OUR RELATION TO THE MILKY WAY
We now approach what may be termed the very heart of the subject of our
inquiry, the determination of how we are actually situated within this vast
but finite universe, and how that position is likely to affect our globe as
being the theatre of the development of life up to its highest forms.
We begin with our relation to the Milky Way (which we have fully described
in our fourth chapter), because it is by far the most important feature in
the whole heavens. Sir John Herschel termed it 'the ground-plane of the
sidereal system'; and the more it is studied the more we become convinced
that the whole of the stellar universe--stars, clusters of stars, and
nebulæ--are in some way connected with it, and are probably dependent on it
or controlled by it. Not only does it contain a greater number of stars of
the higher magnitudes than any other part of the heavens of equal extent,
but it also comprises a great preponderance of star-clusters, and a great
extent of diffused nebulous matter, besides the innumerable myriads of
minute stars which produce its characteristic cloud-like appearance. It is
also the region of those strange outbursts forming new stars; while gaseous
stars of enormous bulk--some probably a thousand or even ten thousand
times that of our sun, and of intense heat and brilliancy--are more
abundant there than in any other part of the heavens. It is now almost
certain that these enormous stars and the myriads of minute stars just
visible with the largest telescopes, are actually intermingled, and
together constitute its essential features; in which case the fainter stars
are really small and cannot be far apart, forming, as it were, the first
aggregations of the nebulous substratum, and perhaps supplying the fuel
which keeps up the intense brilliancy of the giant suns. If this is so,
then the Galaxy must be the theatre of operation of vast forces, and of
continuous combinations of matter, which escape our notice owing to its
enormous distance from us. Among its millions of minute telescopic stars,
hundreds or thousands may appear or disappear yearly without being
perceived by us, till the photographic charts are completed and can be
minutely scrutinised at short intervals. As undoubted changes have occurred
in many of the larger nebulæ during the last fifty years, we may anticipate
that analogous changes will soon be noted in the stars and the nebulous
masses of the Milky Way. Dr. Isaac Roberts has even observed changes in
nebulæ after such a short interval as eight years.
THE MILKY WAY A GREAT CIRCLE
Notwithstanding all its irregularities, its divisions, and its diverging
branches, astronomers are generally agreed that the Milky Way forms a great
circle in the heavens. Sir John Herschel, whose knowledge of it was
unrivalled, stated that its course 'conforms, as nearly as the
indefiniteness of its boundary will allow it to be fixed, to that of a
great circle'; and he gives the Right Ascension and Declination of the
points where it crosses the equinoctial, in figures which define those
points as being exactly opposite each other. He also defines its northern
and southern poles by other figures, so as to show that they are the poles
of a great circle. And after referring to Struve's view that it was _not_ a
great circle, he says, 'I retain my own opinion.' Professor Newcomb says
that its position 'is nearly always near a great circle of the sphere'; and
again he says: 'that we are in the galactic plane itself seems to be shown
in two ways: (1) the equality in the counts of stars on the two sides of
this plane all the way to its poles; and (2) the fact that the central line
of the Galaxy is a great circle, which it would not be if we viewed it from
one side of its central plane' (_The Stars_, p. 317). Miss Clerke, in her
_History of Astronomy_, speaks of 'our situation _in_ the galactic plane'
as one of the undisputed facts of astronomy; while Sir Norman Lockyer, in a
lecture delivered in 1899, said, 'the middle line of the Milky Way is
really not distinguishable from a great circle,' and again in the same
lecture--'but the recent work, chiefly of Gould in Argentina, has shown
that it practically is a great circle.'[4]
About this fact, then, there can be no dispute. A great circle is a circle
dividing the celestial sphere into two equal portions, as seen from the
earth, and therefore the plane of this circle must pass through the earth.
Of course the whole thing is on such a vast scale, the Milky Way varying
from ten to thirty degrees wide, that the plane of its circular course
cannot be determined with minute accuracy. But this is of little
importance. When carefully laid down on a chart, as in that of Mr. Sidney
Waters (see end of volume), we can see that its central line does follow a
very even circular course, conforming 'as nearly as may be' to a great
circle. We are therefore certainly well within the space that would be
enclosed if its northern and southern margins were connected together
across the vast intervening abyss, and in all probability not far from the
central plane of that enclosed space.
