too hot for life;
back further still, we should find the earth and all the planets
red-hot; and back further still, to an exceedingly remote epoch, when
the planets would be heated just as much as our sun is now. In a still
earlier stage the whole solar system is thought to have been one vast
mass of glowing gas, from which the present forms of the sun, with the
planets and their satellites, have been gradually evolved. We cannot be
sure that the course of events has been what is here indicated; but
there are sufficient grounds for thinking that this doctrine
substantially represents what has actually occurred.
Many of the features in the solar system harmonise with the supposition
that the origin of the system has been that suggested by the nebular
theory. We have already had occasion in an earlier chapter to allude to
the fact that all the planets perform their revolutions around the sun
in the same direction. It is also to be observed that the rotation of
the planets on their axes, as well as the movements of the satellites
around their primaries, all follow the same law, with two slight
exceptions in the case of the Uranian and Neptunian systems. A
coincidence so remarkable naturally suggests the necessity for some
physical explanation. Such an explanation is offered by the nebular
theory. Suppose that countless ages ago a mighty nebula was slowly
rotating and slowly contracting. In the process of contraction, portions
of the condensed matter of the nebula would be left behind. These
portions would still revolve around the central mass, and each portion
would rotate on its axis in the same direction. As the process of
contraction proceeded, it would follow from dynamical principles that
the velocity of rotation would increase; and thus at length these
portions would consolidate into planets, while the central mass would
gradually contract to form the sun. By a similar process on a smaller
scale the systems of satellites were evolved from the contracting
primary. These satellites would also revolve in the same direction, and
thus the characteristic features of the solar system could be accounted
for.
The nebular origin of the solar system receives considerable countenance
from the study of the sidereal heavens. We have already dwelt upon the
resemblance between the sun and the stars. If, then, our sun has passed
through such changes as the nebular theory requires, may we not
anticipate that similar phenomena should be met with in other stars? If
this be so, it is reasonable to suppose that the evolution of some of
the stars may not have progressed so far as has that of the sun, and
thus we may be able actually to witness stars in the earlier phases of
their development. Let us see how far the telescope responds to these
anticipations.
The field of view of a large telescope usually discloses a number of
stars scattered over a black background of sky; but the blackness of
the background is not uniform: the practised eye of the skilled observer
will detect in some parts of the heavens a faint luminosity. This will
sometimes be visible over the whole extent of the field, or it may even
occupy several fields. Years may pass on, and still there is no
perceptible change. There can be no illusion, and the conclusion is
irresistible that the object is a stupendous mass of faintly luminous
glowing gas or vapour. This is the simplest type of nebula; it is
characterised by extreme faintness, and seems composed of matter of the
utmost tenuity. On the other hand we are occasionally presented with the
beautiful and striking phenomenon of a definite and brilliant star
surrounded by a luminous atmosphere. Between these two extreme types of
a faint diffused mass on the one hand, and a bright star with a nebula
surrounding it on the other, a graduated series of various other nebulae
can be arranged. We thus have a series of links passing by imperceptible
gradations from the most faintly diffused nebulae on the one side, into
stars on the other.
The nebulae seemed to Herschel to be vast masses of phosphorescent
vapour. This vapour gradually cools down, and ultimately condenses into
a star, or a cluster of stars. When the varied forms of nebulae were
classified, it almost seemed as if the different links in the process
could be actually witnessed. In the vast faint nebulae the process of
condensation had just begun; in the smaller and brighter nebulae the
condensation had advanced farther; while in others, the star, or stars,
arising from the condensation had already become visible.
But, it may be asked, how did Herschel know this? what is his evidence?
Let us answer this question by an illustration. Go into a forest, and
look at a noble old oak which has weathered the storm for centuries;
have we any doubt that the oak-tree was once a young small plant, and
that it grew stage by stage until it reached maturity? Yet no one has
ever followed an oak-tree through its various stages; the brief span of
human life has not been long enough to do so. The reason why we believe
the oak-tree to have passed through all these stages is, because we are
familiar with oak-trees of every gradation in size, from the seedling
up to the noble veteran. Having seen this gradation in a vast multitude
of trees, we are convinced that each individual passes through all these
stages.
It was by a similar train of reasoning that Herschel was led to adopt
the view of the origin of the stars which we have endeavoured to
describe. The astronomer's life is not long enough, the life of the
human race might not be long enough, to watch the process by which a
nebula condenses down so as to form a solid body. But by looking at one
nebula after another, the astronomer thinks he is able to detect the
various stages which connect the nebula in its original form with the
final form. He is thus led to believe that each of the nebulae passes, in
the course of ages, through these stages. And thus Herschel adopted the
opinion that stars--some, many, or all--have each originated from what
was once a glowing nebula.
Such a speculation may captivate the imagination, but it must be
carefully distinguished from the truths of astronomy, properly so
called. Remote posterity may perhaps obtain evidence on the subject
which to us is inaccessible: our knowledge of nebulae is too recent.
There has not yet been time enough to detect any appreciable changes:
for the study of nebulae can only be said to date from Messier's
Catalogue in 1771.
Since Herschel's time, no doubt, many careful drawings and observations
of the nebulae have been obtained; but still the interval has been much
too short, and the earlier observations are too imperfect, to enable any
changes in the nebulae to be investigated with sufficient accuracy. If
the human race lasts for very many centuries, and if our present
observations are preserved during that time for comparison, then
Herschel's theory may perhaps be satisfactorily tested.
A hundred years have passed since Laplace, with some diffidence, set
forth his hypothesis as to the mode of formation of the solar system. On
the whole it must be said that this "nebular hypothesis" has stood the
test of advancing science well, though some slight modifications have
become necessary in the light of more recent discoveries. Laplace (and
Herschel also) seems to have considered a primitive nebula to consist of
a "fiery mist" or glowing gas at a very high temperature. But this is by
no means necessary, as we have seen that the gradual contraction of the
vast mass supplies energy which may be converted into heat, and the
spectroscopic evidence seems also to point to the existence of a
moderate temperature in the gaseous nebulae, which must be considered to
be representatives of the hypothetical primitive chaos out of which our
sun and planets have been evolved. Another point which has been
reconsidered is the formation of the various planets. It was formerly
thought that the rotation of the original mass had by degrees caused a
number of rings of different dimensions to be separated from the central
part, the material of which rings in time collected into single planets.
The ring of Saturn was held to be a proof of this process, since we here
have a ring, the condensation of which into one or more satellites has
somehow been arrested. But while it is not impossible that matter in the
shape of rings may have been left behind during the contraction of the
nebulous mass (indeed, the minor planets between Mars and Jupiter have
perhaps originated in this way), it seems likely that the larger planets
were formed from the agglomeration of matter at a point on the equator
of the rotating nebula.
The actual steps of the process by which the primeval nebula became
transformed into the solar system seem to lie beyond reach of discovery.