From spectrum analysis it seems to be established that most of them
contain sodium and magnesium, while a few indicate the presence of
strontium and iron, and in some rare cases there are traces of coal-gas.
Some of the nuclei give a continuous spectrum, and others a spectrum of
lines. The trail always gives a spectrum of bright lines which indicates
its gaseous state. The traces of coal-gas rarely seen in meteors are,
however, of great importance, as it identifies them more closely with
the comets, which generally show a similar spectrum. The continuous
spectra exhibited by some nuclei would indicate that they are
incandescent and either solid or liquid; but it is difficult to conclude
from their spectra what is their true nature, since we do not know
exactly what part the terrestrial atmosphere may play in producing the
results.
The mass of the shooting-stars is not known with certainty, but the fact
that during great meteoric showers, none are seen to reach the surface
of the Earth, all being consumed in a few seconds, sufficiently
indicates that it must be very small. It has been calculated that those
equal to Venus in apparent size and brilliancy may weigh several pounds,
while the faint ones would weigh only a few grains.
If the shooting-stars have even such a mass as that here attributed to
some of them, the extraordinary motions which I have described above
seem to be unaccountable. The change of direction of a heavy mass moving
swiftly cannot be sudden. The semi-circular, the wavy and the angular
orbits observed could not be described, it would seem, by such a mass
animated with a great velocity. Although the meteors are said to be
ignited by the transformation of part of their progressive motion into
molecular motion, yet it is not observed that the velocity of the
falling stars diminishes when they are about to disappear. The luminous
trails they leave in the atmosphere do not appear to be endowed with any
motion, but remain for a time in their original positions. These facts
are apparently opposed to the hypothesis that such meteors have any
appreciable mass. The extraordinary motions exhibited by some meteors
seem to indicate that some unsuspected force resides in these bodies,
and causes them to deviate from the laws of ordinary motion.
Although it is very probable that the ordinary shooting-stars have no
appreciable mass, yet it is known that very heavy meteoric masses
sometimes fall at the surface of the Earth. Such falls are generally
preceded by the sudden apparition in the sky of a large, and usually
very brilliant fire-ball, which traverses the air at a great speed,
sometimes leaving behind it a luminous trail, after which it explodes
with a loud sound, and heavy fiery meteoric fragments, diverging in all
directions, fall at the surface of the Earth. The name of _Aerolites_ or
_Meteorolites_ is given to these ponderous fragments. As these meteors,
before they explode and fall to the ground, have many points of
resemblance with the shooting-stars, they are generally supposed to be
connected with them, and to have a similar cometary origin. The fact
that the aerolites differ widely from each other in constitution, and
are all composed of substances found on the Earth, associated with other
facts given below, would rather seem to indicate a terrestrial than a
celestial origin.
If the aerolites belong to the same class of bodies as the falling
stars, differing from them only in size and mass, it is difficult to see
why so very few should fall upon the Earth during the great meteoric
showers, when thousands of shooting-stars traverse our atmosphere. In
Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
falls of aerolites and fire-balls which have been observed at the time
of the periodic meteoric showers of the 10th of August and the 13th of
November, during a period of 221 years for the Perseids, or August
showers, and of 318 years for the Leonids, or November showers. During
221 years, 10 falls of aerolites have been witnessed simultaneously with
the fall of the Perseids; while during 318 years, only 4 such falls have
been recorded as having occurred at the time of the Leonid shower. If
there is any close connection between the shooting-stars and the
aerolites, we should expect to find a maximum in their fall at the time
of the great meteoric displays. So far, no maxima or minima have yet
been discovered in the fall of aerolites; they do not seem, like
meteoric showers, to be governed by a law of periodicity.