THE FORM OF THE MILKY WAY AND OUR POSITION ON ITS PLANE
Although the Galaxy forms a great circle in the heavens from our point of
view, it by no means follows that it is circular in plan. Being unequal in
width and irregular in outline, it might be elliptic or even angular in
shape without being at all obviously so to us. If we were standing in an
open plain or field two or three miles in diameter, and bounded in every
direction by woods of very irregular height and density and great diversity
of tint, we should find it difficult to judge of the shape of the field,
which might be either a true circle, an oval, a hexagon, or quite irregular
in outline, without our being able to detect the exact shape unless some
parts were very much nearer to us than others. Again, just as the woods
bounding the field might be either a narrow belt of nearly uniform width,
or might in some places be only a few yards wide and in others stretch out
for miles, so there have been many opinions as to the width of the Milky
Way in the direction of its plane, that is, in the direction in which we
look towards it. Lately, however, as the result of long-continued
observation and study, astronomers are fairly well agreed as to its general
form and extent, as will be seen by the following statements of fact and
reasoning.
Miss Clerke, after giving the various views of many astronomers--and as the
historian of modern astronomy her opinion has much weight--considers that
the most probable view of it is, that it is really very much what it seems
to us--an immense ring with streaming appendages extending from the main
body in all directions, producing the very complex effect we see. The
belief seems to be now spreading that the whole universe of stars is
spherical or spheroidal, the Milky Way being its equator, and therefore in
all probability circular or nearly so in plan; and it is also held that it
must be rotating--perhaps very slowly--as nothing else can be supposed to
have led to the formation of such a vast ring, or can preserve it when
formed.
Professor Newcomb considers, from the numbers of the stars in all
directions towards the Milky Way being approximately equal, that there
cannot be much difference in our distance from it in various directions. It
would follow that its plan is approximately circular or broadly elliptic.
The existence of ring-nebulæ may be held to render such a form probable.
Sir Norman Lockyer gives facts which tend in the same direction. In an
article in _Nature_ of November 8th, 1900, he says: 'We find that the
gaseous stars are not only confined to the Milky Way, but they are the most
remote in every direction, in every galactic longitude; all of them have
the smallest proper motion.' And again, referring to the hottest stars
being equally remote on all sides of us, he says: 'It is because we are in
the centre, because the solar system is in the centre, that the observed
effect arises.' He also considers that the ring-nebula in Lyra nearly
represents the form of our whole system; and he adds: 'We practically know
that in our system the centre is the region of least disturbance, and
therefore cooler conditions.'
These various facts and conclusions of some of the most eminent astronomers
all point to one definite inference, that our position, or that of the
solar system, is not very far from the centre of the vast ring of stars
constituting the Milky Way, while the same facts imply a nearly circular
form to this ring. Here, more than as regards our position in the plane of
the Galaxy, there is no possibility of precise determination; but it is
quite certain that if we were situated very far away from the centre, say,
for instance, one-fourth of its diameter from one side of it and
three-fourths from the other, the appearances would not be what they are,
and we should easily detect the excentricity of our position. Even if we
were one-third the diameter from one side and two-thirds from the other, it
will, I think, be admitted that this also would have been ascertained by
the various methods of research now available. We must, therefore, be
somewhere between the actual centre and a circle whose radius is one-third
of the distance to the Milky Way. But if we are about midway between these
two positions, we shall only be one-sixth of the radius or one-twelfth of
the diameter of the Milky Way from its exact centre; and if we form part of
a cluster or group of stars slowly revolving around that centre, we should
probably obtain all the advantages, if any, that may arise from a nearly
central position in the entire star-system.
