THE NEBULAE
1. I have attempted to show that, even if we suppose the other bodies of
the universe to resemble the Earth, so far as to seem, by their
materials, forms, and motions, no less fitted than she is to be the
abodes of life; yet that, knowing what we do of man, we can believe that
the Earth is tenanted by a race who are the _special_ objects of God's
care. Even if the tendency of the analogies of creation were, to incline
us to suppose that the other planets are as well suited as our globe, to
have inhabitants, still it would require a great amount of evidence, to
make us believe that they have such inhabitants as we are; while yet
such evidence is altogether wanting. Even if we knew that the stars were
the centres of revolving systems, we should have an immense difficulty
in believing that an Earth, with such a population as ours, revolves
about any of them. If astronomy made a plurality of worlds probable, we
have strong reasonings, drawn from other subjects, to think that the
other worlds are not like ours.
2. The admirers of astronomical triumphs may perhaps be disposed to say,
that when so much has been discovered, we may be allowed to complete the
scheme by the exercise of fancy. I have attempted to show that we are
not in such a state of ignorance, when we look at other relations of
the earth and of man, as to allow us to do this. But now we may go a
little onwards in our argument; and may ask, whether Astronomy really
does what is here claimed for her:--whether she carries us so securely
to the bounds of the visible universe, that our Fancy may take up the
task, and people the space thus explored:--whether the bodies which
Astronomy has examined, be really as fitted as our Earth, to sustain a
population of living things:--whether the most distant objects in the
universe do really seem to be systems, or the beginnings of
systems:--whether Astronomy herself may not incline in favor of the
condition of man, as being the sole creature of his kind?
3. In making this inquiry, it will of course be understood, that I do so
with the highest admiration for the vast discoveries which Astronomy has
really made; and for the marvellous skill and invention of the great men
who have, in all ages of the world, and not least, in our time, been the
authors of such discoveries. From the time when Galileo first discovered
the system of Jupiter's satellites, to the last scrutiny of the
structure of a nebula by Lord Rosse's gigantic telescope, the history of
the telescopic exploration of the sky, has been a history of genius
felicitously employed in revealing wonders. In this history, the noble
labors of the first and the second Herschel relative to the distribution
of the fixed stars, the forms and classes of nebulae, and the phenomena
of double stars, especially bear upon our present speculations; to which
we may add, the examination of the aspect of each planet, by various
observers, as Schroeter, and of the moon by others, from Huyghens to
Maedler and Beer. The achievements which are most likely to occur to the
reader's mind are those of the Earl of Rosse; as being the latest
addition to our knowledge, and the result of the greatest instrumental
powers. By the energy and ingenuity of that eminent person, an eye is
directed to the heavens, having a pupil of six feet diameter, with the
most complete optical structure, and the power of ranging about for its
objects over a great extent of sky; and thus the quantity of light which
the eye receives from any point of the heavens is augmented, it may be,
fifty thousand times. The rising Moon is seen from the Observatory in
Ireland with the same increase of size and light, as if her solid globe,
two thousand miles in diameter, retaining all its illumination, really
rested upon the summits of the Alps, to be gazed at by the naked eye. An
object which appears to the naked eye a single star, may, by this
telescope, so far as its power of seeing is concerned, be resolved into
fifty thousand stars, each of the same brightness as the obvious star.
What seems to the unassisted vision a nebula, a patch of diluted light,
in which no distinct luminous point can be detected, may, by such an
instrument, be discriminated or resolved into a number of bright dots;
as the stippled shades of an engraving are resolved into dots by the
application of a powerful magnifying glass. Similar results of the
application of great telescopic power had of course been attained long
previously; but, as the nature of scientific research is, each step adds
something to our means of knowledge; and the last addition assumes,
includes, and augments the knowledge which we possessed before. The
discussions in which we are engaged, belong to the very boundary region
of science;--to the frontier where knowledge, at least astronomical
knowledge, ends, and ignorance begins. Such discoveries, therefore, as
those made by Lord Rosse's telescope, require our special notice here.
4. We may begin, at what appears to us the outskirts of creation, the
Nebulae. At one time it was conceived by astronomers in general, that
these patches of diffused light, which are seen by them in such
profusion in the sky, are not luminous bodies of regular terms and
definite boundaries, apparently solid, as the stars are supposed to be;
but really, as even to good telescopes many of them seem, masses of
luminous cloud or vapor, loosely held together, as clouds and vapors
are, and not capable by any powers of vision of being resolved into
distinct visible elements. This opinion was for a time so confidentially
entertained, that there was founded upon it an hypothesis, that these
were gaseous masses, out of which suns and systems might afterwards be
formed, by the concentration of these luminous vapors into a solid
central sun, more intensely luminous; while detached portions of the
mass, flying off, and cooling down so as to be no longer self-luminous,
might revolve round the central body, as planets and satellites. This is
the _Nebular Hypothesis_, suggested by the elder Herschel, and adopted
by the great mathematician Laplace.
5. But the result of the optical scrutiny of the nebulae by more modern
observers, especially by Lord Rosse in Ireland, and Mr. Bond in America,
has been, that many celestial objects which were regarded before as
truly nebulous, have been resolved into stars; and this resolution has
been extended to so many cases of nebulae, of such various kinds, as to
have produced a strong suspicion in the minds of astronomers that _all_
the nebulae, however different in their appearance, may really be
resolved into stars, if they be attacked with optical powers
sufficiently great.
