THE FIXED STARS.
1. We appear, in the last chapter, to have cleared away the supposed
inhabitants of the outskirts of creation, so far as the Nebulae are the
outskirts of creation. We must now approach a little nearer, in
appearance at least, to our own system. We must consider the Fixed
Stars; and examine any evidence which we may be able to discover, as to
the probability of their containing, in themselves or in accompanying
bodies, as planets, inhabitants of any kind. Any special evidence which
we can discern on this subject, either way, is indeed slight. On the one
side we have the asserted analogy of the parts of the universe; of which
point we have spoken, and may have more to say hereafter. Each Fixed
Star is conceived to be of the nature of our Sun; and therefore, like
him, the centre of a planetary system. On the other side, it is
extremely difficult to find any special facts relative to the nature of
the fixed stars, which may enable us in any degree to judge how far they
really are of a like nature with the Sun, and how far this resemblance
goes. We may, however, notice a few features in the starry heavens, with
which, in the absence of any stronger grounds, we may be allowed to
connect our speculations on such questions. The assiduous scrutiny of
the stars which has been pursued by the most eminent astronomers, and
the reflections which their researches have suggested to them, may have
a new interest, when discussed under this point of view.
2. Next after the Nebulae, the cases which may most naturally engage our
attention, are Clusters of stars. The cases, indeed, in which these
clusters are the closest, and the stars the smallest, and in which,
therefore, it is only by the aid of a good telescope that they are
resolved into stars, do not differ from the resolvable nebulae, except in
the degree of optical power which is required to resolve them. We may,
therefore, it would seem, apply to such clusters, what we have said of
resolvable nebulae: that when they are thus, by the application of
telescopic power, resolved into bright points, it seems to be a very
bold assumption to assume, without further proof, that these bright
points are suns, distant from each other as far as we are from the
nearest stars. The boldness of such an assumption appears to be felt by
our wisest astronomers.[1] That several of the clusters which are
visible, some of them appearing as if the component stars were gathered
together in a nearly spherical form, are systems bound together by some
special force, or some common origin, we may regard, with those
astronomers, as in the highest degree probable. With respect to the
stability of the form of such a system, a curious remark has been made
by Sir John Herschel,[2] that if we suppose a globular space filled with
equal stars, uniformly dispersed through it, the particular stars might
go on forever, describing ellipses about the centre of the globe, in all
directions, and of all sizes; and all completing their revolutions in
the same time. This follows, because, as Newton has shown, in such a
case, the compound force which tends to the centre of the sphere would
be everywhere proportional to the distance from the centre; and under
the action of such a force, ellipses about the centre would be
described, all the periods being of the same amount. This kind of
symmetrical and simple systematic motion, presented by Newton as a mere
exemplification of the results of his mechanical principles, is perhaps
realized, approximately at least, in some of the globular clusters. The
motions will be swift or slow, according to the total mass of the
groups. If, for instance, our Sun were thus broken into fragments, so as
to fill the sphere girdled by the earth's orbit, all the fragments would
revolve round the centre in a year. Now, there is no symptom, in any
cluster, of its parts moving nearly so fast as this; and therefore we
have, it would seem, evidence that the groups are much less dense than
would be the space so filled with fragments of the sun. The slowness of
the motions, in this case, as in the nebulae, is evidence of the weakness
of the forces, and therefore, of the rarity of the mass; and till we
have some gyratory motion discovered in these groups, we have nothing to
limit our supposition of the extreme tenuity of their total substance.
3. Let us then go on to the cases in which we have proof of such
gyratory motions in the stars; for such are not wanting. Fifty years
ago, Herschel the father, had already ascertained that there are certain
pairs of stars, very near each other (so near, indeed, that to the
unassisted eye they are seen as single stars only,) and which revolve
about each other. These Binary Sidereal Systems have since been examined
with immense diligence and profound skill by Herschel the son, and
others; and the number of such binary systems has been found, by such
observers, to be very considerable. The periods of their revolutions are
of various lengths, from 30 or 40 years to several hundreds of years.
Some of those pairs which have the shortest periods, have already,
since the nature of their movements was discovered, performed more than
a complete revolution;[3] thus leaving no room for doubting that their
motions are really of this gyratory kind. Not only the fact, but the law
of this orbital motion, has been investigated; and the investigations,
which naturally were commenced on the hypothesis that these distant
bodies were governed by that Law of universal Gravitation, which
prevails throughout the solar system, and so completely explains the
minutest features of its motions, have ended in establishing the reality
of that Law, for several Binary Systems, with as complete evidence as
that which carries its operations to the orbits of Uranus and Neptune.
4. Being able thus to discern, in distant regions of the universe,
bodies revolving about each other, we have the means of determining, as
we do in our own solar system, the masses of the bodies so revolving.
