THE GASEOUS NEBULÆ.
When we reach the irresolvable nebulæ, we unquestionably have
approached the creative period of solar systems and in many cases
of whole galaxies. These are multifarious in form, but all can be
reduced to a few comprehensive types. In determining the question
as to whether these irresolvable nebulæ were composed of distinct
stars like the Milky Way, but too distant to be resolved from their
mist-like light into discrete stars by the most powerful telescopes,
or whether they were gaseous in constitution,--that is, composed
of diffused gaseous elements not condensed into solar bodies,--the
spectroscope became the final and infallible test. Of this instrument,
thus used, Professor Proctor, in his "Star-Clouds and Star-Mist," says,
"A very few words will explain the whole matter to readers who remember
the three fundamental laws of this new mode of investigation,--viz.,
that, first, light from a burning solid or liquid source gives the
rainbow-colored streak of light commonly known as the prismatic
spectrum; secondly, when vapors surround such a source of light, the
rainbow-colored streak is crossed by dark lines; and, thirdly, when the
source of light is gas, there is no longer a rainbow-colored streak,
but merely a finite number of bright lines." Dr. Huggins selected for
investigation the small planetary nebula in the Dragon. He says, "When
I had directed the telescope armed with the spectrum apparatus to this
nebula, I at first suspected that some derangement of the instrument
had taken place, for no spectrum was seen, but only a short line of
light. I then found that the light of this nebula, unlike any other
extra-terrestrial light which had yet been subjected by me to prismatic
analysis, was of definite colors, and therefore could not form a
spectrum. A great part of the light is monochromatic, and so remains
concentrated in a bright line occupying a position in the spectrum
corresponding to its color. Careful examination showed a narrower and
much fainter line near the one first discovered. Beyond this point,
about three times as far from the first line, was a third exceedingly
faint line. From the position of one of the bright lines it is inferred
the gas nitrogen is one of the constituents of the nebula; another line
indicates the existence of the gas hydrogen in that far-off system;
the third line has not yet been associated with any known terrestrial
element, though it is near one belonging to the metal barium, and
still nearer one belonging to oxygen; a fourth line occasionally seen
belongs to hydrogen." Professor Proctor says, "Dr. Huggins examined
a large number of the planetary nebulæ (so called), obtaining in
each case a spectrum which indicates gaseity. In some cases only one
line could be seen, in others two, more commonly three, and in a few
instances four. When these lines were seen they invariably corresponded
in position with those already described. The single line sometimes
seen corresponded with the brightest line of the three; and when a
second line was visible, this also was no new line, but agreed with
the second brightest line in the three-line spectrum. The fourth line
was seen only in the spectrum of a very bright, small, blue planetary
nebula, but was later observed in other cases, and especially in the
great Orion nebula." At this time the latter was not visible, but when
Dr. Huggins had opportunity to examine it, he says, "The telescopic
observations of this nebula seem to show that it is suitable to a
crucial test of the usually received opinion that the resolution of
a nebula into bright stellar points is a certain indication that the
nebula consists of discrete stars." Professor Proctor says, "A simple
glance resolved the difficulty. The light from the brightest part of
the nebula--the very part which under Lord Rosse's great reflector
blazed with innumerable points of light--gave a spectrum identical in
all respects with that which Huggins had obtained from the planetary
nebulæ. Thus, what had been deemed boldness in Herschel--namely,
that he should have associated the wildest and most fantastic nebula
in the heavens with the circular and (in ordinary telescopes) almost
uniformly luminous planetary nebulæ--was unexpectedly confirmed." The
spectrum of this nebula has more recently been photographed by a long
exposure in the camera of the prepared plate. Of the result, Professor
Proctor thus speaks, "The nebula is seen to be in great part gaseous,
and, where gaseous, to shine in the main with the tints described
above; but parts of the nebula are not gaseous, and those portions
which are so are not all constituted in the same manner.... That
portion which is called the fish's mouth gives a continuous spectrum;
in other words, the same spectrum which we obtain from a star or a
star-cluster. This is the spectrum arising from a glowing solid or
liquid mass, or if from a gaseous body, then the gaseous body must be
in a state of great compression.... But the stars thus forming must
be immersed in the glowing gas forming the general substance of the
nebula.... It would be absurd to suppose that the nebula is a flat
surface; ... nebulous matter lies also, in all probability (certainly
one might fairly say), between us and the stellar aggregration as
well as on the farther side." Further, the same author says, "If,
as is probable, the luminosity of the gaseous portion of the Orion
nebula is accompanied by but a relatively small proportion of heat,
then the rays from the violet and ultra-violet part of the spectrum
are likely to give us much more complete information respecting the
constitution of these nebulous masses than can be derived from the
visible part of the spectrum."
