The Visible Universe
Some researches on the distribution of stars in the sky have recently been
made at the Harvard Observatory (U.S.A.). The principal results are:--(1)
The number of stars on any "given area of the Milky Way is about twice as
great as in an equal area of any other region." (2) This ratio does not
increase for faint stars down to the 12th magnitude. (3) "The Milky Way
covers about one-third of the sky and contains about half of the stars."
(4) There are about 10,000 stars of magnitude 6·6 or brighter, 100,000
down to magnitude 8·7, one million to magnitude 11, and two millions to
magnitude 11·9. It is estimated that there are about 18 millions of stars
down to the 15th magnitude visible in a telescope of 15 inches
aperture.[456]
According to Prof. Kapteyn's researches on stellar distribution, he finds
that going out from the earth into space, the "star density"--that is,
the number of stars per unit volume of space--is fairly constant until we
reach a distance of about 200 "light years." From this point the density
gradually diminishes out to a distance of 2500 "light years," at which
distance it is reduced to about one-fifth of the density in the sun's
vicinity.[457]
In a letter to the late Mr. Proctor (_Knowledge_, November, 1885, p. 21),
Sir John Herschel suggested that our Galaxy (or stellar system) "contained
within itself miniatures of itself." This beautiful idea is probably true.
In his account of the greater "Magellanic cloud," Sir John Herschel
describes one of the numerous objects it contains as follows:--
"Very bright, very large; oval; very gradually pretty, much brighter
in the middle; a beautiful nebula; it has very much the resemblance to
the Nubecula Major itself as seen with the naked eye, but it is far
brighter and more impressive in its general aspect as if it were
doubled in intensity. Note--July 29, 1837. I well remember this
observation, it was the result of repeated comparisons between the
object seen in the telescope and the actual nubecula as seen high in
the sky on the meridian, and no vague estimate carelessly set down.
And who can say whether in this object, magnified and analysed by
telescopes infinitely superior to what we now possess, there may not
exist all the complexity of detail that the nubecula itself presents
to our examination?"[458]
The late Lord Kelvin, in a remarkable address delivered before the
Physical Science Section of the British Association at its meeting at
Glasgow in 1901, considered the probable quantity of matter contained in
our Visible Universe. He takes a sphere of radius represented by the
distance of a star having a parallax of one-thousandth of a second (or
about 3000 years' journey for light), and he supposes that uniformly
distributed within this sphere there exists a mass of matter equal to 1000
million times the sun's mass. With these data he finds that a body placed
originally at the surface of the sphere would in 5 million years acquire
by gravitational force a velocity of about 12-1/2 miles a second, and
after 25 million of years a velocity of about 67 miles a second. As these
velocities are of the same order as the observed velocities among the
stars, Lord Kelvin concludes that there _is_ probably as much matter in
our universe as would be represented by a thousand million suns. If we
assumed a mass of ten thousand suns the velocities would be much too high.
The most probable estimate of the total number of the visible stars is
about 100 millions; so that if Lord Kelvin's calculations are correct we
seem bound to assume that space contains a number of dark bodies. The
nebulæ, however, probably contain vast masses of matter, and this may
perhaps account--partially, at least--for the large amount of matter
estimated by Lord Kelvin. (See Chapter on "Nebulæ.")
In some notes on photographs of the Milky Way, Prof. Barnard says with
reference to the great nebula near [Greek: r] Ophiuchi, "The peculiarity
of this region has suggested to me the idea that the apparently small
stars forming the ground work of the Milky Way here, are really very small
bodies compared with our own sun"; and again, referring to the region near
[Greek: b] Cygni, "One is specially struck with the apparent extreme
smallness of the general mass of the stars in this region." Again, with
reference to [Greek: ch] Cygni, he says, "The stars here also are
remarkably uniform in size."[459]
Eastman's results for parallax seem to show that "the fainter rather than
the brighter stars are nearest to our system." But this apparent paradox
is considered by Mr. Monck to be very misleading;[460] and the present
writer holds the same opinion.
Prof. Kapteyn finds "that stars whose proper motions exceed 0"·05 are not
more numerous in the Milky Way than in other parts of the sky; or, in
other words, if only the stars having proper motions of 0"·05 or upwards
were mapped, there would be no aggregation of stars showing the existence
of the Milky Way."[461]
With reference to the number of stars visible on photographs, the late Dr.