This question of our situation within the great circle of the Milky Way is
of considerable importance from the point of view I am here suggesting, so
that every fact bearing upon it should be noted; and there is one which has
not, I think, been given the full weight due to it. It is generally
admitted that the greater brilliancy of some parts of the Milky Way is no
indication of nearness, because surfaces possess equal brilliancy from
whatever distance they are seen. Thus each planet has its special
brilliancy or reflective power, technically termed its 'albedo,' and this
remains the same at all distances if the other conditions are similar. But
notwithstanding this well-known fact, Sir John Herschel's remark that the
greater brightness of the southern Milky Way 'conveys strongly the
impression of greater proximity,' and therefore, that we are excentrically
placed in its plane, has been adopted by many writers as if it were the
statement of a fact, or at least a clearly expressed opinion, instead of
being a mere 'impression,' and really a misleading one. I therefore wish to
adduce a phenomenon which has a real bearing on the question. It is evident
that, if the Milky Way were actually of uniform width throughout, then
differences of apparent width would indicate differences of distance. In
the parts nearer to us it would appear wider, where more remote, narrower;
but in these opposite directions there would not necessarily be any
differences in brightness. We should, however, expect that in the parts
nearer to us the lucid stars, as well as those within any definite limits
of magnitude, would be either more numerous or more wide apart on the
average. No such difference as this, however, has been recorded; but there
_is_ a peculiar correspondence in the opposite portions of the Galaxy which
is very suggestive. In the beautiful charts of the Nebulæ and Star Clusters
by the late Mr. Sidney Waters, published by the Royal Astronomical Society
and here reproduced by their permission (see end of volume), the Milky Way
is delineated in its whole extent with great detail and from the best
authorities. These charts show us that, in both hemispheres, it reaches its
maximum extension on the right and left margins of the charts, where it is
almost equal in extent; while in the centre of each chart, that is at its
nearest points to the north and south poles respectively, it is at its
narrowest portion; and, although this part in the southern hemisphere is
brightest and most strongly defined, yet the actual extent, including the
fainter portions, is, again, not very unequal in the opposite segments.
Here we have a remarkable and significant symmetry in the proportions of
the Milky Way, which, taken in connection with the nearly symmetrical
scattering of the stars in all parts of the vast ring, is strongly
suggestive of a nearly circular form and of our nearly central position
within its plane. There is one other feature in this delineation of the
Milky Way which is worthy of notice. It has been the universal practice to
speak of it as being double through a considerable portion of its extent,
and all the usual star-maps show the division greatly exaggerated,
especially in the northern hemisphere; and this division was considered so
important as to lead to the cloven-disc theory of its form, or that it
consisted of two separate irregular rings, the nearer one partly hiding the
more distant; while various spiral combinations were held by others to be
the best way of explaining its complex appearance. But this newer map,
reduced from a large one by Lord Rosse's astronomer, Dr. Boeddicker, who
devoted five years to its delineation, shows us that there is no actual
division in any portion of it in the northern hemisphere, but that
everywhere, throughout its whole width, it consists of numerous
intermingled streams and branches, varying greatly in luminosity, and with
many faint or barely distinguishable extensions along its margins, yet
forming one unmistakable nebulous belt; and the same general character
applies to it in the southern hemisphere as delineated by Dr. Gould.
Another feature, which is well shown to the eye by these more accurate
maps, is the regular curvature of the central line of the Milky Way. We can
judge of this almost sufficiently by the eye; but if, with a pair of
compasses, we find the proper radius and centre of curvature, we shall see
that the true circular curve is always in the very centre of the nebulous
mass, and the same radius applied in the same manner to the opposite
hemisphere gives a similar result. It will be noted that as the Milky Way
is obliquely situated on these charts, the centre of the curve will be
about in R.A. 0h. 40m. in the map of the southern hemisphere, and in R.A.
12h. 40m. in that of the northern hemisphere; while the radius of curvature
will be about the length of the chord of eight hours of R.A. as measured on
the margin of the maps. This great regularity of curve of the central line
of the Galaxy strongly suggests rotation as the only means by which it
could have originated and be maintained.
THE SOLAR CLUSTER
Astronomers are now generally agreed that there is a cluster of stars of
which our sun forms a part, though its exact dimensions, form, and limits
are still under discussion. Sir William Herschel long ago arrived at the
conclusion that the Milky Way 'consists of stars very differently scattered
from those immediately around us.' Dr. Gould believed that there were about
five hundred bright stars much nearer to us than the Milky Way, which he
termed the solar cluster. And Miss Clerke observes that the actual
existence of such a cluster is indicated by the fact that 'an enumeration
of the stars in photometric order discloses a systematic excess of stars
brighter than the 4th magnitude, making it certain that there is an actual
condensation in the neighbourhood of the sun--that the average allowance of
cubical space per star is smaller within a sphere enclosing him with a
radius, say, of 140 light-years, than further away.'[5]
But the most interesting inquiry into this subject is that by Professor
Kapteyn of Gröningen, one of the most painstaking students of the
distribution of the stars. He founds his conclusions mainly on the proper
motions of the stars, this being the best general indication of distance in
the absence of actual determination of parallax. He made use of the proper
motions and the spectra of more than two thousand stars, and he finds that
a considerable body of stars having large proper motions, and also
presenting the solar type of spectra, surround our sun in all directions,
and show no increased density, as the more distant stars do, towards the
Milky Way. He finds also that towards the centre of this cluster stars are
far closer together than near its outer limits (he says there are
ninety-eight times as many), that it is roughly spherical in shape, and
that the maximum compression is, as nearly as can be ascertained, at the
centre of the circle of the Milky Way, while the sun is at some distance
away from this central point.[6]
It is a very suggestive fact that most of the stars belonging to this
cluster have spectra of the solar type, which indicates that they are of
the same general chemical constitution as our sun, and are also at about
the same stage of evolution; and this may well have arisen from their
origin in a great nebulous mass situated at or near the centre of the
galactic plane, and probably revolving round their common centre of
gravity.