A very remarkable peculiarity of the aerolites is that they seem to have
a tendency to fall in certain regions. Such are the southern part of
France, the north of Italy, Hindostan, the central states of North
America, and Mexico and Brazil. There is a curious contrast existing
between the quick cometary motion of the aerolites before their
explosion, and the comparatively slow motion of their fragments as they
reach the Earth; motion which seems to be no greater than that
corresponding to their natural fall impeded by the resistance of the
air. In general, their penetration into the soil upon which they fall
does not at all correspond to the great velocity with which they move in
the atmosphere. The fragmentary structures of the aerolites, their
identity of substance with that of our globe, their great resemblance to
the volcanic minerals of the Earth, and the fractures and faults which
some of them exhibit, do not correspond at all with the idea that they
are cometary particles fallen on the Earth. As far as their structure
and appearance is concerned, they seem rather to be a volcanic product
of the interior of the Earth than parts of disintegrated comets. It must
be admitted that their identity with the shooting-stars is far from
established, and that they are still involved in mystery.
The so-called meteoric dust gathered at sea and on high mountains may
have various origins, and may be partly furnished by volcanic dust
carried to great distances in the atmosphere.
Since millions of shooting-stars penetrate our atmosphere every year and
remain in it, becoming definitively a part of the Earth, it follows
that, no matter how small may be the quantity of matter of which they
are composed, they must gradually increase the volume and mass of our
globe, although the increase may be exceedingly slow. Supposing every
one of the shooting-stars penetrating our atmosphere to contain one
cubic millimeter of matter, it has been calculated that it would take
nearly 35,000 years to make a deposit one centimeter in thickness all
over the surface of our globe. Insignificant as this may appear, it is
probable that the quantity of matter of meteoric origin which is added
to our globe is much less than has just been supposed.
THE MILKY-WAY OR GALAXY
PLATE XIII
During clear nights, when the Moon is below the horizon, the starry
vault is greatly adorned by an immense belt of soft white light,
spanning the heavens from one point of the horizon to the opposite
point, and girdling the celestial sphere in its delicate folds. Every
one is familiar with this remarkable celestial object, called the
_Milky-way_ or _Galaxy_.
Seen with the naked eye, the Galaxy appears as an irregular, narrow,
nebulous belt, apparently composed of cloud-like luminous masses of
different forms and sizes, separated by comparatively dark intervals.
These cloud-like masses vary much in luminous intensity, and while some
among them are very bright and conspicuous, others are so faint that
they are hard to recognize. In general, the brightest parts of the
Milky-way are situated along the middle of its belt, while its borders,
which are usually very faint, gradually vanish in the sky. Some parts of
the Galaxy, however, show very little of the cloudy structure so
characteristic of other parts, being almost uniform throughout, except
towards the borders, which are always fainter. These parts showing
greater uniformity are also the faintest.
Such is the general appearance of the Milky-way on ordinary nights, but
on rare occasions, when the atmosphere is particularly pure, it presents
one of the grandest sights that can be imagined. At such favorable
moments I have seen the Galaxy gleaming with light, and appearing as if
composed of star-dust or of precious stones. The strange belt then
appeared all mottled over and fleecy, its large cloud-like masses being
subdivided into numerous small, irregular cloudlets of great brilliancy,
which appeared projected upon a soft luminous background.
The width of the Galaxy is far from being uniform; while in some places
it is only 4° or 5°, in others it is 15° and even more. In some
places it appears wavy in outline, at others quite straight; then it
contracts, to expand a few degrees distant; while at other places it
sends off branches and loops, varying in form, size and direction, some
of which are quite prominent, while others are very faint.
Although very irregular in form, the general appearance of the galactic
belt is that of a regular curve occupying one of the great circles of
the celestial sphere. The Milky-way completely encircles the heavens,
but, of course, only one-half is visible at any one moment, since our
globe prevents the other half from being seen. If, for a moment, we
imagine ourselves left in space, our globe having vanished from under
our feet, we should then see the whole Galaxy forming a continuous belt
in the heavens, at the centre of which we should apparently be situated.
While only one-half of the galactic belt can be seen at once from any
point on the Earth, yet, according to the position of the observer, a
larger or smaller portion of the whole can be seen at different times.