Isaac Roberts says--
"So far as I am able at present to judge, under the atmospheric
conditions prevalent in this country, the limit of the photographic
method of delineation will be reached at stellar, or nebular, light of
the feebleness of about 18th-magnitude stars. The reason for this
inference is that the general illumination of the atmosphere by
starlight concentrated upon a film by the instrument will mask the
light of objects that are fainter than about 18th-magnitude
stars."[462]
With reference to blank spaces in the sky, the late Mr. Norman Pogson
remarked--
"Near S Ophiuchi we find one of the most remarkable vacuities in this
hemisphere--an elliptic space of about 65' in length in the direction
of R.A., and 40' in width, in which there exists _no_ star larger than
the 13th magnitude ... it is impossible to turn a large telescope in
that direction and, if I may so express it, view such black darkness,
without a feeling that we are here searching into the remote regions
of space, far beyond the limits of our own sidereal system."[463]
Prof. Barnard describes some regions in the constellation Taurus
containing "dark lanes" in a groundwork of faint nebulosity. He gives two
beautiful photographs of the regions referred to, and says that the dark
holes and lanes are apparently darker than the sky in the immediate
vicinity. He says, "A very singular feature in this connection is that the
stars also are absent in general from the lanes." A close examination of
these photographs has given the present writer the impression that the
dark lanes and spots are _in_ the nebulosity, and that the nebulosity is
mixed up with the stars. This would account for the fact that the stars
are in general absent from the dark lanes. For if there is an intimate
relation between the stars and the nebulosity, it would follow that where
there is no nebulosity in this particular region there would be no stars.
Prof. Barnard adds that the nebulosity is easily visible in a 12-inch
telescope.[464]
With reference to the life of the universe, Prof. F. R. Moulton well
says--
"The lifetime of a man seems fairly long, and the epoch when Troy was
besieged, or when the Pharaohs piled up the pyramids in the valley of
the Nile, or when our ancestors separated on the high plateaux of
Asia, seems extremely remote, but these intervals are only moments
compared to the immense periods required for geological evolutions and
the enormously greater ones consumed in the developement of worlds
from widely extended nebulous masses. We recognize the existence of
only those forces whose immediate consequences are appreciable, and it
may be that those whose effects are yet unseen are really of the
highest importance. A little creature whose life extended over only
two or three hours of a summer's day might be led, if he were
sufficiently endowed with intelligence, to infer that passing clouds
were the chief influence at work in changing the climate instead of
perceiving that the sun's slow motion across the sky would bring on
the night and its southward motion the winter."[465]
In a review of my book _Astronomical Essays_ in _The Observatory_,
September, 1907, the following words occur. They seem to form a good and
sufficient answer to people who ask, What is there beyond our visible
universe? "If the stellar universe is contained in a sphere of say 1000
stellar units radius, what is there beyond? To this the astronomer will
reply that theories and hypotheses are put forward for the purpose of
explaining observed facts; when there are no facts to be explained, no
theory is required. As there are no observed facts as to what exists
beyond the farthest stars, the mind of the astronomer is a complete blank
on the subject. Popular imagination can fill up the blank as it pleases."
With these remarks I fully concur.
In his address to the British Association, Prof. G. H. Darwin (now Sir
George Darwin) said--
"Man is but a microscopic being relatively to astronomical space, and
he lives on a puny planet circling round a star of inferior rank. Does
it not, then, seem futile to imagine that he can discover the origin
and tendency of the Universe as to expect a housefly to instruct us as
to the theory of the motions of the planets? And yet, so long as he
shall last, he will pursue his search, and will no doubt discover many
wonderful things which are still hidden. We may indeed be amazed at
all that man has been able to find out, but the immeasurable magnitude
of the undiscovered will throughout all time remain to humble his
pride. Our children's children will still be gazing and marvelling at
the starry heavens, but the riddle will never be read."