As Kapteyn's result was based on materials which were not so full or
reliable as those now available, Professor S. Newcomb has examined the
question himself, using two recent lists of stars, one limited to those
having proper motions of 10" a century, of which there are 295, and the
other of nearly 1500 stars with 'appreciable proper motions.' They are
situated in two zones, each about 5° in breadth and cutting across the
Milky Way in different parts of its course. They afford, therefore, a good
test of the distribution of these nearer stars with regard to the Galaxy.
The result is, that on the average these stars are not more numerous in or
near the Milky Way than elsewhere; and Professor Newcomb expresses himself
on this point as follows:--'The conclusion is interesting and important. If
we should blot out from the sky all the stars having no proper motion large
enough to be detected, we should find remaining stars of all magnitudes;
but they would be scattered almost uniformly over the sky, and show little
or no tendency to crowd towards the Galaxy, unless, perhaps, in the region
near 19h. of Right Ascension.'[7]
A little consideration will show that, as the stars of all magnitudes which
are, on the average, nearest to us are spread over the sky in 'all
directions' and 'almost uniformly,' this necessarily implies that they
form a cluster or group, and that our sun is somewhere not very far from
the centre of this group. Again, Professor Newcomb refers to 'the
remarkable equality in the number of stars in opposite directions from us.
We do not detect any marked difference between the numbers lying round the
opposite poles of the Galaxy, nor, so far as known, between the
star-density in different regions at equal distances from the Milky Way'
(_The Stars_, p. 315). And again he refers to the same question at p. 317,
where he says: 'So far as we can judge from the enumeration of the stars in
all directions, and from the aspect of the Milky Way, our system is near
the centre of the stellar universe.'
It will, I think, now be clear to my readers that the four main
astronomical propositions stated in my article which appeared in the New
York _Independent_ and in the _Fortnightly Review_, and which were either
denied or declared to be unproved by my astronomical critics, have been
shown to be supported by so many converging lines of evidence, that it is
no longer possible to deny that they are, at least provisionally, fairly
well established. These facts are, (1) that the stellar universe is not of
infinite extent; (2) that our sun is situated in the central plane of the
Milky Way; (3) that it is also situated near to the centre of that plane;
(4) that we are surrounded by a group or cluster of stars of unknown
extent, which occupy a place not far removed from the centre of the
galactic plane, and therefore, near to the centre of our universe of stars.
Not only are these four propositions each supported by converging lines of
evidence, including some which I believe have not before been adduced in
their support, but a number of astronomers, admittedly of the first rank,
have arrived at the same conclusions as to the bearing of the evidence, and
have expressed their convictions in the clearest manner, as quoted by me.
It is _their_ conclusions which I appeal to and adopt; yet my two chief
astronomical critics positively deny that there is any valid evidence of
the finiteness of the stellar universe, which one of them terms 'a myth,'
and he even accuses _me_ of having started it. Both of them, however, agree
in stating very strongly one objection to my main thesis--that our central
position (not necessarily at the precise centre) in the stellar universe
has a meaning and a purpose, in connection with the development of life and
of man upon this earth, and, _so far as we know_, here only. With this one
objection, the only one that in my opinion has the slightest weight, I will
now proceed to deal.