In high northern or southern latitudes but little more than half can be
seen even by continuous observations; but as we approach the equatorial
regions, more and more of it becomes visible, until the whole may be
seen at different hours and seasons. In the latitudes of the northern
states, about two-thirds of the Galaxy is visible, the rest remaining
hidden below the horizon; but from the southern states very nearly the
whole can be seen. The half of the Milky-way visible at any one time
from any latitude on the Earth never entirely sets below the horizon,
although in some places it may be so near the horizon as to be rendered
invisible by vapors. In the latitude of Cambridge, when in its lowest
position, the summit of its arc is still about 12° or 15° above the
northern horizon. The great circle of the celestial sphere, occupied by
the galactic belt, is inclined at an angle of about 63° to the
celestial equator, and intersects this great circle on one side in the
constellation Monoceros in 6h. 47m., and on the opposite side in the
constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
that its northern pole is situated in the constellation Coma Berenices
in R. A. 12h. 47m., declination N. 27°, and the southern in the
constellation Cetus in R. A. 0h. 47m., declination S. 27°.
According to the seasons and to the hours of the night at which it is
observed, the galactic arch presents different inclinations in the sky.
Owing to its inclination to the equator of the celestial sphere, its
opposite parts exhibit opposite inclinations when they pass the meridian
of a place. That part of the Galaxy which is represented on Plate XIII.,
and which intersects the celestial equator in the constellation Aquila,
is inclined to the left or towards the east, when it is on the meridian;
while the opposite part, situated in Monoceros, is inclined to the
right, or towards the west, when it reaches the meridian. The former
passes the meridian in the evening in the summer and autumn months; the
latter, in the winter and spring months.
[PLATE XIII.--PART OF THE MILKY WAY.
From a study made during the years 1874, 1875 and 1876]
By beginning at its northernmost part, represented at the upper part of
Plate XIII., situated in "the chair" of the constellation Cassiopeia,
and descending southwardly, and continuing in the same direction until
the whole circle is completed, the course of the Milky-way through the
constellations may be briefly described as follows: From Cassiopeia's
chair, the Galaxy, forming two streams, descends south, passing partly
through Lacerta on the left, and Cepheus on the right; at this last
point it approaches nearest to the polar star. Then it enters Cygnus,
where it becomes very complicated and bright, and where several large
cloudy masses are seen terminating its left branch, which passes to the
right, near the bright star Deneb, the leader of this constellation.
Below Deneb, the Galaxy is apparently disconnected and separated from
the northern part by a narrow, irregular dark gap. From this rupture,
the Milky-way divides into two great streams separated by an irregular
dark rift. An immense branch extends to the right, which, after having
formed an important luminous mass between the stars _γ_ and _β_,
continues its southward progress through parts of Lyra, Vulpecula,
Hercules, Aquila and Ophiuchus, where it gradually terminates a few
degrees south of the equator. The main stream on the left, after having
formed a bright mass around _ε_ Cygni, passes through Vulpecula and
then Aquila, where it crosses the equinoctial just below the star _η_
after having involved in its nebulosity the bright star Altair, the
leader of Aquila. In the southern hemisphere the Galaxy becomes very
complicated and forms a succession of very bright, irregular masses, the
upper one being in Scutum Sobieskii, while the others are respectively
situated in Sagittarius and in Scorpio; the last, just a little above
our horizon, being always considerably dimmed by vapors. From Scutum
Sobieskii, the Galaxy expands considerably on the right, and sends a
branch into Scorpio, in which the fiery red star Antares is somewhat
involved.
Continuing its course below our horizon, the Milky-way enters Ara and
Norma, and then, passing partly through Circinus, Centaurus and Musca,
it reaches the Southern Cross, after having been divided by the large
dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
Crux the Milky-way attains its maximum of brightness, which there
surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
approach to the south pole of the heavens. It then enters Carina and
Vela, where it spreads out like a fan, and terminates in this last
constellation, before reaching _λ_, being once more interrupted by a
dark and very irregular gap, on a line with the two stars _γ_ and _λ_.