6. If this were to be assumed as done, and if each of the separate
points, into which the nebulae are thus resolved, were conceived to be a
star, which looks so small only because it is so distant, and which
really is as likely to have a system of planets revolving about it, as
is a star of the first magnitude:--we should then have a view of the
immensity of the visible universe, such as I presented to the reader in
the beginning of this essay. All the distant nebulae appear as nebulae,
only because they are so distant; if truly seen, they are groups of
stars, of which each may be as important as our sun, being, like it, the
centre of a planetary system. And thus, a patch of the heavens, one
hundredth or one thousandth part of the visible breadth of our sun, may
contain in it more life, not only than exists in the solar system, but
in as many such systems as the unassisted eye can see stars in the
heavens, on the clearest winter night.
7. This is a stupendous view of the greatness of the creation; and, to
many persons, its very majesty, derived from magnitude and number, will
make it so striking and acceptable, that, once apprehended, they will
feel as if there were a kind of irreverence in disturbing it. But if
this view be really not tenable when more closely examined, it is, after
all, not wise to connect our feelings of religious reverence with it, so
that they shall suffer a shock when we are obliged to reject it. I may
add, that we may entertain an undoubting trust that any view of the
creation which is found to be true, will also be found to supply
material for reverential contemplation. I venture to hope that we may,
by further examination, be led to a reverence of a deeper and more
solemn character than a mere wonder at the immensity of space and
number.
8. But whatever the result may be, let us consider the evidence for this
view. It assumes that all the Nebulae are resolvable into stars, and that
they appear as nebulae only because they are more distant than the region
in which they can appear as stars. Are there any facts, any phenomena in
the heavens, which may help us to determine whether this is a probable
opinion?
9. It is most satisfactory for us, when we can, in such inquiries, know
the thoughts which have suggested themselves to the minds of those who
have examined the phenomena with the most complete knowledge, the
greatest care, and the best advantages; and have speculated upon these
phenomena in a way both profound and unprejudiced. Some remarks of Sir
John Herschel, recommended by these precious characters, seem to me to
bear strongly upon the question which I have just had to ask:--Do all
the nebulae owe their nebulous appearance to their being too distant to
be seen as groups of distinct stars, though they really are such groups?
10. Herschel, in the visit which he made to the Cape of Good Hope, for
the purpose of erecting to his father the most splendid monument that
son ever erected,--the completed survey of the vault of heaven,--had
full opportunity of studying a certain pair of remarkable bright spaces
of the skies, filled with a cloudy light, which lie near the southern
pole; and which, having been unavoidably noticed by the first Antarctic
voyagers, are called the _Magellanic Clouds_. When the larger of these
two clouds is examined through powerful telescopes, it presents, we are
told, a constitution of uncommon complexity: "large patches and tracts
of nebulosity in every stage of resolution, from light, irresolvable
with eighteen inches of reflecting aperture, up to perfectly separated
stars like the Milky Way, and clustering groups sufficiently insulated
and condensed to come under the designation of irregular, and in some
cases pretty rich clusters. But besides these, there are also nebulae in
abundance, both regular and irregular; globular clusters in every stage
of condensation, and objects of a nebulous character quite peculiar, and
which have no analogies in any other region of the heavens."[1] He goes
on to say, that these nebulae and clusters are far more crowded in this
space than they are in any other, even the most crowded parts, of the
nebulous heavens. This _Nubecula Major_, as it is termed, is of a round
or oval form, and its diameter is about six degrees, so that it is about
twelve times the apparent diameter of the moon. The _Nubecula Minor_ is
a smaller patch of the same kind. If we suppose the space occupied by
the various objects which the nubecula major includes, to be, in a
general way, spherical, its nearest and most remote parts must (as its
angular size proves) differ in their distance from us by little more
than a tenth part of our distance from its centre. That the two nubeculae
are thus approximately spherical spaces, is in the highest degree
probable; not only from the peculiarity of their contents, which
suggests the notion of a peculiar group of objects, collected into a
limited space; but from the barrenness, as to such objects, of the sky
in the neighborhood of these Magellanic Clouds. To suppose (the only
other possible supposition) that they are two columns of space, with
their ends turned towards us, and their lengths hundreds and thousands
of times their breadths, would be too fantastical a proceeding to be
tolerated; and would, after all, not explain the facts without further
altogether arbitrary assumptions.