THE SUN'S MOTION THROUGH SPACE
The two astronomers who did me the honour to criticise my original article
laid the greatest stress on the fact, that even if I had proved that the
sun now occupied a nearly central position in the great star-system, it was
really of no importance whatever, because, at the rate the sun was
travelling, 'five million years ago we were deep in the actual stream of
the Milky Way; five million years hence we shall have completely crossed
the gulf which it encircles, and again be a member of one of its
constituent groups, but on the opposite side. And ten million years are
regarded by geologists and biologists as but a trifle on account to meet
their demands upon the bank of Time.' Thus speaks one of my critics. The
other is equally crushing. He says:--'If there is a centre to the visible
universe, and if we occupy it to-day, we certainly did not do so yesterday,
and shall not do so to-morrow. The Solar System is known to be moving among
the stars with a velocity which would carry us to Sirius within 100,000
years, if we happened to be travelling in his direction, as we are not. In
the 50 or 100 million years during which, according to geologists, this
earth has been a habitable globe, we must have passed by thousands of stars
on the right hand and on the left.... In his eagerness to limit the
universe in space, Dr. Wallace has surely forgotten that it is equally
important, for his purpose, to limit it in time; but incomparably more
difficult in the face of ascertained facts.... Indeed, so far from our
having tranquilly enjoyed a central position in unbroken continuity for
scores or perhaps hundreds of millions of years, we should in that time
have traversed the universe from boundary to boundary.'[8]
Now the average reader of these two criticisms, taking account of the high
official position of both writers, would accept their statements of the
case as being demonstrated facts, requiring no qualification whatever, and
would conclude that my whole argument had been thereby rendered worthless,
and all that I founded upon it a fantastic dream. But if, on the other
hand, I can show that their stated facts as to the sun's motion are by no
means demonstrated, because founded upon assumptions which may be quite
erroneous; and further, that if the facts should turn out to be
substantially correct, they have both omitted to state well-known and
admitted qualifications which render the conclusions they derive from the
facts very doubtful, then the average reader will learn the valuable lesson
that official advocacy, whether in medicine, law, or science is never to be
accepted till the other side of the case has been heard. Let us see,
therefore, what the facts really are.
Professor Simon Newcomb calculates that, if there are one hundred million
stars in the stellar universe each five times the mass of our sun, and
spread over a space which light would require thirty thousand years to
cross, then any mass traversing such a system with a velocity of more than
twenty-five miles a second, would fly off into infinite space never to
return. Now as there are many stars which have, apparently, very much more
than this velocity, it would follow that the visible universe is unstable.
It also implies that these great velocities were not acquired in the system
itself, but that the bodies which possess them must have entered it from
without, thus requiring other universes as the feeders of our universe.
For the accuracy of the above statement the authority of Professor Newcomb
is an ample guarantee; but there may be modifications required in the data
on which it is founded, and these may greatly alter the result. If I do not
mistake, the estimate of a hundred million stars is founded on actual
counts or estimates of stars of successive magnitudes in different parts of
the heavens, and it does not include either those of the denser star
clusters nor the countless millions just beyond the reach of telescopes in
the Milky Way. Neither does it make allowance for the dark stars supposed
by some astronomers to be many times more numerous than the bright ones,
nor for the vast number of the nebulæ, great and small, in calculating the
total mass of the stellar system.[9] In his latest work Professor Newcomb
says, 'The total number of stars is to be counted by hundreds of millions';
and hence the controlling power of the system on bodies within it will be
many times greater than that given above, and might even be ample to retain
within its bounds such a rapidly moving star as Arcturus, which is believed
to be travelling at the rate of more than three hundred miles a second. But
there is another very important limitation to the conclusions to be drawn
from Professor Newcomb's calculation. It assumes the stars to be nearly
uniformly distributed through the whole of the space to which the system
extends. But the facts are very different. The existence of clusters, some
of which comprise many thousands of stars, is one example of irregularity
of distribution, and any one of these larger clusters would probably be able
to change the course of even the swiftest stars passing near it. The larger
nebulæ might have the same effect, since the late Mr. Ranyard, taking all
his data so as to produce a minimum result, calculated the probable mass
of the Orion nebula to be four and a half million times that of the sun,
and there may be many other nebulæ equally large. But far more important is
the fact of the vast ring of the Milky Way, which is now universally held
by astronomers to be, not only apparently but really, more densely crowded
with stars and also with vast masses of nebulous matter than any other part
of the heavens, so that it may possibly comprise within itself a very large
proportion of the whole of the matter of the visible universe. This is
rendered more probable by the fact that the great majority of star-clusters
lie along its course, most of the huge gaseous stars belong to it, while
the occurrence there only of 'new stars' is evidence of a superabundance of
matter in various forms leading to frequent heat-producing collisions, just
as the frequent occurrence of meteoric showers on our earth is evidence of
the superabundance of meteoric matter in the solar system.
