NEBULÆ—APPARENT AND REAL.
The Globular Star-clusters—Structure of these Objects—Variability of
Stars in the Cluster—Telescopic Resemblance of a Cluster to a
Nebula—Resolution of a Nebula—Supposition that all Nebulæ may be
Clusters—A Criterion for distinguishing a Nebula and a Cluster—Dark
Lines on a bright Background characterise the Structure of a
Star—Bright Lines on a dark Background characterise the Structure of
a Nebula—Characteristics of the Spectrum of a true Nebula and of a
Resolvable Nebula—Spectra of the Sun and Capella—Spectra of the
Nebula in Orion and of a White Star compared—Number of Lines in a
Nebular Spectrum—Criterion of a Nebular Spectrum—Spiral Nebula not
Gaseous—Solar Spectra during an Eclipse—Bearing on the Nebular
Theory—Herschel’s Work—The Objection to the Theory—The Objection
Removed in 1864.
THERE is perhaps hardly any telescopic object more pleasing or more
instructive than a globular cluster of stars when viewed through an
instrument sufficiently powerful to do justice to the spectacle. There
are several star-clusters of the class designated as “globular.” The
most famous of these, or, at all events, the one best known to northern
astronomers, is found in the constellation of Hercules, and is for most
purposes sufficiently described by the expression, “The Cluster in
Hercules.” The genuine lover of Nature finds it hard to withhold an
exclamation of wonder and admiration when for the first time, or even
for the hundredth time, the Cluster in Hercules is adequately displayed
in the field of a first-class telescope.
[Illustration: Fig. 9.—THE CLUSTER IN HERCULES.
(_Photographed by Dr. W. E. Wilson, F.R.S._)]
In Fig. 9 is a photograph of this celebrated object, which was taken by
Dr. W. E. Wilson, F.R.S., at his observatory at Daramona, in Ireland.
The picture has been obtained from an enlargement of the original
photograph taken with the telescope in Mr. Wilson’s observatory. It is,
however, precisely as Nature has given it, except for this enlargement.
You will note that towards the margin of the cluster the several stars
are seen separately, and in many cases with admirable distinctness. We
do, however, occasionally find two or more stars so close together that
their images overlap; and, indeed, in the centre of the cluster the
stars are so close together that it is impossible to differentiate them,
so as to see them as individual points of light. We need have no doubt,
however, that the cluster is mainly composed of separate stars, although
the difficulties interposed by our atmosphere, added to the necessary
imperfections of our appliances, make it impossible for us to
discriminate the individual stars.
In looking at a star group of this particular kind the observer may
perhaps be reminded of a swarm of bees in flight from the hive, for the
stars in the cluster are, on a vast scale, apparently associated in the
same way as the bees, on a small scale, are associated in the swarm. We
may also compare the stars in the cluster to the bees in the swarm in
another respect. Each bee in the swarm is in incessant movement. There
can be no doubt that each star in a globular cluster is unceasingly
changing its position with reference to the others. The distance by
which the cluster is separated from the earth renders it impossible for
us to see those movements, at all events within those narrow limits of
time over which our observations have as yet extended; but the laws of
mechanics assure us that the mutual attraction of the stars in this
cluster must give rise to incessant movements, and that this must be the
case notwithstanding the fact that the relative places of the stars in
the cluster show no alteration that can be recognised from one year’s
end to another.
I may, however, mention that though there may be no movements in these
stars great enough to be observed, yet the brightness of some of them
shows most remarkable fluctuations. The investigations of Professor
Bailey and other astronomers have, indeed, disclosed such curious
variability in the brightness of some of these stars that if it were not
for the exceedingly high authority by which this phenomenon has been
guaranteed we should, perhaps, almost hesitate to believe so startling a
fact. It has, however, been most certainly proved that many of the stars
in certain globular clusters pass through a series of periodical changes
of lustre. The period is a very short one as compared with the periods
of better known variable stars, for in this case twenty-four hours are
more than sufficient for a complete cycle of changes, and it not
infrequently happens that in the course of a single quarter of an hour a
star will lose or gain brightness to the extent of a whole magnitude.