The ancient philosopher Lucretius said--
"Globed from the atoms falling slow or swift
I see the suns, I see the systems lift
Their forms; and even the system and the suns
Shall go back slowly to the eternal drift."[466]
But it has been well said that the structure of the universe "has a
fascination of its own for most readers quite apart from any real progress
which may be made towards its solution."[467]
The Milky Way itself, Mr. Stratonoff considers to be an agglomeration of
immense condensations, or stellar clouds, which are scattered round the
region of the galactic equator. These clouds, or masses of stars,
sometimes leave spaces between them, and sometimes they overlap, and in
this way he accounts for the great rifts, like the Coal Sack, which allow
us to see through this great circle of light. He finds other
condensations of stars; the nearest is one of which our sun is a member,
chiefly composed of stars of the higher magnitudes which "thin out rapidly
as the Milky Way is approached." There are other condensations: one in
stars of magnitudes 6·5 to 8·5; and a third, farther off, in stars of
magnitudes 7·6 to 8. These may be called opera-glass, or field-glass
stars.
Stratonoff finds that stars with spectra of the first type (class A, B, C,
and D of Harvard) which include the Sirian and Orion stars, are
principally situated near the Milky Way, while those of type II. (which
includes the solar stars) "are principally condensed in a region
coinciding roughly with the terrestrial pole, and only show a slight
increase, as compared with other stars, as the galaxy is approached."[468]
Prof. Kapteyn thinks that "undoubtedly one of the greatest difficulties,
if not the greatest of all, in the way of obtaining an understanding of
the real distribution of the stars in space, lies in our uncertainty about
the amount of loss suffered by the light of the stars on its way to the
observer."[469] He says, "There can be little doubt in my opinion, about
the existence of absorption in space, and I think that even a good guess
as to the order of its amount can be made. For, first we know that space
contains an enormous mass of meteoric matter. This matter must necessarily
intercept some part of the star-light."
This absorption, however, seems to be comparatively small. Kapteyn finds a
value of 0·016 (about 1/60th) of a magnitude for a star at a distance
corresponding to a parallax of one-tenth of a second (about 33 "light
years"). This is a quantity almost imperceptible in the most delicate
photometer. But for very great distances--such as 3000 "light years"--the
absorption would evidently become very considerable, and would account
satisfactorily for the gradual "thinning out" of the fainter stars. If
this were fully proved, we should have to consider the fainter stars of
the Milky Way to be in all probability fairly large suns, the light of
which is reduced by absorption.
That some of the ancients knew that the Milky Way is composed of stars is
shown by the following lines translated from Ovid:--
"A way there is in heaven's extended plain
Which when the skies are clear is seen below
And mortals, by the name of Milky, know;
The groundwork is of stars, through which the road
Lies open to great Jupiter's abode."[470]
From an examination of the distribution of the faint stars composing the
Milky Way, and those shown in Argelander's charts of stars down to the
9-1/2 magnitude, Easton finds that there is "a real connection between the
distribution of 9th and 10th magnitude stars, and that of the faint stars
of the Milky Way, and that consequently the faint or very faint stars of
the galactic zone are at a distance which does not greatly exceed that of
the 9th and 10th magnitude stars."[471] A similar conclusion was, I think,
arrived at by Proctor many years ago. Now let us consider the meaning of
this result. Taking stars of the 15th magnitude, if their faintness were
merely due to greater distance, their actual brightness--if of the same
size--would imply that they are at 10 times the distance of stars of the
10th magnitude. But if at the same distance from us, a 10th magnitude star
would be 100 times brighter than a 15th magnitude star, and if of the same
density and "intrinsic brightness" (or luminosity of surface) the 10th
magnitude would have 10 times the diameter of the fainter star, and hence
its volume would be 1000 times greater (10{3}), and this great difference
is not perhaps improbable.
The constitution of the Milky Way is not the same in all its parts. The
bright spot between [Greek: b] and [Greek: g] Cygni is due to relatively
bright stars. Others equally dense but fainter regions in Auriga and
Monoceros are only evident in stars of the 8th and 9th magnitude, and the
light of the well-known luminous spot in "Sobieski's Shield," closely
south of [Greek: l] Aquilæ, is due to stars below magnitude 9-1/2.
The correspondence in distribution between the stars of Argelander's
charts and the fainter stars of the Milky Way shows, as Easton points out,
that Herschel's hypothesis of a uniform distribution of stars of
approximately equal size is quite untenable.
It has been suggested that the Milky Way may perhaps form a ring of stars
with the sun placed nearly, but not exactly, in the centre of the ring.