11. It appears, then, that, in these groups, there are stars of various
magnitudes, clusters of various forms, nebulae regular and irregular,
nebulous tracts and patches of peculiar character; and all so disposed,
that the most distant of them, whichever these may be, are not more than
one-tenth more distant than the nearest. If the nearest star in this
space be at nine times the distance of Sirius, the farthest nebulae,
contained in the same space, will not be at more than ten times the
distance of Sirius. Of course, the doctrine that nebulae are seen as
nebulae, merely because they are so distant, requires us to assume all
nebulae to be hundreds and thousands of times more distant than the
smallest stars. If stars of the eighth magnitude (which are hardly
visible to the naked eye) be eight times as remote as Sirius, a nebula
containing a thousand stars, which is invisible to the naked eye, must
be more than eight thousand times as remote as Sirius. And thus if, in
the whole galaxy, we reckon only the stars as far as the eighth
magnitude, and suppose all the stars of the galaxy to form a nebula,
which is visible to the spectators in a distant nebula, only as their
nebula is visible to us; we must place them at eight thousand times two
hundred thousand times the distance of the Sun; and, even so, we are
obviously vastly understating the calculation. These are the gigantic
estimates with which some astronomical speculators have been in the
habit of overwhelming the minds of their listeners; and these views have
given a kind of majesty to the aspect of the nebulae; and have led some
persons to speak of the discovery of every new streak of nebulous light
in the starry heavens, as a discovery of new worlds, and still new
worlds. But the Magellanic Clouds show us very clearly that all these
calculations are entirely baseless. In those regions of space, there
coexists, in a limited compass, and in indiscriminate position, stars,
clusters of stars, nebulae, regular and irregular, and nebulous streaks
and patches. These, then, are different kinds of things in themselves,
not merely different to us. There are such things as nebulae side by side
with stars, and with clusters of stars. Nebulous matter resolvable
occurs close to nebulous matter irresolvable. The last and widest step
by which the dimensions of the universe have been expanded in the
notions of eager speculators, is checked by a completer knowledge and a
sager spirit of speculation. Whatever inference we may draw from the
resolvability of some of the nebulae, we may not draw this
inference;--that they are more distant, and contain a larger array of
systems and of worlds, in proportion as they are difficult to resolve.
12. But indeed, if we consider this process, of the resolution of nebulae
into luminous points, on its own ground, without looking to such facts
as I have just adduced, it will be difficult, or impossible, to assign
any reason why it should lead to such inferences as have been drawn from
it. Let us look at this matter more clearly. An astronomer, armed with a
powerful telescope, _resolves_ a nebula, discerns that a luminous cloud
is composed of shining dots:--but what are these dots? Into _what_ does
he resolve the nebula? Into _Stars_, it is commonly said. Let us not
wrangle about words. By all means let these dots be Stars, if we know
about what we are speaking: if a _Star_ merely mean a luminous dot in
the sky. But that these stars shall resemble, in their nature, stars of
the first magnitude, and that such stars shall resemble our Sun, are
surely very bold structures of assumption to build on such a basis. Some
nebulae are resolvable; are resolvable into distinct points; certainly a
very curious, probably an important discovery. We may hereafter learn
that _all_ nebulae are resolvable into distinct points: that would be a
still more curious discovery. But what would it amount to? What would be
the simple way of expressing it, without hypothesis, and without
assumption? Plainly this: that the substance of all nebulae is not
continuous, but discrete;--separable, and separate into distinct
luminous elements;--nebulae are, it would then seem, as it were, of a
curdled or granulated texture; they have run into _lumps_ of light, or
have been formed originally of such lumps. Highly curious. But what are
these lumps? How large are they? At what distances? Of what structure?
Of what use? It would seem that he must be a bold man who undertakes to
answer these questions. Certainly he must appear to ordinary thinkers to
be _very_ bold, who, in reply, says, gravely and confidently, as if he
had unquestionable authority for his teaching:--"These lumps, O man, are
Suns; they are distant from each other as far as the Dog-star is from
us; each has its system of Planets, which revolve around it; and each of
these Planets is the seat of an animal and vegetable creation. Among
these Planets, some, we do not yet know how many, are occupied by
rational and responsible creatures, like Man; and the only matter which
perplexes us, holding this belief on astronomical grounds, is, that we
do not quite see how to put our theology into its due place and form in
our system."
13. In discussing such matters as these, where our knowledge and our
ignorance are so curiously blended together, and where it is so
difficult to make men feel that so much ignorance can lie so close to so
much knowledge;--to make them believe that they have been allowed to
discover so much, and yet are not allowed to discover more:--we may be
permitted to illustrate our meaning, by supposing a case of blended
knowledge and ignorance, of real and imaginary discovery. Suppose that
there were carried from a scientific to a more ignorant nation,
excellent maps of the world, finely engraved; the mountain-ranges shaded
in the most delicate manner, and the sheet crowded with information of
all kinds, in writing large, small, and microscopic. Suppose also, that
when these maps had been studied with the naked eye, so as to establish
a profound respect for the knowledge and skill of the author of them,
some of those who perused them should be furnished with good
microscopes, so as to carry their examination further than before. They
might then find that, in several parts, what before appeared to be
merely crooked lines, was really writing, stating, it may be, the amount
of population of a province, or the date of foundation of a town. To
exhaust all the information thus contained on the maps, might be a work
of considerable time and labor. But suppose that, when this was done, a
body of resolute microscopists should insist that the information which
the map contained was not exhausted: that they should continue peering
perseveringly at the lines which formed the shading of the mountains,
maintaining that these lines also were writing, if only it might be
deciphered; and should go on increasing, with immense labor and
ingenuity, the powers of their microscopes, in order to discover the
legend contained in these unmeaning lines. We should, perhaps, have here
an image of the employment of these astronomers, who now go on looking
in nebulae for worlds. And we may notice in passing, that several of the
arguments which are used by such astronomers, might be used, and would
be used, by our microscopists:--how improbable it was that a person so
full of knowledge, and so able to convey it, as the author of the maps
was known to be, should not have a design and purpose in every line that
he drew: what a waste of space it would be to leave any part of the
sheet blank of information; and the like. To which the reply is to us
obvious; that the design of shading the mountains was design enough; and
that the information conveyed was all that was necessary or convenient.