The phenomenon referred to is at the present moment engaging the careful
attention of astronomers; but it offers a problem of which, indeed, it
is not at present easy to see the solution.
Our immediate concern, however, with the globular star-clusters relates
to a point hardly of such refinement as that to which I have just
referred; it is one of a much more elementary nature. The photograph in
the figure may be considered to represent the Cluster in Hercules as it
would be seen with a telescope of very considerable visual power, for
the object would assume a different appearance in a telescope which was
not first class. The perfection of a really powerful instrument is
tested by its capability of exhibiting as two separate points a pair of
stars which are excessively close together, and which in an instrument
of inferior power cannot be distinguished, but seem fused into a single
object. The defining power of a telescope—that is to say, its capability
for separating close double stars—is increased with the size of the
instrument, always granting, of course, that there is equal optical
perfection in both cases. It follows that the more powerful the
telescope the more numerous are the stars which can be seen separately
in a globular cluster.
If, however, a small telescope be used, or a telescope which, though of
considerable size, has not the high optical perfection that is demanded
in the best modern instruments, then adjacent stars are not always to be
seen separately. It may be that the telescope, on account of its small
size, cannot separate the objects sufficiently, or it may be that the
imperfections of the telescope do not present the star as a point of
light, but rather as a more or less diffused, luminous disc. In either
case it may happen that a star overlaps other stars in its immediate
neighbourhood, and consequently an object which is really a cluster of
separate stars may fail altogether to present the appearance of a
cluster.
I have been alluding to something which, as every astronomer knows, is
of practical importance in the observatory. Like every one else who has
ever used a telescope, I have myself seen the Cluster of Hercules with
just the same misty appearance in a small telescope that an undoubted
nebula possesses in the very finest instrument. It is, accordingly,
sometimes impossible, merely by observation with a small instrument, to
distinguish between what is certainly a cluster of stars and what is
certainly a nebula. It has indeed not infrequently happened that an
observer with a small telescope has discovered what appeared to him to
be a nebula, and he has recorded it as such; and yet when the same
object was subsequently examined with an instrument of greater defining
power the nebulous character has been seen to have been wrongly
attributed. The object in such a case is proved to be nothing more than
a cluster of stars, of which the individual members are either
intrinsically faint or exceedingly remote; it certainly is not a mass of
that fire-mist or gaseous material which alone is entitled to be called
a nebula.
It is therefore a question of importance in practical astronomy to
decide whether objects which appear to be nebulæ are really entitled to
the name, or whether the nebulous appearance may not be an optical
illusion. The operation by which an object previously deemed to be a
nebula is shown by the application of increased telescopic power to be a
cluster of stars is commonly known as the resolution of a nebula. About
fifty years ago the mighty six-foot reflecting telescope of Lord Rosse,
and other great instruments, were largely employed on this work. It was,
indeed, at that time held to be one of the special tasks which came most
legitimately within the province of the big telescopes, to show that the
so-called nebulæ of earlier observers were resolvable into star-clusters
under the superior powers now brought to bear upon them.
The success with which this process was applied to many reputed nebulæ,
which were thereby shown to be not entitled to the name, led not
unnaturally to a certain conjecture. It was admitted that certain
objects which had successfully resisted the resolving powers of inferior
instruments were forced to confess themselves as mere star-clusters when
greatly increased telescopic power was brought to bear on them; and it
was conjectured that similar success would attend the attempts to
resolve still other nebulæ. It was even supposed that every object
described as a nebula could only be entitled to bear that designation
provisionally, only indeed until some telescope of sufficient power
should have been brought to bear on it. It seemed not unreasonable to
surmise that every one of the so-called nebulæ is a cluster of stars,
even though a telescope sufficiently powerful to effect its resolution
might never be actually forthcoming.
