STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.
After the death of Herschel there was little done in the direction of
furthering our knowledge of stellar distribution, or the construction of
the heavens. Here, as elsewhere, Herschel’s immediate successor was his
son, whose star-gauges, both in England and in South Africa, were a
worthy sequel to those of his father; but John Herschel, in his books on
astronomy, reproduced his father’s disc-theory, unaware that the elder
Herschel had himself abandoned it. The work of the younger Herschel was
entirely supplementary to that of his father.
To Wilhelm Struve belongs the credit of showing the disc-theory to be
untenable, and of demonstrating that Herschel had abandoned it. This he
was able to do after a perusal of Herschel’s papers, presented to him by
John Herschel. Having demonstrated this, he undertook a series of
investigations which resulted in his famous theory of the Universe. This
was published in his work ‘Études d’Astronomie Stellaire,’ which was
published in 1847. His researches were based on the star-catalogues of
Bessel, Piazzi, and others; and dealing with 52,199 stars, he discussed
the number of stars in each zone of Right Ascension. He found, in the
words of Mr Gore, “that the numbers increase from hour i to hour vi,
where they attain a maximum. They then diminish to a minimum at hour
xiii, and rise to another but smaller maximum at hour xviii, again
decreasing to a second minimum at hour xxii. As the hours vi and xviii
are those crossed by the Milky Way, the result is very significant.” He
concluded the Galaxy to be produced by a collection of
irregularly-condensed clusters, the stars condensed in parallel planes.
Next, he considered the Universe as perhaps infinitely extended in the
direction of the Galaxy, and accordingly he put forward the idea that
the light from the fainter and more distant stars was extinguished in
its passage through the ether of space, which he regarded as imperfectly
transparent. The theory, as Struve propounded it, was disposed of by Sir
John Herschel, who remarked that we were not permitted to believe that
at one part of the sky our view was limited by extinction, while at
another a clear view right through the Galaxy could be had; and by
_Robert Grant_ (1814-1892), director of the Glasgow Observatory, who
showed that, were the theory true, the Galaxy should present a uniform
appearance throughout its course. On the whole, Struve’s theory was no
improvement on Herschel’s; for, as Encke pointed out, Struve’s theory
was built on five assumptions, all of which were questionable.
At the time of Struve’s investigation Mädler, at Dorpat, was engaged in
an attempt to solve the question of the construction of the heavens by
quite another method, that of stellar proper motion. He determined to
investigate the subject of proper motion in order to discover the
central body of the Milky Way. If such a centre existed, however, the
motions near it would be somewhat different from those in the Solar
System. In our Solar System the planets nearest the Sun move swiftest,
owing to the strength of the force of gravitation. In the Sidereal
System, on the other hand, the movements at the centre, as Mädler
pointed out, would be slowest. As there would be no very large
preponderating body, the mutual attractions of the different stars would
cause the bodies at the boundaries of the Universe to move faster than
those at the centre, the central sun—the object of Mädler’s search—being
in a state of rest relative to the Sidereal System. Mädler accordingly
began to search the heavens for a region of sluggish proper motions.
In the constellation Taurus, Mädler noticed that the proper motions of
the stars were very slow. The idea occurred to him that the bright red
star Aldebaran might be the central sun, but its very large proper
motion was obviously against this inference. Star after star was now
subjected by Mädler to the most careful scrutiny. At length, after a
laborious investigation, he announced that the star which fulfilled the
conditions of a central body was Alcyone, the brightest of the Pleiades,
a group possessed of no proper motion except that due to the sun’s drift
in the opposite direction. In 1846 Mädler published his hypothesis in
his elaborate work, ‘The Central Sun.’ He announced that his
observations had led him to the conclusion that Alcyone occupied the
centre of gravity of the Sidereal System, and was the point round which
the stars of the Galaxy were all revolving. His profound imagination,
however, did not stop here. This speculation led him to the sublime
thought that the centre of the Universe was the Abode of the Creator. In
1847 Struve rejected Mädler’s theory as “much too hazardous,” and this
has been the general opinion of astronomers. Mädler’s theory is now
regarded as quite untenable.