But for this purpose, we must know their distance from each other; which
is, to our vision, exceedingly small, requiring, as we have said, high
magnifying powers to make it visible at all. And again, to know what
linear distance this small visible distance represents, we must know the
distance of the stars from us, which is, for every star, as we know,
immensely great; and for most, we are destitute of all means of
determining how great it is. There are, however, some of these binary
systems, in which astronomers conceive that they have sufficiently
ascertained the value of both these elements, (the distance of the two
stars from each other, and from us,) to enable them to proceed with the
calculation of which I have spoken; the determination of the masses of
the revolving bodies. In the case of the star _Alpha Centauri_, the
first star in the constellation of the Centaur, the period is reckoned
to be 77 years; and as, by the same calculator, the apparent semi-axis
of the orbit described is stated at 15 seconds of space, while the
annual parallax of each star is about one second, it is evident that the
orbit must have a radius about 15 times the radius of the earth's orbit;
that is, an orbit greater than that of Saturn, and approaching to that
of Uranus. In the solar system, a revolution in such an orbit would
occupy a time greater than that of Saturn, which is 30 years, and less
than that of Uranus, which is about 80 years: it would, in fact, be
about 58 years. And since, in the binary star, the period is greater
than this, namely 77 years, the attraction which holds together its two
elements must be less than that which holds together the Sun and a
planet at the same distance; and therefore the masses of the two stars
together are considerably less than the mass of our sun.
5. A like conclusion is derived from another of these conspicuous double
stars, namely, the one termed by astronomers _61 Cygni_; of which the
annual parallax has lately been ascertained to be one-third of a second
of space, while the distance of the two stars is 15 seconds. Here
therefore we have an orbit 45 times the size of the Earth's orbit;
larger than that of the newly-discovered planet Neptune, whose orbit is
30 times as large as the earth's, and his period nearly 165 years. The
period of 61 Cygni is however, it appears, probably not short of 500
years; and hence it is calculated that the sum of the masses of the two
stars which make up this pair is about one-third of the mass of our
Sun.[4]
6. These results give some countenance to the opinion, that the quantity
of luminous matter, in other systems, does not differ very considerably
from the mass of our Sun. It differs in these cases as 1 to 3, or
thereabouts. In what degree of condensation, however, the matter of
these binary systems is, compared with that of our solar system, we have
no means whatever of knowing. Each of the two stars may have its
luminous matter diffused through a globe as large as the earth's orbit;
and in that case, would probably not be more dense than the tail of a
comet.[5] It is observed by astronomers, that in the pairs of binary
stars which we have mentioned, the two stars of each pair are of
different colors; the stars being of a high yellow, approaching to
orange color,[6] but the smaller individual being in each case of a
deeper tint. This might suggest to us the conjecture that the smaller
mass had cooled further below the point of high luminosity than the
larger; but that both these degrees of light belong to a condition still
progressive, and probably still gaseous. Without attaching any great
value to such conjectures, they appear to be at least as well authorized
as the supposition that each of these stars, thus different, is
nevertheless precisely in the condition of our sun.
7. But, even granting that each of the individuals of this pair were a
sun like ours, in the nature of its material and its state of
condensation, is it probable that it resembles our Sun also in having
planets revolving about it? A system of planets revolving around or
among a pair of suns, which are, at the same time, revolving about one
another, is so complex a scheme, so impossible to arrange in a stable
manner, that the assumption of the existence of such schemes, without a
vestige of evidence, can hardly require confutation. No doubt, if we
were really required to provide such a binary system of suns with
attendant planets, this would be best done by putting the planets so
near to one sun, that they should not be sensibly affected by the other;
and this is accordingly what has been proposed.[7] For, as has been well
said of the supposed planets, in making this proposal, "Unless closely
nestled under the protecting wing of their immediate superior, the sweep
of the other sun in his perihelion passage round their own, might carry
them off, or whirl them into orbits utterly inconsistent with the
existence of their inhabitants." To assume the existence of the
inhabitants, in spite of such dangers, and to provide against the
dangers by placing them so close to one sun as to be out of the reach of
the other, though the whole distance of the two may not, and as we have
seen, in some cases does not, exceed the dimensions of our solar system,
is showing them all the favor which is possible. But in making this
provision, it is overlooked that it may not be possible to keep them in
permanent orbits so near to the selected centre: their sun may be a vast
sphere of luminous vapor; and the planets, plunged into this atmosphere,
may, instead of describing regular orbits, plough their way in spiral
paths through the nebulous abyss to its central nucleus.
8. Clustered stars, then, and double stars, appear to give us but little
promise of inhabitants. We must next turn our attention to the single
stars, as the most hopeful cases. Indeed, it is certain that no one
would have thought of regarding the individual stars of clusters, or of
pairs, as the centres of planetary systems, if the view of insulated
stars, as the centres of such systems, had not already become familiar,
and, we may say, established. What, then, is the probability of that
view? Is there good evidence that the Fixed Stars, or some of them,
really have planets revolving round them? What is the kind of proof
which we have of this?
9. To this we must reply, that the only proof that the fixed stars are
the centres of planetary systems, resides in the assumption that those
stars are _like the Sun_;--resemble him in their qualities and nature,
and therefore, it is inferred, must have the same offices, and the same
appendages. They are, as the Sun is, independent sources of light, and
thence, probably, of heat; and therefore they must have attendant
planets, to which they can impart their light and heat; and these
planets must have inhabitants, who live under and enjoy those
influences. This is, probably, the kind of reasoning on which those
rely, who regard the fixed stars as so many worlds, or centres of
families of worlds.
10. Everything in this argument, therefore, depends upon this: that the
Stars are _like the Sun_; and we must consider, what evidence we have of
the exactness of this likeness.
