are fewest in regions the most distant from the galactic belt; while
from these regions, which correspond to the pole of the Galaxy, they
gradually increase in number in approaching the Milky-way. The star
density was found to be extremely variable, and while some of the
telescopic gaugings detected either no star at all, or only one or two,
other gaugings gave 500 stars and even more. The average number of stars
in a field of view of his telescope, obtained for the six zones, each of
15°, into which Herschel divided up the portion of his observing belt,
extending from the Galaxy to its pole, is as follows: In the first zone,
commencing at 90° from the galactic belt and extending towards it, 4
stars per telescopic field were found; 5 in the second; 8 in the third;
14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
in the Galaxy itself. Very nearly similar results were afterwards found
by Sir John Herschel, for corresponding regions in the southern
hemisphere.
From these studies, Herschel concluded that the stellar system is of the
general form supposed by the Kantian theory, and that its diameter must
be five times as extended in the direction of the galactic plane, as it
is in a direction perpendicular to it. To explain the great branch sent
out by the Galaxy in Cygnus, he supposed a great cleft dividing the
system edgewise, about half way from its circumference to its centre.
From suppositions founded on the apparent magnitude and arrangement of
stars, he estimated that it would take light about 7,000 years to reach
us from the extremities of the Galaxy, and therefore 14,000 years to
travel across the system, from one border to the opposite one.
But Herschel's theory concerning the Milky-way rested on the erroneous
assumption that the stars are uniformly distributed in space, and also
that his telescopes penetrated through the entire depth of the Galaxy.
Further study showed him that his telescope of twenty feet, and even his
great forty-foot telescope, which was estimated to penetrate to a
distance 2,300 times that of stars of the first magnitude, failed to
resolve some parts of the Galaxy into stars. Meanwhile, the structure of
the Milky-way being better known, the irregular condensation of its
stars became apparent, while the mutual relation existing between binary
and multiple systems of stars, as also between the stars which form
clusters, was recognized, as showing evidence of closer association
between certain groups of stars than between the stars in general.
Herschel's system, which rested on the assumption of the uniform
distribution of the stars in space, and on the supposition that the
telescopes used for his gauges penetrated through the greater depths of
the Galaxy, being thus found to contradict the facts, was gradually
abandoned by its author, who adopted another method of estimating the
relative distances of the stars observed in his gaugings.
This method, founded on photometric principles, consisted in judging the
penetrating power of his telescope by the brightness of the stars, and
not, as formerly, by the number which they brought into view. He then
studied by this new method the structure of the Milky-way and the
probable distance of the clustering masses of which it is formed,
concluding that the portion of the Galaxy traversing the constellation
Orion is the nearest to us. This last result seems indicated by the fact
that this portion of the Milky-way is the faintest and the most uniform
of all the galactic belt.
More recently Otto Struve investigated the same subject, and arrived at
very nearly similar conclusions, which may be briefly stated as follows:
The galactic system is composed of a countless number of stars,
spreading out on all sides along a very extended plane. These stars,
which are very unevenly distributed, show a decided tendency to cluster
together into individual groups of different sizes and forms, separated
by comparatively vacant spaces. This layer where the stars congregate in
such vast numbers may be conceived as a very irregular flat disk,
sending many branches in various directions, and having a diameter eight
or ten times its thickness. The size of this starry disk cannot be
determined, since it is unfathomable in some directions, even when
examined with the largest telescopes. The Sun, with its attending
planets, is involved in this immense congregation of suns, of which it
forms but a small particle, occupying a position at some distance from
the principal plane of the Galaxy. According to Struve, this distance is
approximately equal to 208,000 times the radius of the Earth's orbit.
The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
of the whole number visible in the heavens being involved in this great
belt. In conclusion, our Sun is only one of the individual stars which
constitute the galactic system, and each of these stars itself is a sun
similar to our Sun. These individual suns are not independent, but are
associated in groups varying in number from a few to several thousands,
the Galaxy itself being nothing but an immense aggregation of such
clusters, whose whole number of individual suns probably ranges between
thirty and fifty millions. In this vast system our globe is so
insignificant that it cannot even be regarded as one of its members.
According to Dr. Gould, there are reasons to believe that our Sun is a
member of a small, flattened, bifid cluster, composed of more than 400
stars, ranging between the first and seventh magnitude, its position in
this small system being eccentric, but not very far from the galactic
plane.
The study of the Milky-way, of which Plate XIII. is only a part, was
undertaken to answer a friendly appeal made by Mr. A. Marth, in the
Monthly Notices of the Royal Astronomical Society, in 1872. I take
pleasure in offering him my thanks for the suggestion, and for the
facility afforded me in this study by his "List of Co-ordinates of Stars
within and near the Milky-way," which was published with it.