Herschel’s earlier idea that the nebulæ were external galaxies was long
held by the majority of astronomers, in preference to his later and more
advanced ideas. The supposed resolution of the nebulæ by Lord Rosse’s
telescope gave support to this external galaxy theory. It was clearly
shown, however, by _William Whewell_ (1794-1866) in 1853, and by
_Herbert Spencer_ (1820-1903) in 1858, that the systematic distribution
of the nebulæ in regard to the stars precluded the possibility of their
being external galaxies. This was confirmed by the spectroscopic
discovery of the gaseous nature of some of the nebulæ, and by the later
researches of R. A. Proctor. Not only did Proctor make fresh
discoveries, but it fell to him to clear away the erroneous ideas
regarding the construction of the heavens, and to put the study on a new
basis. In 1870 Proctor plotted on a single chart all the stars, to the
number of 324,198, contained in Argelander’s ‘Durchmusterung’ charts.
This work gave the death-blow to the “disc-theory.” In his own words,
“In the very regions where the Herschelian gauges showed the minutest
telescopic stars to be most crowded, my chart of 324,198 stars shows the
stars of the higher orders (down to the eleventh magnitude) to be so
crowded, that by their mere aggregation within the mass they show the
Milky Way with all its streams and clusterings. It is utterly impossible
that excessively remote stars could seem to be clustered exactly where
relatively near stars were richly spread.”
Proctor showed also that in all probability the stars composing the
nebulous light of the Galaxy are much smaller than the brighter stars,
and not at such a great distance as their faintness would lead us to
suppose,—a conclusion confirmed by the work of Celoria. Proctor was not
so fortunate in theorising as in direct investigation. He thought that
the Magellanic clouds were probably external galaxies; and further, he
put forward the idea that the Milky Way is a spiral, the gaps and
coal-sacks being due to loops in the stream, but neither of these ideas
has found favour with astronomers. But the chief work accomplished by
Proctor was a revision of our knowledge of the Universe, which he thus
describes: “Within one and the same region coexist stars of many orders
of real magnitude, the greatest being thousands of times larger than the
least. All the nebulæ hitherto discovered, whether gaseous and stellar,
irregular, planetary, ring-formed, or elliptic, exist within the limits
of the Sidereal System.”
Proctor’s discovery of the excess of bright stars on the Galaxy was
confirmed by _Jean Charles Houzeau_ (1820-1888), director of the
Brussels Observatory. Some time later J. E. Gore carefully examined the
positions of all the brighter stars in the northern and southern
hemisphere. Following this, he made an enumeration of the stars in the
atlas of Heis and in the charts constructed by Harding; the outcome of
the investigation being to show that stars of each individual magnitude
taken separately tend to aggregate on the Galaxy, the aggregation being
noticed even in first-magnitude stars. Gore further pointed out many
cases of close connection between the lucid stars and the galactic
light. A similar investigation was undertaken by Schiaparelli in 1889.
Schiaparelli, basing his work on the catalogue of Gould and the
photometric measures of Pickering, constructed a series of planispheres
which demonstrated the crowding of the lucid stars towards the plane of
the Galaxy. These investigations were still further continued by Simon
Newcomb, who demonstrated that “the darker regions of the Galaxy are
only slightly richer in stars visible to the naked eye than other parts
of the heavens, while the bright areas are between 60 and 100 per cent
richer than the dark areas.” The Dutch astronomer, _Charles Easton_,
finds a connection between the distribution of ninth-magnitude stars and
the luminous and obscure spots in the Galaxy.
It was noticed by Gould, from observations made at Cordova, that “a belt
or stream of bright stars appears to girdle the heavens very nearly in a
great circle which intersects the Milky Way.” According to Gould, the
belt includes Orion, Canis Major, Argo, Crux, Centaurus, Lupus, and
Scorpio in the southern hemisphere, and Taurus, Perseus, Cassiopeia,
Cepheus, Cygnus, and Lyra in the northern. This was interpreted by
Celoria as indicating the existence of two galactic rings, but Gould
considered the zone of bright stars to form with the Sun a subordinate
cluster of about five hundred stars within the Galaxy.
Perhaps the most elaborate investigations on the structure of the
Universe have been those of Kapteyn, commenced in 1891. In that year he
demonstrated that stars are bluer and more easily photographed in the
Galaxy than elsewhere, a discovery independently made by Gill at the
Cape, and Pickering at Harvard. In 1893 Kapteyn announced his
conclusions, derived from a novel method of studying the distance of the
stars from their proper motions. In order to reach a definite idea of
the distances of the stars, he made use of the component of the proper
motion, measured at right angles to a great circle of the sphere which
passes through a given star and the apex of the solar motion. He found
that stars of the first spectral type have smaller proper motions than
those of the second, indicating that stars of the second type are on the
average nearer to the Solar System than those of the first, the near
vicinity containing almost exclusively second-type stars. Kapteyn
concluded that the group of second-type stars formed one system, named
the solar cluster, which he considered to be roughly spherical in shape.