11. The Stars are like the Sun in this, that they shine with an
independent light, not with a borrowed light, as the planets shine. In
this, however, the stars resemble, not only the Sun, but the nebulous
patches in the sky, and the tails of comets; for these also, in all
probability, shine with an original light. Probably it will hardly be
urged that we see, by the very appearance of the stars, that they are of
the nature of the Sun: for the appearance of luminaries in the sky is so
far from enabling us to discriminate the nature of their light, that to
a common eye, a planet and a fixed star appear alike as stars. There is
no obvious distinction between the original light of the stars and the
reflected light of the planets. The stars, then, being like the sun in
being luminous, does it follow that they are, like the sun, definite
dense masses?[8] Or are they, or many of them, luminous masses in a far
more diffused state; visually contracted to points, by the immense
distance from us at which they are?
12. We have seen that some of those stars, which we have the best means
of examining, are, in mass, one third, or less, of our Sun. If such a
mass, at the distance of the fixed stars, were diffused through a sphere
equal in radius to the earth's orbit, it would still appear to us as a
point; as is evident by this, that the fixed stars, for the most part,
have no discoverable annual parallax; that is, the earth's orbit appears
to them a point. If one of the fixed stars, Sirius, for instance, be in
this diffused condition, such a circumstance will not, mechanically
speaking, prevent his having planets revolving round him; for, as we
have said, the attraction of his whole mass, in whatever state of
spherical diffusion, will be the same as if it were collected at the
centre. But such a state of diffusion will make him so unlike our Sun,
as much to break the force of the presumption that he must have planets
because our Sun has. If the luminous matter of the stars gradually
cools, grows dark, and solidifies, such diffusion would imply that the
time of solidification is not yet begun; and therefore that the solid
planets which accompany the luminous central body are not yet brought
into being. If there be any truth in this hypothetical account of the
changes, through which the matter of the stars successively passes; and
if, by such changes, planetary systems are formed; how many of the fixed
stars may never yet have reached the planetary state! how many, for want
of some necessary mechanical condition, may never give rise to permanent
orbits at all!
13. And that the matter of the stars does go through changes, we have
evidence, in many such changes which have actually been observed;[9]
and perhaps in the different colors of different stars; which may, not
improbably, arise from their being at different stages of their
progress. That planetary systems, once formed, go through mighty
changes, we have evidence in the view which geology gives us of the
history of this earth; and in that view, we see also, how unique, and
how far elevated in its purpose, the last period of this history may be,
compared with the preceding periods; and, up to the present time at
least, how comparatively brief in its duration. If, therefore, stellar
globes can become planetary systems in the progress of ages, it will not
be at all inconsistent with what we know of the order of nature, that
only a few, or even that only one, should have yet reached that
condition. All the others, but the one, may be systems yet unformed, or
fragments struck off in the forming of the one. If any one is not
satisfied with this account of the degree of resemblance between the
fixed stars and the sun, but would make the likeness greater than this;
we have only to say, that the proof that it is so lies upon him. Such a
resemblance as we have supposed, is all that the facts suggest. That the
stars are independent luminaries, we see; but whether they are as dense
as the sun, or globes a hundred or a thousand times as rare, we have no
means whatever of knowing. And, to assume that besides these luminous
bodies which we see, there are dark bodies which we do not see,
revolving round the others in permanent orbits, which require special
mechanical conditions; and to suppose this, in order that we may build
upon this assumption a still larger one, that of living inhabitants of
these dark bodies; is a hypothetical procedure, which it seems strange
that we should have to combat, at the present stage of the history of
science, and in dealing with those whose minds have been disciplined by
the previous events in the progress of astronomy.
14. Let us consider, however, further, how far astronomy authorizes us
to regard the Fixed Stars as being, like our Sun, the centres of systems
of Planets. Those who hold this, consider them as having a permanent
condition of brightness, as our Sun has had for an indefinite period, so
far as we have any knowledge on the subject. Yet, as we have said, no
small number of the stars undergo changes of brightness; and some of
them undergo such changes, in a manner which is not discernibly
periodical; and which must therefore be regarded as progressive. This
phenomenon countenances the opinion of such a progress from one material
condition to another; which, we have seen, is suggested by the analogy
of the probable formation of our own solar system. The very star which
is so often taken as the probable centre of a system, Sirius, has, in
the course of the last 2,000 years, changed its light from red to white.
Ptolemy notes it as a red star: in Tycho's time it was already, as it is
now, a white one.[10] The star _Eta Argus_ changes both its degree of
light and its color; ranging, in seemingly irregular intervals of time,
from the fourth to the first magnitude,[11] and from yellow to red.
Several other examples of the like kind have been observed. Mr. Hind[12]
gives an example in which he has, quite recently, observed in two years
a star change its color from very red to bluish. These variable
unperiodical stars are probably very numerous. Also, some stars,
observed of old, are now become invisible. "The lost Pleiad," by the
loss of which the cluster, called the Seven Stars, offers now only six
to the naked eye, is an example of a change of this kind already noted
in ancient times. There are several others, of which the extinction is
recognized by astronomers as proved.[13] In other cases, new stars have
appeared, and have then seemed to die away and vanish. The appearance of
a new star in the time of the Greek astronomer Hipparchus, induced him
to construct his famous Catalogue of the Stars. Others are recorded to
have appeared in the middle ages. The first which was observed by modern
astronomers was the celebrated star seen by Tycho Brahe in 1572. It
appeared suddenly in the constellation Cassiopeia, was fixed in its
place like the neighboring stars, had no nebula or tail, exceeded in
splendor all other stars, being as bright as Venus when she is nearest
the earth. It soon began to diminish in brightness, and passing through
various diminishing degrees of magnitude, vanished altogether after
seventeen months. This star also passed through various colors; being
first white, then yellow, then red. In like manner, in 1604, a new star
of great magnitude blazed forth in the constellation Serpentarius; and
was seen by Kepler. And this also, like that of 1572, after a few
months, declined and vanished.