It is recognised by mathematicians that within any great system of bodies
subject to the law of gravitation there can be no such thing as motion of
any of them in a straight line; neither can any amount of motion arise
within such a system through the action of gravitation alone capable of
carrying any of its masses out of the system. The ultimate tendency must be
towards concentration rather than towards dispersal.
It seems, therefore, only reasonable to consider whatever motions and
whatever velocities we find among the stars, as having been produced by the
gravitative power of the larger aggregations, modified perhaps by
electrical repulsive forces, by collisions, and by the results of those
collisions; and we may look to the changes now visibly going on in some of
the nebulæ and clusters as indications of the forces that have probably
brought about the actual condition of the whole stellar universe.
If we examine the beautiful photographs of nebulæ by Dr. Roberts and other
observers, we find that they are of many forms. Some are extremely
irregular and almost like patches of cirrus clouds, but a large number are
either distinctly spiral in form, or show indications of becoming spiral,
and this has been found to be the case even with some of the large
irregular nebula. Then again we have numerous ring-formed nebulæ, usually
with a star involved in dense nebulosity in the centre, separated by a dark
space of various widths from the outer ring. All these kinds of nebulæ have
stars involved in them, and apparently forming part of their structure,
while others which do not differ in appearance from ordinary stars are
believed by Dr. Roberts to lie between us and the nebula. In the case of
many of the spiral nebulæ, stars are often strung along the coils of the
spiral, while other curved lines of stars are seen just outside the nebula,
so that it is impossible to avoid the conclusion that both are really
connected with it, the outer lines of stars indicating a former greater
extension of the nebula whose material has been used up in the growth of
these stars. Some of these spiral nebulæ show beautifully regular
convolutions, and these usually have a large central star like mass, as in
M. 100 Comæ and I. 84 Comæ, in Vol. II. Pl. 14 of Dr. Roberts's
photographs. The straight white streaks across the nebula of the Pleiades
and some others are believed by Dr. Roberts to be indications of spiral
nebulæ seen edgewise. In other cases, clusters of stars are more or less
nebulous, and the arrangement of the stars seems to indicate their
development from a spiral nebula. It is to be noted that many of the
objects classed as planetary nebulæ by Sir John Herschel are shown by the
best photographs to be really of the ring-type, though often with a very
narrow division between the ring and the central mass. This form may
therefore be of frequent occurrence.
But if this annular form with some kind of central nucleus, often very
large, is produced under certain conditions by the action of the ordinary
laws of motion upon more or less extensive masses of discrete matter, why
may not the same laws acting upon similar matter once dispersed over the
whole extent of the existing stellar universe, or even beyond what are now
its farthest limits, have led to the aggregation of the vast annular
formation of the Milky Way, with all the subordinate centres of
concentration or dispersal to be found within or around it? And if this is
a reasonable conception, may we not hope that by a concentration of
attention upon a few of the best marked and most favourably situated
annular and spiral systems, sufficient knowledge of their internal motions
may be obtained which may serve as a guide to the kind of motion we may
expect to find in the great galactic ring and its subordinate stars? We may
then perhaps discover which now seem so erratic, are really all parts of a
series of orbital movements limited and controlled by the forces of the
great system to which they belong, so that, if not mathematically stable,
they may yet be sufficiently so to endure for some thousand millions of
years.
It is a suggestive fact that the calculated position of the 'solar
apex'--the point towards which our sun appears to move--is now found to be
much more nearly in the plane of the Milky Way than the position first
assigned to it, and Professor Newcomb adopts, as most likely to be
accurate, a point near the bright star Vega in the constellation Lyra.
Other calculators have placed it still farther east, while Rancken and Otto
Stumpe assign it a position actually in the Milky Way; and Mr. G.C. Bompas
concludes that the sun's plane of motion nearly coincides with that of the
Galaxy. M. Rancken found that 106 stars near the Milky Way showed, in their
very small proper motions, a drift along it in a direction from Cassiopeiæ
towards Orion, and this, it is supposed, may be partly due to our sun's
motion in an opposite direction.