It is noteworthy that this second rupture of continuity of the Galaxy in
Vela is very nearly opposite, or at about 180° from the break near
Deneb in Cygnus.
Continuing its course on the other side of the break, the Milky-way
again spreads out into the shape of a fan, grows narrower in entering
Puppis, where it is longitudinally divided by darkish channels. It then
passes above our southern horizon, becoming visible to us, passing
through part of Canis Major, where its border just grazes the brilliant
star Sirius. But from Puppis it gradually diminishes in brightness and
complication, becoming faint and uniform. It enters Monoceros and Orion,
where it again crosses the equator a little above _δ_, the northernmost
of the three bright stars in the belt of Orion. Continuing its northward
course it passes through Gemini, extending as far as Castor and Pollux,
and then entering Auriga, where it begins to increase in brightness and
in complication of structure. It passes partly through Camelopardus and
into Perseus, where an important branch proceeds from its southern
border.
This branch beginning near the star _θ_, advances towards the
celebrated variable star Algol, around which it is quite bright and
complicated. Continuing its course in the same direction, the branch
rapidly loses its brightness, becoming very faint a little below Algol,
and passing through _ζ_ Persei, it enters Taurus, leaving the Pleiades
on its extreme southern margin; and after having passed through _ε_
where it branches off, it rapidly curves towards the main stream, which
it joins near _ζ_ Tauri, thus forming an immense loop. The ramification
projecting near _ε_ Tauri involves in its nebulosity the ruddy star
Aldebaran and the scattered group of the Hyades. It then advances
towards the three bright stars _δ_, _ε_ and _ζ_ of the belt of Orion,
which, together with the sextuple star _θ_ Orionis, are involved in its
faint nebulosity, and joins the main stream on the equinoctial, having
thus formed a second loop, whose interior part is comparatively free
from nebulosity, and contains the fine stars Betelgeuse and Bellatrix.
That portion of the main galactic stream which is comprised between the
star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
by a very irregular, narrow, darkish cleft, comparatively devoid of
nebulosity, which, however, is interrupted at some points. This dark gap
sends short branches north and south, the most important of which are
situated near _ζ_ Cephei and _β_ Cassiopeiæ. Another branch runs from
_γ_ beyond _ε_ of the constellation last mentioned. The main stream of
the Galaxy after leaving Perseus, enters Cassiopeia, and sending short
branches into Andromeda, it completes its immense circle in Cassiopeia's
chair, where this description was begun.
When examined through the telescope, the appearance of the Milky-way
completely changes, and its nebulous light is resolved into an immense
number of stars, too faint to be individually seen with the naked eye.
When Galileo first directed the telescope to the galactic belt, its
nebulous, cloud-like masses were at once resolved into stars, even by
the feeble magnifying power of his instrument. When, much later, Sir
William Herschel undertook his celebrated star-gaugings of the Galaxy,
millions of stars blazed out in his powerful telescopes. The stars
composing this great nebulous belt are so numerous that it is impossible
to arrive at any definite idea as to their number. From his soundings
Herschel estimated at 116,000 the number of stars which, on one
occasion, passed through the field of his telescope in 15 minutes, by
the simple effect of the diurnal motion of the heavens; and on another
occasion, a number estimated at 250,000 crossed the field in 41 minutes.
In a space of 50, comprised between _β_ and _γ_ Cygni, shown on Plate
XIII., he found no less than 331,000 stars. Prof. Struve has estimated
at 20,500,000 the number of stars seen in the Milky-way through the
twenty-foot telescope employed by Herschel in his star-gaugings. Great
as this number may seem, it is yet far below the truth; as the great
modern telescopes, according to Professor Newcomb, would very probably
double the number of stars seen through Herschel's largest telescope,
and detect from thirty to fifty millions of stars in the Milky-way.
Although the telescope resolves the Galaxy into millions of stars, yet
the largest instruments fail to penetrate its immense depths. The
forty-foot telescope of Herschel, and even the giant telescope of Lord
Rosse, have failed to resolve the Milky-way entirely into stars, the
most distant ones appearing in them as nebulosities upon which the
nearer stars are seen projected, the galactic stratum being unfathomable
by the largest telescopes yet made.