15. These appearances led Tycho to frame an hypothesis like that which
Sir William Herschel afterwards proposed, that the stars are formed by
the condensation of luminous nebulous matter. Nor is it easy to think of
such phenomena (of which several others have been observed, though none
so conspicuous as these), without regarding them as showing that the
matter of the fixed stars, occasionally at least, passes through changes
of consistence as great as would be the condensation and extinction of a
luminous vapor. And if such changes have been but few within the
recorded period of man's observation of the stars, we must recollect how
small that period is, compared with the period during which the stars
have existed. The stars themselves give us testimony of their having
been in being for millions of years. For according to the best estimates
we can form of their distances, the time which light would employ in
reaching us from the most remote of them, would be millions of years;
and, therefore, we now see those remote stars by means of the light
emitted from them millions of years ago. And if, in the 2,000 years
during which such observations are recorded, only 200 stars have
undergone such changes in a degree visible to the earth's inhabitants;
in a million of years, change going on at the same rate, 100,000 stars
would exhibit visible progressive change, showing that they had not yet
reached a permanent condition. And how much of change may go on in any
star without its being in any degree perceptible to the most exact
astronomical scrutiny!
16. The tendency of these considerations is, to lead us to think that
the fixed stars are not generally in that permanent condition in which
our sun is; and which appears to be alone consistent with the existence
of a system such as the solar system.[14] These views, therefore, fall
in with that which we have been led to by this consideration of the
Nebulae: that the Solar System is in a more complete and advanced state,
as a system, than many at least of the stellar systems can be; it may
be, than any other.
17. It has been alleged, as a proof of the likeness of the Fixed Stars
to our Sun, that like him, they revolve upon their axes.[15] This has
been supposed to be proved with regard to many of them, by their having
periodical recurrences of fainter and brighter lustre; as if they were
revolving orbs, with one side darkened by spots. Such facts are not very
numerous or definite in the heavens. _Omicron_[16] in the constellation
_Cetus_, is the longest known of them; and is held to revolve in 831
days. From the curious phenomena now spoken of, it has been called _Mira
Ceti_.[17] _Algol_, the second star (_Beta_) of _Perseus_, called also
_Caput Medusae_, is another, with a period of 2 days 21 hours; and in
this case, the obscuration of the light, and the restoration of it, are
so sudden, that from the time when it was first remarked, (by Goodricke,
in 1782,) it suggested the hypothesis of an opaque body revolving round
the star. The star _Delta_, in the constellation _Cephus_, is another,
with a period of 5 days 9 hours. The star _Beta_ in the _Lyre_, has a
period of 6 days 10 hours, or perhaps 12 days 21 hours, one revolution
having been taken for two. Another such star is _Eta Aquilae_, with a
period of 7 days 4 hours. These five are all the periodical stars of
which astronomers can speak with precision.[18] But about thirty more
are supposed to be subject to such change, though their periods, epochs,
and phases of brightness, cannot at present be given exactly.
18. That these periodical changes in certain of the fixed stars are a
curious and interesting astronomical fact, is indisputable. Nothing can
be more probable also, than that it indicates, in the stellar masses, a
revolution on their axes; which cannot surprise us, seeing that
revolution upon an axis is, so far as we know, a universal law of all
the large compact masses of matter which exist in the universe; and may
be conceived to be a result derived from their origin, and a condition
of any permanent or nearly permanent figure. But this can prove little
or nothing as to their being like the sun, in any way which implies
their having inhabitants, in themselves or in accompanying planets. The
rotation of our Sun is not, in any intelligible way, connected with its
having near it the inhabited Earth.
19. If we were to suppose some of the stars to be centres of planetary
systems, we can hardly suppose it likely that these alone rotate, and
that the others stand still. Probably all the stars rotate, more or less
regularly, according as they are permanent or variable in form; but the
most regular may still have no planets; and if they have, those planets
may be as blank of inhabitants as our moon will be proved to be.
20. The revolution of Algol seems to approach the nearest to a fact in
favor of a star being the centre of a revolving system; and from the
first, as we have said, the periodical change, and the sudden darkening
and brightening of this luminary, suggested the supposition of an opaque
body revolving about it. But this body cannot be a planet. The planets
which revolve about our Sun are not, any of them, nor all of them
together, large enough to produce a perceptible obscuration of his
light, to a spectator outside the system. But in Algol, the phenomena
are very different from this.[19] The star is usually visible as a star
of the second magnitude; but during each period of 2 days 21 hours, (or
69 hours,) it suffers a kind of eclipse, which reduces it to a star of
the fourth magnitude. During this eclipse, the star diminishes in
splendor for 3-1/2 hours; is at its lowest brightness for a quarter of
an hour; and then, in 3-1/2 hours more, is restored to its original
splendor. According to these numbers, if the obscuration be produced by
a dark body revolving round a central luminary, and describing a
circular orbit, as the regular recurrence of the obscuration implies,
the space of the orbit during which the eclipsing body is interposed
must be about one-ninth of the circumference; for the obscuration
occupies 7-1/4 hours out of 69. And therefore the space during which the
eclipsing body obscures the central one, must be about one _sixth_ of
the _diameter_ of its orbit. But in order that the revolving body may,
through this space, obscure the central one, the latter must extend over
this space, namely, one sixth of the diameter of the orbit. But we may
remark that there is no proof, in the phenomena, that the darkening body
is detached from the bright mass. The effect would be the same if the
dark mass were a part of the revolving star itself. It may be that the
star has not yet assumed a spherical form, but is an oblong nebular mass
with one part (perhaps from being thinner in texture) cooled down and
become opaque. And the amount of obscuration, reducing the star from the
second to the fourth magnitude, implies that the obscuring mass is large
(perhaps one half the diameter, or much more) compared with the luminous
mass. If this be a probable hypothesis to account for the phenomena,
they are much more against than for the supposition of the star being
the centre of seats of habitation. And even if we have a planet nearly
as large as its sun, revolving at the distance of only six of the sun's
radii, how unlike is this to the solar system!