But were it really a ring of uniform width with the sun eccentrically
placed within it, we should expect to find the Milky Way wider at its
nearest part, and gradually narrowing towards the opposite point. Now,
Herschel's "gages" and Celoria's counts show that the Galaxy is wider in
Aquila than in Monoceros. This is confirmed by Easton, who says, "_for the
faint stars taken as a whole, the Milky Way is widest in its brightest
part_" (the italics are Easton's). From this we should conclude that the
Milky Way is nearer to us in the direction of Aquila than in that of
Monoceros. Sir John Herschel suggested that the southern parts of the
galactic zone are nearer to us on account of their greater _brightness_ in
those regions.[472] But greater width is a safer test of distance than
relative brightness. For it may be easily shown than the _intrinsic_
brightness of an area containing a large number of stars would be the
same for _all_ distances (neglecting the supposed absorption of light in
space). For suppose any given area crowded with stars to be removed to a
greater distance. The light of each star would be diminished inversely as
the square of the distance. But the given area would also be diminished
_directly_ as the square of the distance, so we should have a diminished
amount of light on an equally diminished area, and hence the intrinsic
brightness, or luminosity of the area per unit of surface, would remain
unaltered. The increased brightness of the Milky Way in Aquila is
accounted for by the fact that Herschel's "gages" show an increased number
of stars, and hence the brightness in Aquila and Sagittarius does not
necessarily imply that the Milky Way is nearer to us in those parts, but
that it is richer in small stars than in other regions.
Easton is of opinion that the annular hypothesis of the Milky Way is
inconsistent with our present knowledge of the galactic phenomena, and he
suggests that its actual constitution resembles more that of a spiral
nebula.[473] On this hypothesis the increase in the number of stars in the
regions above referred to may be due to our seeing one branch of the
supposed "two-branched spiral" projected on another branch of the same
spiral. This seems supported by Sir John Herschel's observations in the
southern hemisphere, where he found in some places "a tissue as it were of
large stars spread over another of very small ones, the immediate
magnitudes being wanting." Again, portions of the spiral branches may be
richer than others, as photographs of spiral nebulæ seem to indicate.
Celoria, rejecting the hypothesis of a single ring, suggests the existence
of _two_ galactic rings inclined to each other at an angle of about 20°,
one of these including the brighter stars, and the other the fainter. But
this seems to be a more artificial arrangement then the hypothesis of a
spiral. Further, the complicated structure of the Milky Way cannot be well
explained by Celoria's hypothesis of two distinct rings one inside the
other. From analogy the spiral hypothesis seems much more probable.
Considering the Milky Way to represent a colossal spiral nebula viewed
from a point not far removed from the centre of the spiral branches,
Easton suggests that the bright region between [Greek: b] and [Greek: g]
Cygni, which is very rich in comparatively bright stars, may possibly
represent the "_central accumulations of the Milky Way_," that is, the
portion corresponding to the nucleus of a spiral nebula. If this be so,
this portion of the Milky Way should be nearer to us than others. Easton
also thinks that the so-called "solar cluster" of Gould, Kapteyn, and
Schiaparelli may perhaps be "the expression of the central condensation
of the galactic system itself, composed of the most part of suns
comparable with our own, and which would thus embrace most of the bright
stars to the 9th or 10th magnitude. The distance of the galactic streams
and convolutions would thus be comparable with the distances of these
stars." He thinks that the sun lies within a gigantic spiral, "in a
comparatively sparse region between the central nucleus and Orion."
Scheiner thinks that "the irregularities of the Milky Way, especially in
streams, can be quite well accounted for, as Easton has attempted to do,
if they are regarded as a system of spirals, and not as a ring system."
Evidence in favour of the spiral hypothesis of the Milky Way, as advocated
by Easton and Scheiner, may be found in Kapteyn's researches on the proper
motions of the stars. This eminent astronomer finds that stars with
measurable proper motions--and therefore in all probability relatively
near the earth--have mostly spectra of the solar type, and seem to cluster
round "a point adjacent to the sun, in total disregard to the position of
the Milky Way," and that stars with little or no proper motion collect
round the galactic plain. He is also of opinion that the Milky Way
resembles the Andromeda nebula, "the globular nucleus representing the
solar cluster, and the far spreading wings or whorls the compressed layer
of stars enclosed by the rings of the remote Galaxy."
With reference to the plurality of inhabited worlds, it has been well said
by the ancient writer Metrodorus (third century B.C.), "The idea that
there is but a single world in all infinitude would be as absurd as to
suppose that a vast field had been formed to produce a single blade of
wheat."[474] With this opinion the present writer fully concurs.