THE STAR-CLUSTERS
PLATE XIV
It is a well-known fact that the stars visible to the naked eye are very
unequally distributed in the heavens, and that while they are loosely
scattered in some regions, in others they are comparatively numerous,
sometimes forming groups in which they appear quite close together.
In our northern sky are found a few such agglomerations of stars, which
are familiar objects to all observers of celestial objects. In the
constellation Coma Berenices, the stars are small, but quite condensed,
and form a loosely scattered, faint group. In Taurus, the Hyades and the
Pleiades, visible during our winter nights, are conspicuous and familiar
objects which cannot fail to be recognized. In the last group, six stars
may be easily detected by ordinary eyes on any clear night, but more can
sometimes be seen; on rare occasions, when the sky was especially
favorable, I have detected eleven clearly and suspected several others.
The six stars ordinarily visible, are in order of decreasing brightness,
as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
of Celano and Pleione are sometimes obtained.
When the sky is examined with some attention on any clear, moonless
night, small, hazy, luminous patches, having a cometary aspect, are
visible here and there to the naked eye. In the constellation Cancer is
found one of the most conspicuous, called Præsepe, which forms a small
triangle with the two stars _γ_ and _δ_. In Perseus, and involved in
the Milky-way, is found another luminous cloud, situated in the
sword-handle, and almost in a line with the two stars _γ_ and _δ_ of
Cassiopeia's Chair. In the constellation Hercules, another nebulous mass
of light, but fainter, is also visible between the stars _η_ and _ζ_
where it appears as a faint comet, in the depths of space. In Ophiuchus
and Monoceros are likewise found hazy, luminous patches. In the southern
sky, several such objects are also visible to the naked eye, being found
in Sagittarius, in Canis Major and in Puppis; but the most conspicuous
are those in Centaurus and Toucan. That in Centaurus involves the star
_ω_ in its pale diffused nebulosity, and that in Toucan is involved in
the lesser Magellanic cloud.
When the telescope is directed to these nebulous objects, their hazy,
ill-defined aspect disappears, and they are found to consist of
individual stars of different magnitudes, which being more or less
closely grouped together, apparently form a system of their own. These
groups, which are so well adapted to give us an insight into the
structure and the vastness of the stellar universe, are called
_Star-clusters_.
Star-clusters are found of all degrees of aggregation, and while in some
of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
are so loosely scattered that an opera glass, and even the naked eye,
will resolve them; in others, such as in those situated in Hercules,
Aquarius, Toucan and Centaurus, they are so greatly compressed that even
in the largest telescopes they appear as a confused mass of blazing
dust, in which comparatively few individual stars can be distinctly
recognized. Although only about a dozen Star-clusters can be seen in the
sky with the naked eye, yet nearly eleven hundred such objects visible
through the telescope, have been catalogued by astronomers.
The stars composing the different clusters visible in the heavens vary
greatly in number, and while in some clusters there are only a few, in
others they are so numerous and crowded that it would be idle to try to
count them, their number amounting to several thousands. It has been
calculated by Herschel that some clusters are so closely condensed, that
in an area not more than ⅒ part of that covered by the Moon, at least
5,000 stars are agglomerated.
When the group in the Pleiades is seen through the telescope it appears
more important than it does to the naked eye, and several hundreds of
stars are found in it. In a study of Tempel's nebula, which is involved
in the Pleiades, I have mapped out 250 stars, mostly comprised within
this nebula, with the telescope of 6⅓ inches aperture, which I have
used for this study.
As a type of a loose, coarse cluster, that in Perseus is one of the
finest of its class. It appears to the naked eye as a single object, but
in the telescope it has two centres of condensation, around which
cluster a great number of bright stars, forming various curves and
festoons of great beauty. Among its components are found several yellow
and red stars, which give a most beautiful contrast of colors in this
gorgeous and sparkling region. In a study which I have made of this twin
cluster, I have mapped out 664 stars belonging to it, among which are
two yellow and five red stars.
While some clusters, like those just described, are very easily
resolvable into stars with the smallest instruments, others yield with
the greatest difficulty, even to the largest telescopes, in which their
starry nature is barely suspected. Owing to this peculiarity,
star-clusters are usually divided into two principal classes. In the
first class are comprised all the clusters which have been plainly
resolved into stars, and in the second all those which, although not
plainly resolvable with the largest instruments now at our disposal,
show a decided tendency to resolvability, and convey the impression that
an increase of power in telescopes is the only thing needed to resolve
them into stars. Of course this classification, which depends on the
power of telescopes to decide the nature of these objects, is arbitrary,
and a classification based on spectrum analysis is now substituted for
it.