Nor does this illustration at all tend to show that such astronomical
scrutiny, directed intelligently, with a right selection of the points
examined, may not be highly interesting and important. If the
microscopists had examined the map with a view to determine the best way
in which mountains can be indicated by shading, they would have employed
themselves upon a question which has been the subject of multiplied and
instructive discussion in our own day.
14. But to return to the subject of Nebulae, we may further say, with
the most complete confidence, that whether or not nebulous matter be
generally resolvable into shining dots, it cannot possibly be true that
its being, or not being so resolvable by our telescopes, depends merely
upon its smaller or greater distance from the observer. For, in the
first place, that there is matter, to the best assisted eye not
distinguishable from nebulous matter, which is not so resolvable, is
proved by several facts. The tails of Comets often resemble nebulae; so
much so that there are several known nebulae, which are, by the less
experienced explorers of the sky, perpetually mistaken for comets, till
they are proved not to be so, by their having no cometary motion. Such
is the nebula in Andromeda, which is visible to the naked eye.[2] But
the tails and nebulous appendages of comets, though they alter their
appearance very greatly, according to the power of the telescope with
which they are examined, have never been resolved into stars, or any
kind of dots; and seem, by all investigations, to be sheets or cylinders
or cones of luminous vapor, changing their form as they approach to or
recede from the sun, and perhaps by the influence of other causes. Yet
some of them approach very near the earth; all of them come within the
limits of our system. Here, then, we have (probably, at least,) nebulous
matter, which when brought close to the eye, compared with the stellar
nebulae, still appears as nebulous.
15. Again, as another phenomenon, bearing upon the same question, we
have the Zodiacal Light. This is a faint cone of light[3] which, at
certain seasons, may be seen extending from the horizon obliquely
upwards, and following the course of the ecliptic, or rather, of the
sun's equator. It appears to be a lens-shaped envelope of the sun,
extending beyond the orbits of Mercury and Venus, and nearly attaining
that of the earth; and in Sir John Herschel's view, may be regarded as
placing the sun in the list of nebulous stars. No one has ever thought
that this nebulous appearance was resolvable into luminous points; but
if it were, probably not even the most sanguine of speculators on the
multitude of suns would call these points _suns_.
16. But indeed the nebulae themselves, and especially the most remote of
the nebulae, or at least those which most especially require the most
powerful telescopes, offer far more decisive proofs that their
resolvability or non-resolvability,--their apparent constitution as
diffused and vaporous masses,--does not depend upon their distance. A
remarkable fact in the irregular, and in some of the regular nebulae[4]
is, that they consist of long patches and streaks, which stretch out in
various directions, and of which the form[5] and extent vary according
to the visual power which is applied to them. Many of the nebulae and
especially of the fainter ones, entirely change their form with the
optical power of the instrument by which they are scrutinized; so that,
as seen in the mightier telescopes of modern times, the astronomer
scarcely recognizes the figures in which the earlier observers have
recorded what they saw in the same place. Parts which, before, were
separate, are connected by thin bridges of light which are now detected;
and where the nebulous space appeared to be bounded, it sends off long
tails of faint light into the surrounding space. Now, no one can suppose
that these newly-seen portions of the nebula are immensely further off
than the other parts. However little we know of the nature of the
object, we must suppose it to be one connected object, with all its
parts, as to sense, at the same distance from us. Whether therefore it
be resolvable or no, there must be some other reason, besides the
difference of distance, why the brighter parts were seen, while the
fainter parts were not. The obvious reason is, that the latter were not
seen because they were thin films which required more light to see them.
