THE FIRE-MIST.
Evolution of other Bodies in the Universe—The Nebulæ—Estimate of the
Size of the Great Nebula in Orion—Photograph of that Nebula taken at
Lick Observatory—The Dumb-bell Nebula—The Crossley Reflector—The
late Professor Keeler—Astonishing Discovery of New Nebulæ—120,000
Nebulæ—The Continuous Chain from a Fluid Haze of Light to a Star—The
Celestial Evolution.
WE commence this chapter with a scrutiny of the heavens, to see whether,
among the bodies which it contains, we can discover any which appear at
this moment to be in the condition through which our system has passed
in some of its earlier stages.
So far as our unaided vision is concerned, we can see little or nothing
in the skies which will render us assistance in our present endeavour.
The objects that we do see in thousands are, of course, the stars, and,
as we have already pointed out, the stars are sun-like objects, and as
such have advanced many stages beyond the elementary condition. The
stars are therefore not immediately available for the illustration we
require. But when we come to look at the heavens through our telescopes
we presently find that there are objects which were not visible to the
eye, and which are neither stars nor planets. Closer examination of
these objects with the powerful instruments of modern observatories, and
especially with the help of those marvellous appliances which have
enabled us to learn the actual chemistry of the heavenly bodies,
supplies the suggestions that are required.
For not only does the telescope reveal myriads of stars which the naked
eye cannot detect; not only does it reveal wonderful clusters in which
thousands of stars are grouped closely together so as to form spectacles
of indescribable magnificence, when we take into account the intrinsic
splendour of each star-like point, but it also reveals totally different
objects, known as nebulæ. These objects are not stars and are not
composed of stars, but are vast extensions of matter existing in a far
more elementary condition. It is to these curious bodies that we invite
special attention at present. It is believed that they offer a
remarkable illustration of the origin of the solar system. We shall
first consider the best known object of this class. It is the Great
Nebula in Orion.
[Illustration: Fig. 6.—THE GREAT NEBULA IN ORION (Lick Observatory,
California).
(_From the Royal Astronomical Society Series._)]
And here it may be well to give an estimate which will enable us to form
some notion of the size of this object. We are accustomed to recognise
the stars as presenting the appearance of mere points of light; but an
object like the Great Nebula stretches over a wide area of the sky. As
to the actual extent of the space which it occupies we cannot speak with
confidence. The fact is that with every increase in the power of the
telescope the nebula appears to encroach more and more on the darkness
of space around. We give in Fig. 6 a representation of the Great Nebula
as it appears on a photographic plate obtained at the Lick Observatory
in California. But no picture can adequately represent the extraordinary
delicacy of the object and the softness and tenderness with which the
blue nebulous light fades into the black sky around. And it must not be
imagined that the nebula, as seen on this picture, represents the utmost
limits of the object itself. Every prolongation of the exposure, every
increase in the sensitiveness of the plate, show more and more the
extent of the nebula.
We shall, I doubt not, still be within the bounds of truth if we say
that the nebula extends over an area ten times as great as that
represented in this photograph. But we will take only the area of the
object as shown in the photograph for the purpose of our calculation.
Let us say that the nebula, as it is here represented, covers about two
degrees square. I shall not attempt to express in miles the dimensions
of an object so vast. I will try to give a conception of the size of the
Great Nebula in a different manner. Let us employ the dimensions of our
solar system for the purpose of comparison. Let us suppose that we draw,
upon the scale of this celestial photograph, a map which shall represent
the sun in the centre, the earth at her proper distance from the sun,
and Jupiter in his orbit, which is five times the diameter of the
earth’s orbit; and then let us mark the other planets at their
respective distances, even to Neptune, revolving in his great ellipse,
with a diameter thirty times that of the earth’s orbit. Let us then take
the area of the orbit described by Neptune as a unit with which to
measure the size of the Great Nebula in Orion. We shall certainly be
well within the actual truth if we say that a million circles as big as
that described by Neptune would not suffice to cover the area that is
represented on this photograph. This will give some idea of the imposing
dimensions of the Great Nebula in Orion.
But I would not have it to be supposed that the Great Nebula in Orion is
unique, unless in respect to its convenient position. The circumstances
of its situation in space happen to make it a comparatively easy object
for observation by dwellers on the earth. There are, however, very many
other nebulæ, although, with one exception—namely, the Great Nebula in
Andromeda, to which we shall have to refer in a later chapter—they do
not from our point of observation appear to be so brilliant as the
nebula in Orion. The fact is that by large and powerful telescopes
multitudes of these nebulæ are revealed, and the number ever tends to
increase as greater depths in space are sounded. Many of the nebulæ are
objects which possess sufficient detail to merit the particular
attention which they receive from astronomers. It must, however, be
confessed that by far the greater number of these objects are so dimly
discerned that it is impossible to study their individual
characteristics.