In many other parts of the heavens there are groups of stars which have
almost identical proper motions--a phenomenon which the late R.A. Proctor
termed 'star-drift'; and he especially pointed out that five of the stars
of the Great Bear were all drifting in the same direction; and although
this has been denied by later writers, Professor Newcomb, in his recent
book on _The Stars_, declares that Proctor was right, and explains that the
error of his critics was due to not making allowance for the divergence of
the circles of right ascension. The Pleiades are another group, the stars
of which drift in the same direction, and it is a most suggestive fact that
photographs now show this cluster to be embedded in a vast nebula, which,
therefore, has also a proper motion; but some of the smaller stars do not
partake of it. Three stars in Cassiopeiæ also move together, and no doubt
many other similarly connected groups remain to be discovered.
These facts have a very important bearing on the question of the motion of
our sun in space. For this motion has been determined by comparing the
motions of large numbers of stars which are assumed to be wholly
independent of each other, and to move, as it were, at random. Miss A.M.
Clerke, in her _System of the Stars_, puts this point very clearly, as
follows: 'For the assumption that the absolute movements of the stars have
no preference for one direction over another, forms the basis of all
investigations hitherto conducted into the translatory advance of the solar
system. The little fabric of laboriously acquired knowledge regarding it at
once crumbles if that basis has to be removed. In all investigations of the
sun's movement, the movements of the stars have been regarded as casual
irregularities; should they prove to be in any visible degree systematic,
the mode of treatment adopted (and there is no other at present open to us)
becomes invalid, and its results null and void. The point is then of
singular interest, and the evidence bearing upon it deserves our utmost
attention.'
Mr. W.H.S. Monck, a well-known astronomer, takes the same view. He says:
'The proof of this motion rests on the assumption that if we take a
sufficient number of stars, their real motions in all directions will be
equal, and that therefore the apparent preponderances which we observe in
particular directions result from the real motion of the sun. But there is
no impossibility in a systematic motion of the majority of the stars used
in these researches which might reconcile the observed facts with a
motionless sun. And, in the second place, if the sun is not in the exact
centre of gravity of the universe, we might expect him to be moving in an
orbit around this centre of gravity, and our observations on his actual
motion are not sufficiently numerous or accurate to enable us to affirm
that he is moving in a right line rather than such an orbit.'
Now this 'systematic motion,' which would render all calculations as to the
sun's motion inaccurate or even altogether worthless, is by many
astronomers held to be an observed reality. The star-drift, first pointed
out by Proctor, has been shown to exist in many other groups of stars,
while the curious arrangements of stars all over the heavens in straight
lines, or regular curves, or spirals, strongly suggests a wide extension of
the same kind of relation. But even more extensive systematic movements
have been observed or suggested by astronomers. Sir D. Gill, by an
extensive research, believes that he has found indications of a rotation of
the brighter fixed stars as a whole in regard to the fainter fixed stars as
a whole. Mr. Maxwell Hall has also found indications of a movement of a
large group of stars, including our sun, around a common centre, situated
in a direction towards Epsilon Andromedæ, and at a distance of about 490
years of light-travel. These last two motions are not yet established; but
they seem to prove two important facts--(_a_) that eminent astronomers
believe that _some_ systematic motions must exist among the stars, or they
would not devote so much labour to the search for them; and (_b_) that
extensive systematic motions of some kind do exist, or even these results
would not have been obtained.
Mr. W.W. Campbell, of the Lick Observatory, thus remarks on the uncertainty
of determinations of the sun's motions: 'The motion of the solar system is
a purely relative quantity. It refers to specified groups of stars. The
results for various groups may differ widely, and all be correct. It would
be easy to select a group of stars with reference to which the solar motion
would be reversed 180° from the values assigned above' (_Astrophysical
Journal_, vol. xiii. p. 87. 1901).
It must be remembered that, within a uniform cluster of stars, each moving
round the common centre of gravity of the whole cluster, Kepler's laws do
not prevail, the law being that the angular velocities are all identical,
so that the more distant stars move faster than those nearer the centre,
subject to modifications, however, due to the varying density of the
cluster. But if the cluster is nearly globular, there must be stars moving
round the centre in every plane, and this would lead to apparent motions in
many directions as viewed by us, although those which were moving in the
same plane as ourselves would, when compared with remote stars outside the
cluster, appear to be all moving in the same direction and at the same
rate, forming, in fact, one of those drifting systems of stars already
referred to. Again, if in the process of formation of our cluster, smaller
aggregations already having a rotatory motion were drawn into it, this
might lead to their revolving in an opposite direction to those which were
formed from the original nebula, thus increasing the diversities of
apparent motion.