We are led, irresistibly as it seems, to regard the whole mass of such a
nebula, as an aggregation of vaporous rolls and streaks, assuming such
forms as thin volumes of smoke or vapor often assume in our atmosphere,
and assuming, like them, different shapes according to the quantity of
light which comes to us from them. If, as soon as one of these new
filaments or webs of a nebula comes into view, we should say, Here we
have a new array of suns and of worlds, we should judge as
fantastically, as any one who should combine the like imaginations with
the varying cloud-work of a summer-sky. To suppose that all the varied
streaks by which the patch of nebulous light shades off into the
surrounding darkness, and which change their form and extent with every
additional polish which we can give to a reflecting or refracting
surface, disclose, with every new streak, new worlds, is a wanton
indulgence of fancy, to which astronomy gives us no countenance.[6]
17. Undoubtedly all true astronomers, taught caution and temperance of
thought by the discipline of their magnificent science, abstain from
founding such assumptions upon their discoveries. They know how
necessary it is to be upon their guard against the tricks which fancy
plays with the senses; and if they see appearances of which they cannot
interpret the meaning, they are content that they should have no meaning
for them, till the due explanation comes. We have innumerable examples
of this wise and cautious temper, in all periods of astronomy. One has
occurred lately. Several careful astronomers, observing the stars by
day, had been surprised to see globes of light glide across the field of
view of their telescopes, often in rapid succession and in great
numbers. They did not, as may be supposed, rush to the assumption that
these globes were celestial bodies of a new kind, before unseen; and
that from the peculiarity of their appearance and movement, they were
probably inhabited by beings of a peculiar kind. They proceeded very
differently; they altered the focus of their telescopes, looked with
other glasses, made various changes and trials, and finally discovered
that these globes of light were the winged seeds of certain plants which
were wafted through the air; and which, illuminated by the sun, were
made globular by being at distances unsuited to the focus of the
telescope.[7]
18. But perhaps something more may be founded on the ramified and
straggling form which belongs to many of the nebulae. Under the powers of
Lord Rosse's telescope, a considerable number of them assume a shape
consisting of several spiral films diverging from one centre, and
growing broader and fainter as they diverge, so as to resemble a curled
feather, or whirlpool of light.[8] This form, though generally deformed
by irregularities, more or less, is traceable in so many of the nebulae,
that we cannot easily divest ourselves of the persuasion that there is
some general reason for such a form;--that something, in the mechanical
causes which have produced the nebulae, has tended to give them this
shape. Now, when this thought has occurred to us, since mathematicians
have written a great deal concerning the mechanics of the universe, it
is natural to ask, whether any of the problems which they have solved
give a result like that thus presented to our eyes. Do such spirals as
we here see, occur in any of the diagrams which illustrate the possible
motions of celestial bodies? And to this, a person acquainted with
mathematical literature might reply, that in the second Book of Newton's
_Principia_, in the part which has especial reference to the Vortices
of Descartes, such spirals appear upon the page. They represent
the path which a body would describe if, acted upon by a central
force, it had to move in a medium of which the resistance was
considerable;--considerable, that is, in comparison with the other
forces which act; as for example, the forces which deflect the motion
from a straight line. Indeed, that in such a case a body would describe
a spiral, of which the general form would be more or less oval, is
evident on a little consideration. And in this way, for instance,
Encke's comet, which, if the resistance to its motion were insensible,
would go on describing an ellipse about the sun, always returning upon
the same path after every revolution; does really describe a path which,
at each revolution, falls a little within the preceding revolution, and
thus gradually converges to the centre. And if we suppose the comet to
consist of a luminous mass, or a string of masses, which should occupy
a considerable arc of such an orbit, the orbit would be marked by a
track of light, as an oval spiral. Or if such a comet were to separate
into two portions, as we have, with our own eyes, recently seen Biela's
comet do; or into a greater number; then these portions would be
distributed along such a spiral. And if we suppose a large mass of
cometic matter thus to move in a highly resisting medium, and to consist
of patches of different densities, then some would move faster and some
more slowly; but all, in spirals such as have been spoken of; and the
general aspect produced would be, that of the spiral nebulae which I have
endeavored to describe. The luminous matter would be more diffused in
the outer and more condensed in the central parts, because to the centre
of attraction all the spirals converge.
19. This would be so, we say, if the luminous matter moved in a greatly
resisting medium. But what is the measure of _great_ resistance? It is,
as we have already said, that the resistance which opposes the motion
shall bear a considerable proportion to the force which deflects the
motion. But what is that force? Upon the theory of the universal
gravitation of matter, on which theory we here proceed, the force which
deflects the motions of the parts of each system into curves, is the
mutual attraction of the parts of the system; leaving out of the account
the action of other systems, as comparatively insignificant and
insensible. The condition, then, for the production of such spiral
figures as I have spoken of, amounts really to this; that the mutual
attraction of the parts of the luminous matter is slight; or, in other
words, that the matter itself is very thin and rare. In that case,
indeed, we can easily see that such a result would follow. A cloud of
dust, or of smoke, which was thin and light, would make but a little way
through the air, and would soon fall downwards; while a metal bullet
shot horizontally with the same velocity, might fly for miles. Just so,
a loose and vaporous mass of cometic matter would be pulled rapidly
inwards by the attraction to the centre; and supposing it also drawn
into a long train, by the different density of its different parts, it
would trace, in lines of light, a circular or elliptical spiral
converging to the centre of attraction, and resembling one of the
branches of the spiral nebulae. And if several such cometic masses thus
travelled towards the centre, they would exhibit the wheel-like figure
with bent spokes, which is seen in the spiral nebulae. And such a figure
would all the more resemble some of these nebulae, as seen through Lord
Rosse's telescope, if the spirals were accompanied by exterior branches
of thinner and fainter light, which nebulous matter of smaller density
might naturally form. Perhaps too, such matter, when thin, may be
supposed to cool down more rapidly from its state of incandescence; and
thus to become less luminous. If this were so, a great optical power
would of course be required, to make the diverging branches visible at
all.