In the recent work of Professor Ball, "In the High Heavens," that
author says, "There are, however, good grounds for believing that
nebulæ really do undergo some changes, at least as regards brightness;
but whether these changes are such as Herschel's theory would seem
to require is quite another question. Perhaps the best-authenticated
instance is that of the variable nebula in the constellation of
Taurus, discovered by Mr. Hind in 1852. At the time of its discovery
this object was a small nebula about one minute in diameter, with a
central condensation of light. D'Arrest, the distinguished astronomer
of Copenhagen, found in 1861 that this nebula had vanished. On the 29th
of December, 1861, the nebula was again seen in the powerful refractor
at Pulkova, but on December 12, 1863, Mr. Hind failed to detect it
with the telescope by which it had been originally discovered.... In
1868, O. Struve, observing at Pulkova, detected another nebulous spot
in the vicinity of the place of the missing object, but this also
has now vanished. Struve, however, does not consider that the nebula
of 1868 is distinct from Hind's nebula, but he says, 'What I see is
certainly the variable nebula itself, only in altered brightness and
spread over a larger space. Some traces of nebulosity are still to be
seen exactly on the spot where Hind and D'Arrest placed the variable
nebula. It is a remarkable circumstance that this nebula is in the
vicinity of a variable star which changes somewhat irregularly from
the ninth to the twelfth magnitude. At the time of the discovery in
1861 both the star and the nebula were brighter than they have since
become.'... It must be admitted that the changes are such as would
not be expected if Herschel's theory were universally true. Another
remarkable occurrence in modern astronomy may be cited as having
some bearing on the question as to the actual evidence for or against
Herschel's theory. On November 24, 1876, Dr. Schmidt noticed a new star
of the third magnitude in the constellation Cygnus.... The brilliancy
gradually declined until, on the 13th of December, Mr. Hind found it to
be of the sixth magnitude. The spectrum ... exhibited several bright
lines which indicated that the star differed from other stars by the
possession of vast masses of glowing gaseous material.... September 2,
1877, it was then below the tenth magnitude and of a decidedly bluish
tint. Viewed through the spectroscope, its light was almost completely
monochromatic, and appeared to be indistinguishable from that which
is often found to come from nebulæ.... It would seem certain that
we have an instance before us in which a star has changed into a
planetary nebula of small angular diameter.... Professor Pickering,
however, has since found slight traces of a continuous spectrum, but
the object has now become so extremely faint that such observations
are very difficult.... For the nebular theory we require evidence
of the conversion of nebulæ into stars." And not, it may be added,
of stars into nebulæ.
Of the irregular nebulæ, Professor Proctor says, "It may well
chance, as long since suggested by Professor Clark, of Cincinnati,
and as more cautiously hinted by Dr. Huggins, that in the varieties
of constitution observed in the irregular nebulæ, and the evidence
such varieties afford of progressive changes, we may find not merely
direct evidence of the development of suns and sun-systems from the
great masses of nebulous matter, but even what would be a far more
important and impressive result,--actual evidence of the development
of so-called elements from substances really elementary, or, at any
rate, one stage nearer the elementary condition than are our hydrogen,
nitrogen, oxygen, carbon, and so forth. The peculiarity of the spectral
indications of the presence of nitrogen and hydrogen in the nebula,
that only one line of nitrogen and two or three lines of hydrogen are
discernible, instead of a complete spectrum of either element as seen
under any known conditions, seems suggestive of what may be called
a more elemental condition of hydrogen and nitrogen." Whether this
be so, or whether these peculiarities are due to self-obscuration, or
mutual reversal of the familiar lines due to the enormous disturbances
of the nebular mass which must exist, it is certain that there is one
terrestrial substance, at least, which acts invariably, in combination
and chemical affinity, as a simple element in inorganic chemistry,
but which is, in fact, compound,--to wit, the hypothetical radical
ammonium, which is closely allied with the simple alkaline metals
potassium and sodium, forming with them a single group; and yet,
while the others have always remained as fixed, primitive elements,
the hypothetical element ammonium alone is a composite substance
consisting of hydrogen and nitrogen, two of the invariable gaseous
constituents of all these nebulæ. In comets we find, vaguely expressed,
an occasional strongly marked sodium line, and also the spectrum of
carbon; in these gaseous nebulæ we find, as yet, no trace of carbon,
and this element is so closely allied to hydrogen in its chemical
affinities and reactions as to suggest that it may be the same element
or some alloy of it, or in some allotropic form, as we find to be the
case with other simple elements under special conditions. In organic
chemistry--the chemistry of organic life--we find almost innumerable
compound radicals which act as simple elements in combination, but
which we can combine and separate into their constituents at will;
to all intents and purposes, in their various reactions, they behave
as elemental substances, and were it not that our analyses are able to
resolve them, as the spectroscope resolves the nebulæ, we might well
believe that here also we were dealing with simple primary elements. It