The stars composing the Milky-way are very unevenly distributed, as
might easily be supposed from the cloud-like appearance of this belt. In
some regions they are loosely scattered, forming long rows or streams of
various figures, while in others they congregate into star groups and
clusters having all imaginable forms, some being compressed into very
dense globular masses. The intervals left between the clustering masses
are poorer in stars, and indeed some of them are even totally devoid of
stars or nebulosity. Such are the great and small "coal-sacks" in the
southern Galaxy. I have myself detected such a dark space devoid of
stars and nebulosity in one of the brightest parts of the Milky-way, in
the constellation Sagittarius, in about 17h. 45m. right ascension, and
27° 35' south declination. It is a small miniature coal-sack or opening
in the Galaxy, through which the sight penetrates beyond this great
assemblage of stars. Close to this, is another narrow opening near a
small, loose cluster.
Although lacking the optical resources which now enable us to recognize
the structure of the Milky-way, some of the ancient philosophers had
succeeded tolerably well in their speculations regarding its nature. It
was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
was nothing else but a vast and confused assemblage of stars, whose
faint light was the true cause of its milky appearance.
Before the invention of the telescope, no well-founded theory in regard
to the structure of the Milky-way could, of course, be attempted.
Although Kepler entertained different ideas in regard to the structure
of this great belt from those now generally admitted, yet in them may be
found the starting point of the modern conception of the structure of
the Galaxy and of the visible universe. In the view of this great mind,
the Milky-way, with all its stars, formed a vast system, the centre of
which, and of the universe, was occupied by our Sun. Kepler reasoned
that the place of the Sun must be near the centre of the galactic belt,
from the fact this last object appears very nearly as a great circle of
the celestial sphere, and that its luminous intensity is about the same
in all its parts.
Half a century later, another attempt to explain the Milky-way was made
by Wright, of Durham, who rejected the idea of an accidental and
confused distribution of the stars as inconsistent with the appearance
of the Galaxy, and regarded them as arranged along a fundamental plane
corresponding to that of the Milky-way. These ideas which were
subsequently developed and enlarged by Kant, and then by Lambert,
constitute what is now known as Kant's theory. According to this theory,
the stars composing the Galaxy are conceived as being uniformly arranged
between two flat planes of considerable extension, but which are
comparatively near together, the Sun occupying a place not very far from
the centre of this immense starry stratum. As we view this system
crosswise through its thinnest parts, the stars composing it appear
scattered and comparatively few in number, but when we view it
lengthwiser through its most extended parts, they appear condensed and
extremely numerous, thus giving the impression of a luminous belt
encircling the heavens. In the conception of Kant, each star was a sun,
forming the centre of a planetary system. These systems are not
independent, but are kept together by the bonds of universal
gravitation. The Galaxy itself is one of these great systems, its
principal plane being the equivalent of the zodiac in our planetary
system, while a preponderant body, which might be Sirius, is the
equivalent of our Sun, and keeps the galactic system together. In the
universe there are other galaxies, but as they are too distant to be
resolved into stars, they appear as elliptical nebulæ. Such are, in
brief, the grand speculations of Kant and Lambert on the Milky-way, and
the structure of the universe.
Kant's theory rested more on conjectures than on observed facts, and
needed therefore the sanction of direct observations to be established
on a firm basis. With this view, Sir William Herschel investigated the
subject, by a long and laborious series of observations. His plan, which
was that of "star-gauging," consisted in counting all the stars visible
in his twenty-foot telescope, comprised in a wide belt cutting the
Galaxy at right angles, and extending from one of its sides to the
opposite one, thus embracing 180° of the celestial sphere. In this belt
he executed 3,400 telescopic star-gaugings of a quarter of a degree
each, from which he obtained 683 mean gaugings giving the stellar
density of the corresponding regions.