20. There is one additional remark, which we may make, as to the
resemblance of cometary[9] and nebular matter. That cometary matter is
of very small density, we have many reasons to believe:--its
transparency, which allows us to see stars through it undimmed;--the
absence of any mechanical effect, weight, inertia, impulse, or
attraction, in the nearest appulses of comets to planets and
satellites:--and the fact that, in the recent remarkable event in the
cometic history, the separation of Biela's comet into two, the two parts
did not appear to exert any perceptible attraction on each other, any
more than two volumes of dust or of smoke would do on earth. Luminous
cometary matter, then, is very light, that is, has very little weight or
inertia. And luminous nebulous matter is also very light in this sense:
if our account of the cause of spiral nebulae has in it any truth. But
yet, if we suppose the nebulae to be governed by the law of universal
gravitation, the attractive force of the luminous matter upon itself,
must be sufficient to bend the spirals into their forms. How are we to
reconcile this; that the matter is so loose that it falls to the centre
in rapid spirals, and yet that it attracts so strongly that there is a
centre, and an energetic central force to curve the spirals thither? To
this, the reply which we must make is, that the size of the nebular
space is such, that though its rarity is extreme, its whole mass is
considerable. One part does not perceptibly attract another, but the
whole does perceptibly attract every part. This indeed need the less
surprise us, since it is exactly the case with our earth. One stone does
not visibly attract another. It is much indeed for man, if he can make
perceptible the attraction of a mountain upon a plumb-line; or of a
stratum of rock a thousand feet thick upon the going of a pendulum; or
of large masses of metal upon a delicate balance. By such experiments
men of science have endeavored to measure that minute thing, the
attraction of one portion of terrestrial matter upon another; and thus,
to weigh the whole mass of the earth. And equally great, at least, may
be the disproportion between the mutual attraction of two parts of a
nebulous system, and the total central attraction; and thus, though the
former be insensible, the latter may be important.
21. It has been shown by Newton, that if any mass of matter be
distributed in a uniform sphere, or in uniform concentric spherical
shells, the total attraction on a point without the sphere, will be the
same as if the whole mass were collected in that single point, the
centre. Now, proceeding upon the supposition of such a distribution of
the matter in a nebula, (which is a reasonable average supposition,) we
may say, that if our sun were expanded into a nebula reaching to the
extreme bounds of the known solar system, namely, to the
newly-discovered planet Neptune, or even hundreds of times further; the
attraction on an external point would remain the same as it is, while
the attraction on points within the sphere of diffusion would be less
than it is; according to some law, depending upon the degree of
condensation of the nebular matter towards the centre; but still, in the
outer regions of the nebula, not differing much from the present solar
attraction. If we could discover a mass of luminous matter, descending
in a spiral course towards the centre of such a nebula, that is, towards
the sun, we should have a sort of element of the spiral nebulae which
have now attracted so much of the attention of astronomers. But, by an
extraordinary coincidence, recent discoveries have presented to us such
an element. Encke's comet, of which we have just spoken, appears to be
describing such a spiral curve towards the sun. It is found that its
period is, at every revolution, shorter and shorter; the amplitude of
its sweep, at every return within the limits of our observation,
narrower and narrower; so that in the course of revolutions and ages,
however numerous, still, not such as to shake the evidence of the fact,
it will fall into the sun.
22. Here then we are irresistibly driven to calculate what degree of
resemblance there is, between the comet of Encke, and the luminous
elements of the spiral nebulae, which have recently been found to exist
in other regions of the universe. Can we compare its density with
theirs? Can we learn whether the luminous matter in such nebulae is more
diffused or less diffused, than that of the comet of Encke? Can we
compare the mechanical power of getting through space, as we may call
it, that is, the ratio of the inertia to the resistance, in the one
case, and in the other? If we can, the comparison cannot fail, it would
seem, to be very curious and instructive. In this comparison, as in most
others to which cosmical relations conduct us, we must expect that the
numbers to which we are led, will be of very considerable amount. It is
not equality in the density of the two luminous masses which we are to
expect to find; if we can mark their proportions by thousands of times,
we shall have made no small progress in such speculations.
23. The comet of Encke describes a spiral, gradually converging to the
sun; but at what rate converging? In how many revolutions will it reach
the sun? Of how many folds will its spire consist, before it attains the
end of its course? The answer is:--Of very many. The retardation of
Encke's Comet is very small: so small, that it has tasked the highest
powers of modern calculation to detect it. Still, however, it is there:
detected, and generally acknowledged, and confirmed by every revolution
of the comet, which brings it under our notice; that is, commonly, about
every three years. And having this fact, we must make what we can of it,
in reasoning on the condition of the universe. No accuracy of
calculation is necessary for our purpose: it is enough, if we bring into
view the kind of scale of numbers to which calculation would lead us.
24. Encke's comet revolves round the sun in 1,211 days. The period
diminishes at present, by about one-ninth of a day every revolution.
This amount of diminution will change, as the orbit narrows; but for our
purpose, it will be enough to consider it unchangeable. The orbit
therefore will cease to exist in a number of periods expressed by 9
times 1,211; that is, in something more that 10,000 revolutions; and of
course sooner than this, in consequence of its coming in contact with
the body of the sun. In 30,000 years then, it may be, this comet will
complete its spiral, and be absorbed by the central mass. This long
time, this long series of ten thousand revolutions, are long, because
the resistance is so small, compared with the inertia of the moving
mass. However thin, and rare, and unsubstantial the comet may be, the
medium which resists it is much more so.
25. But this spiral, converging to its pole so slowly that it reaches it
only after 10,000 circuits, is very different indeed from the spirals
which we see in the nebulae of which we have spoken. In the most
conspicuous of those, there are only at most three or four circular or
oval sweeps, in each spiral, or even the spiral reaches the centre
before it has completed a single revolution round it. Now, what are we
to infer from this? How is it, that the comet has a spiral of so many
revolutions, and the nebulae of so few? What difference of the mechanical
conditions is indicated by this striking difference of form? Why, while
the Comet thus lingers longer in the outer space, and approaches the sun
by almost imperceptible degrees, does the Nebular Element rush, as it
were, headlong to its centre, and show itself unable to circulate even
for a few revolutions?