is almost certain that great discoveries in this field of chemistry
are not far distant, which will recall with wondering surprise the now
universally exploded fallacies of the "Philosopher's Stone" and the
"Universal Solvent." Indeed, we may find in the electrical energies
of the planets and the self-repulsive force of the electrospheres
of the earth and moon possible grounds for investigating anew some
of the abandoned tenets of astrology, in the hope that the light of
science may disclose some basis, at least, for what, at one time,--and
for nearly all time, in fact,--was the universally accepted belief,
not only of the ignorant, but of those the wisest and most learned of
their day and generation. If the planets by their position can cloud
the sun, nearly a million miles in diameter, with spots, or shed the
brilliance of the aurora borealis over all our skies, may they not
also cloud the embryonic intellect, or charge it with energies for
a career of prosperity or of disaster? May not the unseen currents,
or the electric storms around us, or the vast electrical phenomena
of the sun as well affect the sprouting germs of the husbandman or
some abnormally rapid development of an insect pest as the light,
the warmth, the moisture, or the cold, which, to our coarser vision,
are alone apparent? Fancy and fallacy revel luxuriantly where science
fails, but truth existed long before science was systematized, and the
supercilious condemnation of once generally accepted views without
examination is merely pseudo-science, and scarcely a single grade
higher in the scale than ignorant superstition itself. And every
new advance in knowledge requires a new overhauling of abandoned
material, just as a new advance in metallurgical knowledge enables
us sometimes to work over again our once-rejected mining dumps
with decided profit. Indeed, science itself is but a collection of
observed facts reduced to system, and among the shrewd and practical
miners there is a well-known saying, "The ore is where you find it,"
which has frequently put scientific assertion to the blush.
A study of the beautiful mezzotint plates, from the drawings of
the Earl of Rosse, contained in Professor Nichol's splendid work,
"The Architecture of the Heavens," will clearly disclose the forms, as
revealed by a powerful telescope, of many of these gaseous nebulæ. Of
such nebulæ, Appleton's Cyclopædia says, "nebulæ proper, or those
which have not been definitely resolved, are found in nearly every
quarter of the firmament, though abounding especially near those
regions which have fewest stars. Scarcely any are found near the
Milky Way, and the great mass of them lie in the two opposite spaces
farthest removed from this circle. Their forms are very various,
and often undergo strange and unexpected changes as the power of the
telescope with which they are viewed is increased, so as not to be
recognizable in some cases as the same objects." An example of this
is shown in Plate X. (Figs. 1 and 2) of Professor Nichol's work,
which gives a greatly enlarged view of those shown in Figs. 1 and 2
of Plate IX. (For Fig. 2 of Nichol's Plate X., see illustration of
nebula with double sun, in previous chapter.) Professor Nichol says,
"In every instance examined, save one, the planetary nebulæ are nebulæ
with hollow centers." The inference which this writer makes, that such
a planetary nebula consists of "a grand annular cluster of stars,"
has been since disproved by the discoveries of the spectroscope,
but the telescopic form remains true, and still awaits further
interpretation. While the irresolvable nebulæ seem to seek some
retired spot in space for their processes, like certain animals
when incubating, this rule is not universal. Of this, Appleton's
Cyclopædia says, "The density of nebular distribution increased with
the distance from the galactic zone for the irresolvable nebulæ,
but diminished with that distance for the clusters.... There is not a
gradual condensation of nebulæ towards two opposite regions, near the
poles of the galactic zone, but the nebulæ are gathered into streams,
nodules, and irregular aggregations such as we find in the grouping
of stars.... Between stars and nebulæ their arrangement follows the
law of contrast. There are two remarkable exceptions to this law,--the
Magellanic Clouds. In these, where stars of all orders, from the ninth
magnitude to irresolvable stellar aggregations, are as richly gathered
as in the galactic zone, nebulæ of all orders are also gathered richly,
even more so than anywhere else over the whole heavens." In the same
work, article "Nebula," it is stated of the planetary nebulæ, "There
are several which have perfectly the appearance of a ring, and are
called annular nebulæ.... Some appear to be physically connected in
pairs like double stars. Most of the small nebulæ have the general
appearance of a bright central nucleus enveloped in a nebulous
veil. This nucleus is sometimes concentrated as a star and sometimes
diffused. The enveloping veil is sometimes circular and sometimes
elliptical, with every degree of eccentricity between a circle and a
straight line. There are some which, with a general disposition to
symmetry of form, have great branching arms or filaments with more
or less precision of outline. An example of this is Lord Rosse's Crab
nebula. Another remarkable object is the nebula in Andromeda, which is
visible with the naked eye, and is the only one which was discovered
before the invention of the telescope. Simon Marius (1612) describes
its appearance as that of a candle shining through horn. Besides the
above, which have comparatively regular forms, there are others more
diffused and devoid of symmetry of shape. A remarkable example is the
great nebula in Orion, discovered by Huygens in 1656.... The great
nebula in Argo is another example of this class."