21. In fact, all these periodical stars, in so far as they are
periodical, are proved, not to be like, but to be _unlike_ our sun. It
is true that the sun has spots, by means of which his rotation has been
determined by astronomers. But these spots, besides being so small that
they produce no perceptible alteration in his brightness, and are never,
or very rarely, visible to the naked eye, are not permanent. A star with
a permanent dark side would be very unlike our sun. The largest known of
these stars, _Mira_, as the old astronomers called it, becomes invisible
to the naked eye for 5 months during a period of 11 months. It must,
therefore, have nearly one half its surface quite dark. This is very
unlike the condition of the sun; and is a condition, it would seem, very
little fitted to make this star the centre of a planetary system like
ours.
22. But there are other remarkable phenomena respecting these periodical
stars, which have a bearing on our subject. Their periods are not quite
regular, but are subject to certain variations. Thus it has been
supposed that the period of Mira is subject to a cyclical fluctuation,
embracing 88 of its periods; that is, about 80 years. But this notion of
a cycle of so long a duration, requires confirmation; the fact of
fluctuation in the period is alone certain. In like manner, Algol's
periods are not quite uniform. All these facts agree with our
suggestion, that the periodical stars are bodies of luminous matter
which have not yet assumed a permanent form; and which, therefore, as
they revolve about their axes, and turn to us their darker and their
brighter parts, do so at intervals, and in an order somewhat variable.
And this suggestion appears to be remarkably confirmed, by a result
which recent observations have discovered relative to this star, Algol;
namely, that its periods become shorter and shorter. For if the luminous
matter, which is thus revolving, be gradually gathering into a more
condensed form;--becoming less rare, or more compact; as, for instance,
it would do, if it were collecting itself from an irregular, or
elongated, into a more spherical form; such a shortening of the period
of revolution would take place; for a mass which contracts while it is
revolving, accelerates its rate of revolution, by mechanical principles.
And thus we do appear to have, in this observed acceleration of the
periods of Algol, an evidence that that luminous mass has not yet
reached its final and permanent condition.
23. It is true, it has been conjectured, by high authority,[20] that
this accelerated rapidity of the periods of Algol will not continue; but
will gradually relax, and then be changed to an increase; like many
other cyclical combinations in astronomy. But this conjecture seems to
have little to support it. The cases in which an acceleration of motion
is retarded, checked, and restored, all belong to our Solar System; and
to assume that Algol, like the solar system, has assumed a permanent and
balanced condition, is to take for granted precisely the point in
question. We know of no such cycles among the fixed stars, at least with
any certainty; for the cycle proposed for Mira must be considered as
greatly needing confirmation; considering how long is the cycle, and how
recent the suggestion of its existence.
24. And even in the solar system, we have accelerated motions, in which
no mathematician or astronomer looks for a check or regress of the
acceleration. No one expects that Encke's comet will cease to be
accelerated, and to revolve in periods continually shorter; though all
the other motions hitherto observed in the system are cyclical. In the
case of a fixed star, we have much less reason to look for such a cycle,
than we have in Encke's comet. But further: with regard to the existence
of such a cycle of faster and slower motion in the case of Algol, the
most recent observed facts are strongly against it; for it has been
observed by Argelander, that not only there is a diminution of the
period, but that this diminution proceeds with accelerated rapidity; a
course of events which, in no instance, in the whole of the cosmical
movements, ends in a regression, retardation, and restoration of the
former rate. We are led to believe, therefore, that this remarkable
luminary will go on revolving faster and faster, till its extreme point
of condensation is attained. And in the meantime, we have very strong
reasons to believe that this mutable body is not, like the sun, a
permanent centre of a permanent system; and that any argument drawn from
its supposed likeness to the sun, in favor of the supposition that the
regions which are near it are the seats of habitation, is quite
baseless.
25. There are other phenomena of the Fixed Stars, and other conjectures
of astronomers respecting them, which I need not notice, as they do not
appear to have any bearing upon our subject. Such are the "proper
motions" of the stars, and the explanation which has been suggested of
some of them; that they arise from the stars revolving round other stars
which are dark, and therefore invisible. Such again is the attempt to
show that the Sun, carrying with it the whole Solar System, is in
motion; and the further attempt to show the direction of this motion;
and again, the hypothesis that the Sun itself revolves round some
distant body in space. These minute inquiries and bold conjectures, as
to the movements of the masses of matter which occupy the universe, do
not throw any light on the question whether any part besides the earth
is inhabited; any more than the investigation of the movements of the
ocean, and of their laws, could prove or disprove the existence of
marine plants and animals. They do not on that account cease to be
important and interesting subjects of speculation; but they do not
belong to our subject.