The star-clusters are also divided into globular and irregular clusters,
according to their general form and appearance. The globular clusters,
which are the most numerous, are usually well-defined objects, more or
less circular in their general outlines. The rapid increase of
brightness towards their centres, where the stars composing them are
greatly condensed, readily conveys the impression that the general form
of these sparkling masses is globular. The irregular clusters are not so
rich in stars as the former. Usually their stars are less condensed
towards the centre, and are, for the most part, so loosely and
irregularly distributed, that it is impossible to recognize the outlines
of these clusters or to decide where they terminate. The globular
clusters are usually quite easily resolvable into stars, either partly
or wholly, although some among them do not show the least traces of
resolvability, even in the largest instruments. This may result from
different causes, and may be attributed either to the minuteness of
their components or to their great distance from the Earth, many
star-clusters being at such immense distances that they are beyond our
means of measurement.
As has been shown in the preceding section, the star-clusters are found
in great number in the Galaxy; indeed, it is in this region and in its
vicinity that the greater portion of them are found. In other regions,
with the exception of the Magellanic clouds, where they are found in
great number and in every stage of resolution, the clusters are few and
scattered.
[PLATE XIV.--STAR-CLUSTER IN HERCULES.
From a study made in June, 1877]
The star-cluster in the constellation Hercules, designated as No. 4,230
in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
is one of the brightest and most condensed in the northern hemisphere,
although it is not so extended as several others, its angular diameter
being only 7' or 8'. This object, which was discovered by Halley in
1714, is one of the most beautiful of its class in the heavens.
According to Herschel, it is composed of thousands of stars between the
tenth and fifteenth magnitudes. Undoubtedly the stars composing this
group are very numerous, although those which can be distinctly seen as
individual stars, and whose position can be determined, are not so many
as a superficial look at the object would lead us to suppose. From a
long study of this cluster, which I have made with instruments of
various apertures, I have not been able to identify more stars than are
represented on the plate, although the nebulosity of which this object
mainly consists, and especially the region situated towards its centre,
appeared at times granular and blazing with countless points of light,
too faint and too flickering to be individually recognized. Towards its
centre there is quite an extended region, whose luminous intensity is
very great, and which irresistibly conveys the impression of the
globular structure of this cluster. Besides several outlying appendages,
formed by its nebulosity, the larger stars recognized in this cluster
are scattered and distributed in such a way that they form various
branches, corresponding with those formed by the irresolvable
nebulosity. At least six or seven of these branches and wings are
recognized, some of which are curved and bent in various ways, thus
giving this object a distant resemblance to some crustacean forms.
Although I have looked for it with care, I have failed to recognize the
spiral structure attributed to this object by several observers. Among
the six appendages which I have recognized, some are slightly curved;
but their curves are sometimes in opposite directions, and two branches
of the upper portion make so short a bend that they resemble a claw
rather than a spiral wing. The spectrum of this cluster, like that of
many objects of its class, is continuous, with the red end deficient.
A little to the north-east of this object is found the cluster No.
4,294, which, although smaller and less bright than the preceding, is
still quite interesting. It appears as a distinctly globular cluster
without wings, and much condensed towards its centre. The stars
individually recognized in it, although less bright than those of the
other cluster, are so very curiously distributed in curved lines that
they give a peculiar appearance to this condensed region.
A little to the north of _γ_ Centauri may be found the great _ω_
Centauri cluster, No. 3,531, already referred to above. This magnificent
object, which appears as a blazing globe 20' in diameter, is, according
to Herschel, the richest in the sky, and is resolved into a countless
number of stars from the twelfth to the fifteenth magnitude, which are
greatly compressed towards the centre. The larger stars are so arranged
as to form a sort of net-work, with two dark spaces in the middle.
The great globular cluster No. 52, involved in the lesser Magellanic
cloud, in the constellation Toucan, is a beautiful and remarkable
object. It is composed of three distinct, concentric layers of stars,
varying in brightness and in degree of condensation in each layer. The
central mass, which is the largest and most brilliant, is composed of an
immense number of stars greatly compressed, whose reddish color gives to
this blazing circle a splendid appearance. Around the sparkling centre
is a broad circle, composed of less compressed stars, this circle being
itself involved in another circular layer, where the stars are fainter
and more scattered and gradually fade away.
Many other great globular clusters are found in various parts of the
heavens, among which may be mentioned the cluster No. 4,678, in
Aquarius. This object is composed of several thousand stars of the
fifteenth magnitude, greatly condensed towards the centre, and, as
remarked by Sir J. Herschel, since the brightness of this cluster does
not exceed that of a star of the sixth magnitude, it follows that in
this case several thousand stars of the fifteenth magnitude equal only a
star of the sixth magnitude. In the constellation Serpens the globular
clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
of the most remarkable of this region. The stars composing it, which are
quite large and easily made out separately, form various figures, in
which the square predominates.