The number of nebulæ recognized in all the heavens is upward of five
thousand, and new ones are being constantly discovered. Of these
objects, Professor Nichol says, "The spiral figure is characteristic
of an extensive class of galaxies. Majestic associations of orbs,
arranged in this winding form, with branches issuing like a divergent
geometric curve from a globular cluster." These nebulæ, however, are
not associations of orbs; they are gaseous nebulæ apparently in process
of evolution. This author (Professor Nichol) presents views of such
spiral nebulæ either foreshortened to the view, so as to form a long
ellipse, or with the convolutions of the spiral apparently raised from
the horizontal plane into a conical form, and showing the black streaks
of space which lie between the convolutions, others seen in side view,
others in front, and, in fact, presented to the eye in every position
for observation. The author wrote before the days of the spectroscope,
and that he should conceive these vast objects to be spirals made
up of blazing suns like our Milky Way--vast galaxies, in fact--was
an inevitable conclusion at that time; but we now know that these
spiral nebulæ are gaseous, are apparently in process of manufacture,
and we can see them in their different stages of evolution, and
may perhaps learn something about the processes by which solar
systems and galaxies of suns are formed. Of one of these strange but
exceedingly instructive objects, Professor Ball, in his work "In the
High Heavens," says, "Fig. 3 represents one of the famous spiral nebulæ
(that of Canes Venatici) discovered many years ago by the late Earl of
Rosse. The object is invisible to the naked eye. It seems like a haze
surrounding the stars, which the telescope discloses in considerable
numbers, as shown in the picture. When viewed through an instrument
of sufficient power, a marvellous spectacle is revealed. There are
wisps and patches of glowing cloud-like material which shine not
as our clouds do, by reflecting to us the sunlight. This celestial
cloud is no doubt self-luminous; it is, in fact, composed of vapors so
intensely heated that they glow with fervor. As I write, I have Lord
Rosse's elaborate drawing of this nebula before me, and on the margin
of this stupendous object the nebula fades away so tenderly that it
is almost impossible to say where the luminosity terminates. Probably
this nebula will in some remote age condense down into more solid
substances. It contains, no doubt, enough material to make many
globes as big as our earth. Before, however, it settles down into dark
bodies like the earth, it will have to pass through stages in which
its condensing materials will form bright sun-like bodies. It seems
as if this process of condensation might almost be witnessed at the
present time in some parts of the great object. There are also some
very striking nebulæ which are often spoken of as planetary. They
are literally balls of bluish-colored gas or vapor, apparently more
dense than that which forms the nebula now under consideration. Such
globes are doubtless undergoing condensation, and may be regarded as
incipient worlds." Of these spiral nebulæ it is said, in Appleton's
Cyclopædia, "Many of them had been long known as nebulæ, but their
characteristic spiral form had never been suspected. They have the
appearance of a maelstrom of stellar matter, and are among the most
interesting objects in the heavens." Of their spectra it is said,
"The bright-line spectrum is given by all the irregular nebulæ hitherto
examined and by the planetary nebulæ." That is to say, these nebulæ
are gaseous in constitution, and have not yet reached the stage of
solar condensation which marks the existence of individual suns.