26. In Fontenelle's _Dialogues on the Plurality of Worlds_, a work which
may be considered as having given this subject a place in popular
literature, he illustrates his argument by a comparison, which it may be
worth while to look at for a moment. The speaker who asserts that the
moon, the planets, and the stars, are the seats of habitation, describes
the person, who denies this, as resembling a citizen of Paris, who,
seeing from the towers of Notre Dame the town of Saint Denis, (it being
supposed that no communication between the two places had ever
occurred,) denies that it is inhabited, because he cannot see the
inhabitants. Of course the conclusion is easy, if we may thus take for
granted that what he sees is a town. But we may modify this image, so as
to represent our argument more fairly. Let it be supposed that we
inhabit an island, from which innumerable other islands are visible; but
the art of navigation being quite unknown, we are ignorant whether any
of them are inhabited. In some of these islands, are seen masses more or
less resembling churches; and some of our neighbors assert that these
are churches; that churches must be surrounded by houses; and that
houses must have inhabitants. Others hold that the seeming churches are
only peculiar forms of rocks. In this state of the debate, everything
depends upon the degree of resemblance to churches which the forms
exhibit. But suppose that telescopes are invented, and employed with
diligence upon the questionable shapes. In a long course of careful and
skilful examination, no house is seen, and the rocks do not at all
become more like churches, rather the contrary. So far, it would seem,
the probability of inhabitants in the islands is lessened. But there are
other reasons brought into view. Our island is a long extinct volcano,
with a tranquil and fertile soil; but the other islands are apparently
somewhat different. Some of them are active volcanoes, the volcanic
operations covering, so far as we can discern, the whole island; others
undergo changes, such as weather or earthquakes may produce; but in none
of them can we discover such changes as show the hand of man. For these
islands, it would seem the probability of inhabitants is further
lessened. And so long as we have no better materials than these for
forming a judgment, it would, surely, be accounted rash, to assert that
the islands in general are inhabited; and unreasonable, to blame those
who deny or doubt it. Nor would such blame be justified by adducing
theological or _a priori_ arguments; as, that the analogy of island with
island makes the assumption allowable; or that it is inconsistent with
the plan of the Creator of islands to leave them uninhabited. For we
know that many islands are, or were long, uninhabited. And if ours were
an island occupied by a numerous, well-governed, moral, and religious
race, of which the history was known, and of which the relation to the
Creator was connected with its history; the assumption of a history,
more or less similar to ours, for the inhabitants of the other islands,
whose existence was utterly unproved, would, probably, be generally
deemed a fitter field for the romance-writer than for the philosopher.
It could not, at best, rise above the region of vague conjecture.
27. Fontenelle, in the agreeable book just referred to, says, very
truly, that the formula by which his view is urged on adversaries is,
_Pourquoi non_? which he holds to be a powerful figure of logic. It is,
however, a figure which has this peculiarity, that it may, in most
cases, be used with equal force on either side. When we are asked Why
the Moon, Mercury, Saturn, the system of Sirius, should _not_ be
inhabited by intelligent beings; we may ask, Why the earth in the ages
previous to man might not be so inhabited? The answer would be, that we
have proof _how_ it _was_ inhabited. And as to the fact in the other
case, I shall shortly attempt to give proof that the Moon is certainly
not, and Mercury and Saturn probably not inhabited. With regard to the
Fixed Stars, it is more difficult to reason; because we have the means
of knowing so little of their structure. But in this case also, we might
easily ask on our side, _Pourquoi non_? Why should not the Solar System
be the chief and most complete system in the universe, and the Earth the
principal planet in that System? So far as we yet know, the Sun is the
largest Sun among the stars; and we shall attempt to show, that the
Earth is the largest solid opaque globe in the solar system. Some System
must be the largest and most finished of all; why not ours? Some planet
must be the largest planet; why not the Earth?
28. It should be recollected that there must be some system which is the
most complete of all systems, some planet which is the largest of all
planets. And if that largest planet, in the most complete system, be,
after being for ages tenanted by irrational creatures, at last, and
alone of all, occupied by a rational race, that race must necessarily
have the power of asking such questions as these: Why they should be
alone rational? Why their planet should be alone thus favored? If the
case be ours, we may hope to be then able to answer these questions,
when we can explain the most certain fact which they involve; Why the
Earth was occupied so long by irrational creatures, before the rational
race was placed upon it? The mere power of asking such questions can
prove or disprove nothing; for it is a power which must equally subsist,
whether the human inhabitants of the earth be or be not the only
rational population which the universe contains. If there be a race thus
favored by the Creator, they must, at that stage of their knowledge in
which man now is, be able to doubt, as man does, of the extent and
greatness of the privilege which they enjoy.