Among the nebulæ which possess sufficient individuality to merit study
for our present purpose, I must mention the so-called Dumb-bell. This
most interesting object can be seen in any good telescope. It requires,
however, as indeed do all such objects, an instrument of the highest
power to do it justice; in these modern days, however, the eye
observation of nebulæ through great telescopes has been superseded by
the employment of the photographic plate. I may take this opportunity of
mentioning that a photograph really shows more details in the nebula
than can be perceived even by the most experienced eye when applied to
the most powerful telescope placed in the most favoured situation as to
climate. Those lovers of nature who desire to observe celestial objects
through a great telescope, and have not the opportunity of gratifying
their wishes, may perhaps derive consolation from the fact that a good
photograph actually represents the object much better than any eye can
see it. More of the nebula is to be seen by looking at the photograph
than has actually been directly observed by any astronomer.
We have chosen the Dumb-bell (Fig. 7) and the Great Nebula in Orion as
characteristic examples of this remarkable class of celestial objects;
but there are many others to which I might refer, some of which we
represent in these pages. The Crab Nebula (Fig. 3) and others have been
distinguished by special names; but I must forbear to dwell further on
them, and rather hasten to give the results of recent observations which
have enormously extended our knowledge of the nebulous bodies in the
universe.
Let me first explain the source whence this extraordinary accession to
our knowledge has arisen. We owe it to the astronomers at the Lick
Observatory, that remarkable institution placed on the summit of Mount
Hamilton in California. Many important discoveries had already been made
with the noble instruments with which the famous Lick Observatory had
originally been endowed by its founder; it is, however, by a recent
addition to its magnificent apparatus that the discoveries have been
made which are specially significant for our present purpose.
Many years ago Dr. A. A. Common, the distinguished English astronomer,
constructed an exquisite reflecting telescope of three feet aperture
(Fig. 8). With this telescope Dr. Common himself obtained notable
results in photographing the heavens, and his success earned the award
of the Gold Medal of the Royal Astronomical Society. This telescope
passed into the possession of Mr. E. Crossley, of Halifax, and some time
later Mr. Crossley presented it to the Lick Observatory. The great
mirror, after its voyage across the Atlantic, was duly erected on the
top of Mount Hamilton, and fortunately for science Professor Keeler,
whose early death astronomers of both continents greatly deplore,
devoted himself to the study of the heavens with its aid. He encountered
many difficulties, as might perhaps be expected in such a task as he
proposed. His patience and skill, however, overcame them, and though
death terminated his labours when his great programme had but little
more than commenced, the work he had already accomplished has led to
results of the most striking character. Of the skill that he obtained in
photographing celestial nebulæ we have given illustrations in Figs. 6
and 7.
[Illustration: Fig. 7.—THE DUMB-BELL NEBULA
(Lick Observatory, California).
(_From the Royal Astronomical Society Series._)]
It is not to the individual portraits of notable nebulæ that we are now
about to refer. The most striking characteristic of the sidereal heavens
is not to be found in the fact that in one part of the sky we have a
brilliant Sirius, in another a Capella, and in a third a Canopus, but in
the fact that the heavens wherever we may test them are strewn with
incalculable myriads of stars, many of which appear faint only on
account of their distance and not because they are intrinsically small.
In like manner the remarkable fact with regard to the nebulæ which has
been disclosed by Keeler’s memorable researches with the Crossley
Reflector is the existence not alone of the great nebulæ, but of
unexpected scores of thousands of small nebulæ, or rather, I should say,
of nebulæ which appear small, though doubtless in many cases these
objects are intrinsically quite as splendid as the Dumb-bell Nebula or
the Nebula in Orion. They only seem small in consequence of being many
times further from us than are the more famous objects.