26. Regarding the question as a mechanical problem, the answer must be
this:--It is so, because the nebula is so much more rare than the matter
of the comet, or the resisting medium so much more dense; or combining
the two suppositions, because in the case of the comet, the luminous
matter has _much_ more inertia, more mechanical reality and substance,
than the medium through which it moves; but in the nebula very _little_
more.
27. The numbers of revolutions of the spiral, in the two cases, may not
exactly represent the difference of the proportions; but, as I have
said, they may serve to show the scale of them; and thus we may say,
that if Encke's comet, approaching the centre by 10,000 revolutions, is
100,000 times as dense as the surrounding medium, the elements of the
nebula, which reach the centre in a single revolution, are only ten
times as dense as the medium through which they have to move.[10]
28. Nor does this result (that the bright element of the nebulae is so
few times denser than the medium in which it moves) offer anything which
need surprise us: for, in truth, in a diffused nebula, since we suppose
that its parts have mechanical properties, the nebula itself is a
resisting medium. The rarer parts, which may very naturally have cooled
down in consequence of their rarity, and so, become non-luminous, will
resist the motions of the more dense and still-luminous portions. If we
recur to the supposition, which we lately made, that the Sun were
expanded into a nebulous sphere, reaching the orbit of Neptune, the
diffused matter would offer a far greater resistance to the motions of
comets than they now experience. In that case, Encke's comet might be
brought to the centre after a few revolutions; and if, while it were
thus descending, it were to be drawn out into a string of luminous
masses, as Biela's comet has begun to be, these comets, and any others,
would form separate luminous spiral tracks in the solar system; and
would convert it into a spiral nebula of many branches, like those which
are now the most recent objects of astronomical wonder.
29. It seems allowable to regard it as one of those coincidences, in the
epochs of related yet seeming unconnected discoveries, which have so
often occurred in the history of science; that we should, nearly at the
same time, have had brought to our notice, the prevalence of spiral
nebulae, and the circumstances, in Biela's and in Encke's comets, which
seem to explain them: the one by showing the origin of luminous broken
lines, one part drifting on faster than another, according to its
different density, as is usual in incoherent masses;[11] and the other
by showing the origin of the spiral form of those lines, arising from
the motion being in a resisting medium.
30. But though I have made suppositions by which our Solar System might
become a spiral nebula, undoubtedly it is at present something very
different; and the leading points of difference are very important for
us to consider. And the main point is, that which has already been
cursorily noticed: that instead of consisting of matter all nearly of
the same density, and a great deal of it luminous, our Solar System
consists of kinds of matter immensely different in density, and of large
and regular portions which are not luminous. Instead of a diffused
nebula with vaporous comets trailing spiral tracks through a medium
little rarer than themselves; we have a central sun, and the dark globes
of the solid planets rolling round him, in a medium so rare, that in
thousands of revolutions not a vestige of retardation can be discovered
by the most subtle and persevering researches of astronomers. In the
solar system, the luminous matter is collected into the body of the sun;
the non-luminous matter, into the planets. And the comets and the
resisting medium, which offer a small exception to this account, bear a
proportion to the rest which the power of numbers scarce suffices to
express.
31. Thus with regard to the density of matter in the solar system; we
have supposed, as a mode of expression, that the density of a comet,
Encke's comet for instance, is 100,000 times that of the resisting
medium. Probably this is greatly understated; and probably also we
greatly understate the matter, when we suppose that the tail of a comet
is 100,000 times rarer than the matter of the sun.[12] And thus the
resisting medium would be, at a very low calculation, 10,000 millions of
times more rare than the substance of the sun.
32. And thus we are not, I think, going too far, when we say, that our
Solar System, compared with spiral nebulous systems, is a system
completed and finished, while they are mere confused, indiscriminate,
incoherent masses. In the Nebulae, we have loose matter of a thin and
vaporous constitution, differing as more or less rare, more or less
luminous, in a small degree; diffused over enormous spaces, in
straggling and irregular forms; moving in devious and brief curves, with
no vestige of order or system, or even of separation of different kinds
of bodies. In the Solar System, we have the luminous separated from the
non-luminous, the hot from the cold, the dense from the rare; and all,
luminous and non-luminous, formed into globes, impressed with regular
and orderly motions, which continue the same for innumerable revolutions
and cycles.[13] The spiral nebulae, compared with the solar system,
cannot be considered as other than a kind of chaos; and not even a
chaos, in the sense of a state preceding an orderly and stable system;
for there is no indication, in those objects, of any tendency towards
such a system. If we were to say that they appear mere shapeless masses,
flung off in the work of creating solar systems, we might perhaps
disturb those who are resolved to find everywhere worlds like ours; but
it seems difficult to suggest any other reason for not saying so.
33. The same may be said of the other very irregular nebulae, which
spread out patches and paths of various degrees of brightness; and shoot
out, into surrounding space, faint branches which are of different form
and extent, according to the optical power with which they are seen.