29. The argument that the Fixed Stars are like the Sun, and therefore
the centres of inhabited systems as the Sun is, is sometimes called an
argument from Analogy; and this word _Analogy_ is urged, as giving great
force to the reasoning. But it must be recollected, that precisely the
point in question is, whether there _is_ an analogy. The stars, it is
said, are like the Sun. In what respects? We know of none, except in
being self-luminous; and this they have in common with the nebulae,
which, as we have seen, are not centres of inhabited systems. Nor does
this quality of being self-luminous at all determine the degree of
condensation of a star. Sirius may be less than a hundredth or a
thousandth of the density of the Sun. But the Stars, it may be further
urged, are like the Sun in turning on their axes. To this we reply, that
we know this only of those stars in which, the very phenomenon which
proves their revolution, proves also that they are unlike the Sun, in
having one side darker than the other. Add to which, their revolution is
not connected with the existence of planets, still less of inhabitants
of planets, in any intelligible manner. The resemblance, therefore, so
far as it bears upon the question, is confined to one single point, in
the highest degree ambiguous and inconclusive; and any argument drawn
from this one point of resemblance, has little claim to be termed an
argument from analogy.[21]
30. On a subject on which we know so little, it is difficult to present
any view which deserves to be regarded as an analogy. We see, among the
stars, nebulae more or less condensed, which are possibly, in some cases,
stages of a connected progress towards a definite star; and it may be,
to a star with planets in permanent orbits. We see, in our planet,
evidence of successive stages of a connected series of brute animals,
preceded perhaps by various stages of lifeless chaos. If the histories
of the Sun, and of all the stars, are governed by a common analogy, the
nebulous condensation, and the stages of animal life, may be parts of
the same continued series of events; and different stars may be at
different points of that series. But even on this supposition, but a few
of the stars may be the seats of conscious life, and none, of
intelligence. For among the stars which have condensed to a permanent
form, how many have failed in throwing off a permanent planet! How many
may be in some stage of lifeless chaos! We must needs suppose a vast
number of stages between a nebular chaos and the lowest forms of
conscious life. Perhaps as many as there are fixed stars; and far more
than there are of stars which become fertile of life: so that no two
systems may be at the same stage of the planetary progress. And if this
be so,--our system being so complicated, that we must suppose it
peculiarly developed, having the largest Sun that we know of, and our
Earth being (as we shall hereafter attempt to prove) the largest solid
planet that we know of,--this Earth may be the sole seat of the highest
stage of planetary development.
31. The assumption that there is anything of the nature of a regular law
or order of progress from nebular matter to conscious life,--a law which
extends to all the stars, or to many of them,--is in the highest degree
precarious and unsupported; but since it is sometimes employed in such
speculations as we are pursuing, we may make a remark or two connected
with it. If we suppose, on the planets of other systems, a progress in
some degree analogous to that which geology shows to have occurred on
the Earth, there may be, in those planets, creatures in some way
analogous to our vegetables and animals; but analogy also requires that
they should differ far more from the terrestrial vegetables and animals
of any epoch, than those of one epoch do from those of another; since
they belong to a different stellar system, and probably exist under very
different conditions from any that ever prevailed on the Earth. We are
forbidden, therefore, by analogy, to suppose that on any other planet
there was such an anatomical progression towards the form of man, as we
can discern (according to some eminent physiologists) among the tribes
which have occupied the Earth. Are we to conceive that the creatures on
the planets of other systems are, like the most perfect terrestrial
animals, symmetrical as to right and left, vertebrate, with fore limbs
and hind limbs, heads, organs of sense in their heads, and the like?
Every one can see how rash and fanciful it would be to make such
suppositions. Those who have, in the play of their invention, imagined
inhabitants of other planets, have tried to avoid this servile imitation
of terrestrial forms. Here is Sir Humphry Davy's account of the
inhabitants of Saturn. "I saw moving on the surface below me, immense
masses, the forms of which I find it impossible to describe. They had
systems for locomotion similar to that of the morse or sea-horse, but I
saw with great surprise that they moved from place to place by six
extremely thin membranes, which they used as wings. I saw numerous
convolutions of tubes, more analogous to the trunk of the elephant, than
to anything else I can imagine, occupying what I supposed to be the
upper parts of the body."[22] The attendant Genius informs the narrator,
that though these creatures look like zoophytes, they have a sphere of
sensibility and intellectual enjoyment far superior to that of the
inhabitants of the Earth. If we were to reason upon a work of fancy like
this, we might say, that it was just as easy to ascribe superior
sensibility and intelligence to zoophyte-formed creatures upon the
Earth, as in Saturn. Even fancy cannot aid us in giving consistent form
to the inhabitants of other planets.
32. But even if we could assent to the opinion, as probable, that there
may occur, on some other planet, progressions of organized forms
analogous in some way to that series of animal forms which has appeared
upon the earth, we should still have no ground to assume that this
series must terminate in a rational and intelligent creature like man.
For the introduction of reason and intelligence upon the Earth is no
part nor consequence of the series of animal forms. It is a fact of an
entirely new kind. The transition from brute to man does not come within
the analogy of the transition from brute to brute. The thread of
analogy, even if it could lead us so far, would break here. We may
conceive analogues to other animals, but we could have no analogue to
man, except man. Man is not merely a higher kind of animal; he is a
creature of a superior order, participating in the attributes of a
higher nature; as we have already said, and as we hope hereafter
further to show. Even, therefore, if we were to assume the general
analogy of the Stars and of the Sun, and were to join to that the
information which geology gives us of the history of our own planet;
though we might, on this precarious path, be led to think of other
planets as peopled with unimagined monsters; we should still find a
chasm in our reasoning, if we tried, in this way, to find intelligent
and rational creatures in planets which may revolve round Sirius or
Arcturus.
33. The reasonable view of the matter appears to be this. The assumption
that the Fixed Stars are of exactly the same nature as the Sun, was, at
the first, when their vast distance and probable great size were newly
ascertained, a bold guess; to be confirmed or refuted by subsequent
observations and discoveries. Any appearances, tending in any degree to
confirm this guess, would have deserved the most considerate attention.