Professor Keeler’s experience was a remarkable one. He was photographing
a well-known nebula with the Crossley Reflector, and he was a little
surprised to find that on the same plate which gave him the nebula at
which he was aiming there were no fewer than seven other small nebulous
objects previously unknown to astronomers. It at first appeared to him
that this must be an unusual number of nebulæ to find crowded together
on one plate which covered no more than one square degree of the
heavens, an area about five or six times as large as the area of the
full moon. Subsequent experience, however, showed him that this fact,
however astonishing, was not at all unusual. In fact, he found to his
amazement that, expose the plate where he pleased, he generally obtained
new nebulæ upon it, and sometimes even a much larger number than the
seven which so greatly surprised him at first. I may mention just one or
two instances. There is a well-known and interesting nebula in Pegasus
which Professor Keeler photographed. When he developed the plate, which,
of course, included a considerable region of the heavens in the vicinity
of the particular nebula, he found to his astonishment that, besides the
nebula he wanted, there were not less than twenty other nebulæ on the
plate. But there is a more striking instance even than this. A plate
directed to a part of the constellation of Andromeda, with the object of
taking a portrait of a particular nebula of considerable interest, was
found to contain not only the desired nebula, but no fewer than
thirty-one other new nebulæ and nebulous stars. Nor have we in these
statements exhausted the nebulous contents of these wonderful plates, if
indeed we have rightly interpreted their nature. Professor Keeler tells
us that he finds upon them a considerable number of objects which in all
probability are also nebulæ, though they are so small that the telescope
is unable to reveal them in their true character. Examination does
little more than show these objects as points of light which, however,
are apparently not stars.
In the remarkable paper from which I have taken these facts Professor
Keeler makes an estimate which is founded on the examination of his
plates. If the heavens were to be divided into panels, each one square
degree in area, there would be about forty thousand panels. It follows
that if we desired to photograph the whole heavens, and if each of the
plates was to cover one square degree, forty thousand pictures would be
needed for the representation of the whole celestial sphere. Keeler’s
work convinced him that such plates taken by the Crossley Reflector
would, on an average, each show at least three new nebulæ. He admitted
it is quite possible that there may be regions of the sky in which no
new nebulæ are to be found. But in the regions which he had so far
tested he invariably found more than three nebulæ on each square degree;
indeed, as we have seen, on some of his plates he found a much larger
number of these remarkable objects. He therefore said that he makes but
a very moderate estimate when he gives a hundred and twenty thousand as
the probable number of the new nebulæ within the reach of the
photographic plates of the Crossley Reflector.
The enormous extension which these investigations have given to our
knowledge demands the serious attention of all interested in the
heavens. The discoveries of the earlier astronomers had led to the
knowledge of about six thousand nebulæ; the Crossley Reflector at the
Lick Observatory has now rendered it practically certain that the number
of nebulæ in the heavens must be at least twenty-fold as great as had
been hitherto supposed.
[Illustration: Fig. 8.—THE CROSSLEY REFLECTOR
(CONSTRUCTED BY DR. A. A. COMMON F.R.S. AND NOW AT THE LICK
OBSERVATORY).]
In subsequent chapters we are to present the evidence for the belief
that this earth of ours, as well as the sun and all the other bodies
which form the solar system, did once originate in a nebula. According
to this view the materials which at present are found in the globes of
the solar system were once distributed over a vast extent of space as a
fire-mist, or nebula. It is surely very pertinent to be able to show
that a nebula, such as we suppose to have been the origin of our system,
is not a mere figment of the imagination. No doubt it is impossible for
us now to show the original nebula from which the solar system has been
evolved. It is nevertheless possible, as we have seen, to show that a
hundred and twenty thousand nebulæ are now actually existing of every
grade of magnitude. They range from such magnificent objects as the
Great Nebula in Orion and the Dumb-bell Nebula, down to objects wholly
invisible, not merely to the unaided eye, but even in the most powerful
telescope, and only to be discerned as hazy spots of light on the
photographic plates of an instrument such as the Crossley Reflector.
Though no eye has seen the actual stages in the grand evolution of our
solar system, we may at least witness parallel stages in the evolution
through which some of the myriads of other nebulæ are now passing. We
find some of these nebulæ in that excessively diffused condition in
which they are devoid of visible structure. Material in this form may be
regarded as the primæval nebula. There is at least one of these
extraordinary objects which is larger a great deal than even the Great
Nebula in Orion, but altogether too faint to be seen except by the
photographic plate. Here we find, as it were, the mother-substance in
its most elementary stage of widest possible diffusion, from which
worlds and systems, it may be, are yet to be evolved. From diffused
objects such as shown in Fig. 5 we can pass to other nebulæ in which we
see a certain advance being made in the process by which the nebula is
transformed from the primitive condition. We can point to yet other
nebulæ in which the advance to a further stage of development is more
and more pronounced. Thus the various stages in the evolution of a
system are to be witnessed, not indeed in the transformation of a single
nebula, but by observing a properly arranged series of nebulæ in all
gradations, from the diffused luminous haze to a star with a faint
nebulous surrounding. Such was Herschel’s original argument, and its
cogency has steadily increased from the time he first stated it down to
the present hour.
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