Among the loose irregular clusters, some are very remarkable for the
curious arrangement of their stars. In the constellation Gemini the
cluster No. 1,360, which is visible to the naked eye, is a magnificent
object seen through the telescope, in which its sparkling stars form
curves and festoons of great elegance. The cluster No. 1,467, of the
same constellation, is remarkable for its triangular form. In the
constellation Ara the cluster No. 4,233, composed of loosely scattered
stars, forming various lines and curves, is enclosed on three sides by
nearly straight single lines of stars. In Scorpio the cluster No. 4,224
is still more curious, being composed of a continuous ring of loosely
scattered stars, inside of which is a round, loose cluster, which is
divided into four parts by a dark cross-shaped gap, in which no stars
are visible.
Among the 1,034 objects which are now classified as clusters more or
less resolvable, 565 have been absolutely resolved into stars, and 469
have been only partly resolved, but are considered as belonging to this
class of objects. In Sir J. Herschel's catalogue there are 102 clusters
which are Considered as being globular; among them 30 have been
positively resolved into stars.
The agglomeration of thousands of stars into a globular cluster cannot
be conceived, of course, to be simply the result of chance. This
globular form seems clearly to indicate the existence of some bond of
union, some general attractive force acting between the different
members of these systems, which keeps them together, and condenses them
towards the centre. Herschel regards the loose, irregular clusters as
systems in a less advanced stage of condensation, but gradually
concentrating by their mutual attraction into the globular form.
Although the stars of some globular clusters appear very close together,
they are not necessarily so, and may be separated by great intervals of
space. It has been shown that the clusters are agglomeration of suns,
and that our Sun itself is a member of a cluster composed of several
hundreds of suns, although, from our point of observation, these do not
seem very close together. So far as known, the nearest star to us is a
Centauri, but its distance from the Earth equals 221,000 times the
distance of the Sun from our globe, a distance which cannot be traversed
by light in less than three years and five months. It seems very
probable that if the suns composing the globular clusters appear so near
together, it is because, in the first place, they are at immense
distances from us, and in the second, because they appear nearly in a
line with other suns, which are at a still greater distance from us, and
on which they accordingly are nearly projected. If one should imagine
himself placed at the centre of the cluster in Hercules, for instance,
the stars, which from our Earth seems to be so closely grouped, would
then quite likely appear very loosely scattered around him in the sky,
and would resemble the fixed stars as seen from our terrestrial station.
Judging by their loose and irregular distribution, the easily resolvable
clusters would appear, in general, to be the nearer to us. It is
probable that the globular clusters do not possess, to a very great
degree, the regular form which they ordinarily present to us. It seems
rather more natural to infer that they are irregular, and composed of
many wings and branches, such as are observed in the cluster in
Hercules; but as these appendages would necessarily be much poorer in
stars than the central portions, they would be likely to become
invisible at a great distance, and therefore the object would appear
more or less globular; the globular form being simply given by the close
grouping of the stars in the central portion. It would seem, then, that
in general, the most loosely scattered and irregular clusters are the
nearest to us, while the smallest globular clusters and those resolvable
with most difficulty are the most distant.
In accordance with the theory that the clusters are composed of stars,
the spectrum of these objects is in general continuous; although, in
many cases, the red end of the spectrum is either very faint or
altogether wanting. Many objects presenting in a very high degree the
principal characteristics exhibited by the true star-clusters, namely, a
circular or oval mass, whose luminous intensity is greatly condensed
toward the centre, have not yielded, however, to the resolving power of
the largest telescopes, although their continuous spectrum is in close
agreement with their general resemblance to the star-clusters. Although
such objects may remain irresolvable forever, yet it is highly probable
that they do not materially differ from the resolvable and partly
resolvable clusters, except by their enormous distance from us, which
probably reaches the extreme boundary of our visible universe.
THE NEBULÆ
PLATE XV
Besides the foggy, luminous patches which have just been described, a
few hazy spots of a different kind are also visible to the naked eye on
any clear, moonless night. These objects mainly differ from the former
in this particular, that when viewed through the largest telescopes in
existence they are not resolved into stars, but still retain the same
cloudy appearance which they present to the unassisted eye. On account
of the misty and vaporous appearance which they exhibit, these objects
have been called _Nebulæ_.
Of the 26 nebulous objects visible to the naked eye in the whole
heavens, 19 belong to the class of star-clusters, and 7 to the class of
nebulæ. Among the most conspicuous nebulæ visible to the unassisted
eye, are those in the constellations Argo Navis, Andromeda and Orion.