But there has not been a vestige of any such confirmatory fact. No
planet, nor anything which can fairly be regarded as indicating the
existence of a planet, revolving about a star, has anywhere been
discerned. The discovery of nebulae, of binary systems, of clusters of
stars, of periodical stars, of varying and accelerated periods of such
stars, all seem to point the other way. And if all these facts be held
to be but small in amount, as to the information which they convey,
about the larger, and perhaps nearer stars; still they leave the
original assumption a mere guess, unsupported by all that three
centuries of most diligent, and in other respects successful research,
has been able to bring to light. That Copernicus, that Galileo, that
Kepler, should believe the stars to be Suns, in every sense of the term,
was a natural result of the expansion of thought which their great
discoveries produced, in them and in their contemporaries. Nor are we
yet called upon to withdraw from them our sympathy; or entitled to
contradict their conjecture. But all the knowledge that the succeeding
times have given us; the extreme tenuity of much of the luminous matter
in the skies; the existence of gyratory motion among the stars, quite
different from planetary systems; the absence of any observed motions at
all resembling such systems; the appearance of changes in stars, quite
inconsistent with such permanent systems; the disclosure of the history
of our own planet, as one in which changes have constantly been going
on; the certainty that by far the greater part of the duration of its
existence, it has been tenanted by creatures entirely different from
those which give an interest, and thence, a persuasiveness, to the
belief of inhabitants in worlds appended to each star; the
impossibility, which appears, on the gravest consideration, of
transferring to other worlds such interests as belong to our own race in
this world; all these considerations should, it would seem, have
prevented that old and arbitrary conjecture from growing up, among a
generation professing philosophical caution, and scientific discipline,
into a settled belief.
34. Some of the moral and theological views which tend to encourage and
uphold this belief, may be taken under our more special consideration
hereafter: but here, where we are reasoning principally upon
astronomical grounds, we may conclude what we have to remark about the
Fixed Stars, as the centres of inhabited systems of worlds, by saying;
that it will be time enough to speculate about the inhabitants of the
planets which belong to such systems, when we have ascertained that
there are such planets, or one such planet. When that is done, we can
then apply to them any reasons which may exist, for believing that all,
or many planets, are the seats of habitation of living things. What
reasons of this kind can be adduced, and what is their force with
regard to our own solar system, we must now proceed to discuss.[23]
FOOTNOTES:
[1] Herschel, 866.
[2] Ibid. 866.
[3] Herschel, 846.
[4] Herschel, 848.
[5] That these systems have not condensed to _one_ centre, appears to
imply a less complete degree of condensation than exists in those
systems which have done so.
[6] Herschel, 850.
[7] Herschel, 847.
[8] The density of the sun is about as great as the density of water.
[9] Herschel, 827-832.
[10] _Cosmos_, III. 169, 205, and 641.
[11] Ibid., III. 172 and 252.
[12] _Astron. Soc. Notices_, Dec. 13, 1850.
[13] See Grant's _Hist. of Physical Astronomy_, p. 538.
[14] I am aware of certain speculations, and especially of some recent
ones, tending to show that even our Sun is wasting away by the emission
of light and heat; but these opinions, even if established, do not much
affect our argument one way or the other.
[15] Chalmers' _Astron. Disc._ p. 39.
[16] Hersch. 820.
[17] The periodical character of this star was discovered by David
Fabricius, a parish priest in East Friesland, the father of John
Fabricius, who discovered the solar spots. (_Cosmos_, III. 234.)
[18] Hersch. 825. In Humboldt's _Cosmos_, III. 243, Argelander, who has
most carefully observed and studied these periodical stars, has given a
catalogue containing 24, with the most recent determinations of their
periods.
[19] Hersch. 821. Humboldt (_Cosmos_, III. 238 and 246,) gives the
period as 68 hours 49 minutes, and says that it is 7 or 8 hours in its
less bright state. If we could suppose the times of the warning, and of
the greatest eclipse, given by Herschel, to be exactly determined, as
3-1/2 and 1/4, that is, in the proportion of 14 to 1, the darkening body
must have its effective breadth 14/15 of that of the star. But this is
on the supposition that the orbit of the darkening body has the
spectator's eye in its plane; if this be not so, the darkening body may
be much larger.
[20] Hersch. _Outl. Astr._ 821. Another explanation of the variable
period of Algol, is that the star is moving towards us, and therefore
the light occupies less and less time to reach us.
[21] Humboldt, very justly, regards the force of analogy as tending in
the opposite direction. "After all," he asks, (_Cosmos_, III. 373,) "is
the assumption of satellites to the Fixed Stars so absolutely necessary?
If we were to begin from the outer planets, Jupiter, &c., analogy might
seem to require that all planets have satellites. But yet this is not
true for Mars, Venus, Mercury." To which we may further add the
_twenty-three_ Planetoids. In this case there is a much greater number
of bodies which have not satellites, than which have them.
[22] _Consolations in Travel_. Dial. 1.
[23] What is said in Art. 15, that in consequence of the time employed
in the transmission of visual impressions, our seeing a star is
evidence, not that it exists now, but that it existed, it may be, many
thousands of years ago; may seem, to some readers, to throw doubts upon
reasonings which we have employed. It may be said that a star which was
a mere chaos, when the light, by which we see it, set out from it, may,
in the thousands of years which have since elapsed, have grown into an
orderly world. To which bare possibility, we may oppose another
supposition at least equally possible:--that the distant stars were
sparks or fragments struck off in the formation of the Solar System,
which are really long since extinct; and survive in appearance, only by
the light which they at first emitted.