In 1902 he abandoned this idea, retaining, however, his opinions as to
the relative distances of the different types. That the second-type
stars are nearer to the Sun than the first is, he remarked in a letter
to the writer, incontrovertible.
In the investigation of the motions in, and extent of, the Universe, the
name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace,
in Nova Scotia, he went to the States in 1853. In 1862 he received an
appointment at Washington Observatory, and he retained an official
position until 1897. Throughout his scientific career he has been
specially attracted by the question of the construction of the heavens,
which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s
investigations have shown that some of the stars are not permanent
members of the Sidereal System, among them the swiftly-moving 1830
Groombridge. He has shown that the Stellar Universe does not possess
that form of stability which is seen in the Solar System. Newcomb
considers the Universe to be limited in extent, as opposed to the
opinions of Struve and others, who believed it to be infinite. He has
brought clearly before his readers a calculation, based on the known law
that there are three times as many stars of any given magnitude as of
that immediately brighter, the increase of number compensating for the
decrease of brilliance. Were the Universe infinitely extended, the whole
heavens would shine with the brilliance of the Sun. Newcomb, therefore,
concludes that “that collection of stars which we call the Universe is
limited in extent.”
Positive evidence that this is the case was obtained by Giovanni
Celoria, now director of the Milan Observatory, in the course of a
series of star-gauges at the north galactic pole. Using a small
refractor, showing stars barely to the eleventh magnitude, he found he
could see exactly the same number of stars as Herschel’s large
reflector, indicating that increase of optical power will not increase
the number of stars visible in that direction. Celoria’s observation can
only be explained on the assumption that the Universe is limited in
extent, as otherwise Herschel’s telescope should have shown more stars
than Celoria’s, even granting an extinction of light,—a theory which
Newcomb, Schiaparelli, and others have shown to be quite untenable. That
the Universe is limited in extent is about all that is known for
certain, although even this has been called in question, notably by E.
W. Maunder and H. H. Turner. The problem of the construction of the
heavens is by no means solved, although several more or less probable
theories have been advanced.
A series of investigations on stellar distribution, from 1884 to 1898,
led Hugo Seeliger, director of the Munich Observatory, to some
remarkable deductions. He believes the Universe to be flattened at the
galactic poles. The Galaxy is the zone of stellar condensation, and he
concludes the distance of the Solar System from the inner border of the
zone to be 500 times the distance of Sirius, while the external border
is 1100 times that distance. The Universe is finite in extent, its
limits being about 9000 light years from the Solar System. In Seeliger’s
opinion the extinction of light may come into play beyond our Universe,
and prevent us seeing other collections of stars.
The question of external universes is purely a hypothetical one,
although there is undoubtedly much to be said in its favour. These
universes have never been seen, and we can only speculate as to their
existence. The last word on the subject is by Gore, in 1893, in his
elaborate work, ‘The Visible Universe.’ He regards the Solar System as a
system of the first order, and the Galaxy and its fellow-universes of
the second. He makes a calculation of the possible distance of an
external universe of his second order. He assumes the distance of the
nearest universe from our Galaxy as proportional to that separating the
Sun from α Centauri, and reaches the amazing conclusion that the
distance of the nearest Galaxy is no less than
520,149,600,000,000,000,000 miles,—a distance which light, with its
inconceivable velocity of 186,000 miles a second, would take almost
ninety millions of years to traverse.
These calculations absolutely overwhelm the mind, which is unable to
comprehend such vast distances. Our universe is indeed, as Flammarion
expresses it, a point in the infinite. The calculations of J. E. Gore
represent our highest scientific conception of the universe. He sums up
his investigations with the following words: “Although we must consider
the number of _visible_ stars as strictly finite, the numbers of stars
and systems really existing, but invisible to us, may be practically
infinite. Could we speed our flight through space on angel wings beyond
the confines of our limited universe to a distance so great that the
interval which separates us from the remotest fixed star might be
considered as merely a step on our celestial journey, what further
creations might not then be revealed to our wondering vision? Systems of
a higher order might there be unfolded to our view, compared with which
the whole of our visible heavens might appear like a grain of sand on
the ocean shore,—systems perhaps stretching out to infinity before us,
and reaching at last the glorious ‘mansions’ of the Almighty, the Throne
of the Eternal.”