The evidence now briefly set forth fully justifies, I submit, the remarks
as to the statements of my astronomical critics at the beginning of this
section. They have both given the accepted views as to direction and rate
of movement of our sun without any qualification whatever, as if they were
astronomical facts of the same certainty and the same degree of accuracy as
the sun's distance from the earth; and they will assuredly have been so
understood by the great body of non-mathematical readers. It appears,
however, if the authorities I have quoted are right, that the whole
calculation rests upon certain assumptions, which are certainly to some
extent, and may be to a very large extent, erroneous. This is my reply to
one part of their criticism.
In the next place, they both assert, or imply, not only that the sun's
motion is now in a straight line, but that it has been in a straight line
from some enormously remote period when it first entered the stellar system
on one side, and will so continue to move till it reaches the utmost bounds
of that system on the other side. And this is stated by them both, not as a
possibility, but as a certainty. They use such terms as 'must' and 'will
be,' leaving no room for any doubt whatever. But such a result implies the
abrogation of the law of gravitation, since under its action motion in a
straight line in the midst of thousands or millions of suns of various
sizes is an absolute impossibility; while it also implies that the sun must
have been started on its course from some other system outside the Milky
Way, with such a precise determination of direction as not to collide with,
or even make a near approach to, any one of the suns or clusters of suns,
or vast nebulous masses, during its passage through the very midst of the
stellar universe.
This is my reply to the main point of their criticism, and I think I am
justified in saying that nothing in my whole article is so demonstrably
baseless as the statements I have now examined.
* * * * *
Considering then the whole bearing of the evidence, I refuse to accept the
unsupported dicta of those who would have us believe that our admitted
position not far from the centre of the stellar universe is a mere
temporary coincidence of no significance whatever; or that our sun and
hosts of other similar orbs near to us have come together by an accident,
and are being dispersed into surrounding space, never to meet again. Until
this is proved by indisputable evidence, it seems to me far more probable
that we are moving in an orbit of some kind around the centre of gravity of
a vast cluster, as determined by the investigations of Kapteyn, Newcomb,
and other astronomers; and, consequently, that the nearly central position
we now occupy may be a permanent one. For even if our sun's orbit should
have a diameter a thousand times that of Neptune, it would be but a small
fraction of the diameter of the Milky Way; while so vast is the scale of
our universe, that it might be even a hundred thousand times as great and
still leave us deeply immersed in the solar cluster, and very much nearer
to the dense central portion than to its more diffused outer regions.
Here the subject may be left for the present. After having studied the
evidence afforded by the essential conditions of life-development on the
earth, and the numerous indications that these conditions do not exist on
any of the other planets of the solar system, it may be again touched upon
in a general review of the conclusions arrived at.
FOOTNOTES:
[4] _Nature_, October 26, 1899.
[5] _The System of the Stars_, p. 385.
[6] This account of Professor Kapteyn's research is taken from an article
by Miss A.M. Clerke in _Knowledge_, April 1893.
[7] _The Stars_, p. 256. The region here referred to is that where the
Milky Way has its greatest width (though nearly as wide in the part exactly
opposite), and where it may perhaps extend somewhat in our direction. Miss
A.M. Clerke informs me that in April 1901 Kapteyn withdrew the conclusions
arrived at in 1893, as being founded on illegitimate reasoning as to the
relation of parallaxes to proper motions. But as this relation is still
accepted, under certain limitations, by Professor Newcomb and other
astronomers, who have arrived independently at very similar results, it
seems not improbable that, after all, Professor Kapteyn's conclusions may
not require very much modification. Professor Newcomb also tells us (_The
Stars_, p. 214, footnote) that he has seen the latest of Professor
Kapteyn's papers, down to 1901; but he does not therefore express any doubt
as to his own conclusions as here referred to.
[8] See _Knowledge_ and _The Fortnightly Review_ of April 1903.
[9] Sir R. Ball in an article in _Good Words_ (April 1903) says that
luminosity is an exceptional phenomenon in nature, and that luminous stars
are but the glow-worms and fire-flies of the universe, as compared with the
myriads of other animals.