These irregular forms are incapable of being permanent according to the
laws of mechanics. They are not figures of equilibrium; and, therefore,
must change by the attraction of the matter upon itself. But if the
tenuity of the matter is extreme, and the resistance of the medium in
which it floats considerable, this tendency to change and to
condensation may be almost nullified; and the bright specks may long
keep their straggling forms, as the most fantastically shaped clouds of
a summer-sky often do. It is true, it may be said that the reason why we
see no change in the form of such nebulae, is that our observations have
not endured long enough; all visible changes in the stars requiring an
immense time, according to the gigantic scale of celestial mechanism.
But even this hypothesis (it is no more) tends to establish the extreme
tenuity of the nebulae; for more solid systems, like our solar system,
require, for the preservation of their form, motions which are
perceptible, and indeed conspicuous, in the course of a month; namely,
the motions of the planets. All, therefore, concurs to prove the extreme
tenuity of the substance of irregular nebulae.
34. Nebulae which assume a regular, for instance, a circular or oval
shape, with whatever variation of luminous density from the inner to the
outer parts, may have a form of equilibrium, if their parts have a
proper gyratory motion. Still, we see no reason for supposing that these
differ so much from irregular nebulae, as to be denser bodies, kept in
their forms by rapid motions. We are rather led to believe that, though
perhaps denser than the spiral nebulae, they are still of extremely thin
and vaporous character. It would seem very unlikely that these vast
clouds of luminous vapor should be as dense as the tail of a comet;
since a portion of luminous matter so small as such a tail is, must have
cooled down from its most luminous condition; and must require to be
more dense than nebular matter in order to be visible at all by its own
light.
35. Thus we appear to have good reason to believe that nebulae are vast
masses of incoherent or gaseous matter, of immense tenuity, diffused in
forms more or less irregular, but all of them destitute of any regular
system of solid moving bodies. We seem, therefore, to have made it
certain that _these_ celestial objects at least are not inhabited. No
speculators have been bold enough to place inhabitants in a comet;
except, indeed, some persons who have imagined that such a habitation,
carrying its inmates alternately into the close vicinity of the sun's
surface, and far beyond the orbit of Uranus, and thus exposing them to
the fierce extremes of heat and cold, might be the seat of penal
inflictions on those who had deserved punishment by acts done in their
life on one of the planets. But even to give coherence to this wild
imagination, we must further suppose that the tenants of such
prison-houses, though still sensible to human suffering from extreme
heat and cold, have bodies of the same vaporous and unsubstantial
character as the vehicle in which they are thus carried about the
system; for no frame of solid structure could be sustained by the
incoherent and varying volume of a comet. And probably, to people the
nebulae with such thin and fiery forms, is a mode of providing them with
population, that the most ardent advocates of the plurality of worlds
are not prepared to adopt.
36. So far then as the Nebulae are concerned, the improbability of their
being inhabited, appears to mount to the highest point that can be
conceived. We may, by the indulgence of fancy, people the summer-clouds,
or the beams of the aurora borealis, with living beings, of the same
kind of substance as those bright appearances themselves; and in doing
so, we are not making any bolder assumption than we are, when we stock
the Nebulae with inhabitants, and call them in that sense, "distant
worlds."
FOOTNOTES:
[1] Herschel, _Outl. of Astr._ Art. 893.
[2] Herschel, _Outl. of Astr._ Art. 874, and Plate 11, Fig. 3.
[3] Ibid. Art. 897.
[4] Hersch. 874.
[5] Ibid. 881-8.
[6] At the recent meeting of the British Association (Sept. 1853),
drawings were exhibited of the same nebulae, as seen through Lord Rosse's
large telescope, and through a telescope of three feet aperture. With
the smaller telescopic power, all the characteristic features were lost.
The spiral structure (see next Article but one) has been almost entirely
brought to light by the large telescope.
[7] See monthly Notices of the Royal Astronomical Society, Dec. 13,
1850.
[8] The frontispiece to this volume represents two of these Spiral
Nebulae; those denominated 51 Messier, and 99 Messier, as given by Lord
Rosse in the _Phil. Trans. for 1850_. The former of these two has a
lateral focus, besides the principal focus or pole.
[9] I am aware that some astronomers do not consider it as proved that
cometary matter is entirely self-luminous. Arago found that the light of
a Comet contained a portion of polarized light, thus proving that it had
been reflected (_Cosmos_, I. p. 111, and III. p. 566). But I think the
opinion that the greater part of the light is self-luminous, like the
nebulae, generally prevails. Any other supposition is scarcely consistent
with the rapid changes of brightness which occur in a comet during its
motion to and from the Sun.
[10] We assume here that the number of revolutions to the centre is
greater in proportion as the relative density of the resisting medium is
less; which is by no means mechanically true; but the calculation may
serve, as we have said, to show the scale of the numbers involved.
[11] Humboldt, whom nothing relative to the history of science escapes,
quotes from Seneca a passage in which mention is made of a Comet which
divided into two parts; and from the Chinese Annals, a notice of three
"coupled Comets," which in the year 896 appeared, and described their
paths together. _Cosmos_, III. p. 570, and the notes.
[12] Laplace has proved that the masses of comets are very small. He
reckons their mean mass as very much less than 1-100000th of the Earth's
mass. And hence, considering their great size, we see how rare they must
be. See _Expos. du Syst. du Monde_.
[13] Humboldt repeatedly expresses his conviction that our Solar System
contains a greater variety of forms than other systems. (_Cosmos_, III.
373 and 587.)