I do not, indeed, believe that this opinion as to the ultimate
resolvability of all nebulæ could have been shared by those who had much
practical experience in the actual observation of these objects with the
great telescopes, for the particular classes of nebulæ which in
telescopes of superior powers resolved themselves into groups of stars
had a characteristic appearance. After a little experience the observer
soon learned to recognise those nebulæ which promised to be resolvable.
The object might not indeed be resolvable with the powers at his
disposal, but yet from its appearance he often felt that the nebula
would be probably resolved if ever the time should come that greater
powers were applied to the task.
It is easy to illustrate the question at issue by the help of the
photograph of the Cluster in Hercules in Fig. 9. Each of the stars is
there distinct, except where they are much crowded in the centre. If,
however, the photograph be examined through one of those large lenses
which are often used for the purpose, and if the lens be held very much
out of focus, the stars will not be distinguishable separately, and the
whole object will be merely a haze of light. This illustration may help
to explain how the different optical conditions under which an object is
looked at may exhibit, at one time as a diffused nebula, an object which
in better circumstances is seen to be a star-cluster.
The astronomer who was fortunate enough to have the use of a really
great telescope would not fail to notice that, in addition to the
so-called nebulæ already referred to, which were presumably resolvable,
there were certain other objects, generally characterised by a bluish
hue, which in no circumstances whatever presented the appearance of
being composed of separate stars. We now know for certain that these
bluish objects are not clusters of stars, but that they are in the
strictest sense entitled to the name of nebulæ, and that they are
gaseous masses or mists of fire-cloud. The full demonstration of this
important point was not effected until 1864.
The fact that so very many of the nebulæ were resolved led not
unreasonably to the presumption that all the nebulæ would in due time
also yield. But there were many who could not accept this view, and
there was a long discussion on the subject. At last, however, the
improvements in astronomical methods have cleared up the question. Sir
W. Huggins has shown that there are two totally distinct classes of
nebulæ, or rather of so-called nebulæ. There are certain nebulæ which
can be resolved, and there are certain nebulæ which cannot. A nebula
which can be resolved would be a veritable cluster of stars, and is not
really entitled to the name of nebula; a nebula which cannot be resolved
would be entitled to the name, for it is a volume of gas or of gaseous
material which is itself incandescent. We have been provided with a
beautiful criterion by which we can decide to which of these classes any
nebulous-looking object belongs.
The spectroscope is the instrument which discriminates the two different
classes of objects. This remarkable apparatus, to which we owe so much
in every department of astronomy, receives the beam of light from the
celestial body. The instrument then analyses the light into its
component rays, and conducts each one of those rays separately to a
distinct place on the photographic plate. When the photograph is
developed we find on the various parts of the plate the evidence as to
the class of rays which have entered into the composition of the light
that has been submitted to this very searching form of examination.
The light which comes from a star or any star-like body, including the
sun itself, may first be described. That light, after passing through
the spectroscope and having been conducted to the photographic plate,
will produce a picture of dark lines on a bright background; this is, at
least, the spectrum which a star generally presents. There are, indeed,
many types of stellar spectra, for there are many different kinds of
stars, and each kind of star is conveniently characterised by the
particular spectrum that it yields. If the star be one of small
magnitude, then the lines in its spectrum may be detected, but only with
great difficulty. It not infrequently happens that the photograph of the
spectrum of such a star will show no more than a continuous band of
light without recognisable lines; and this is what occurs in the case of
a resolvable nebula, where the stars are so closely associated that the
spectrum of each separate star cannot be distinguished. The spectrum of
a resolvable nebula is merely a streak of light, which is the joint
effect of all the spectra. The spectrum is then too faint to show the
rainbow hues which present such beautiful features in the spectrum of a
bright star, as they do in the spectrum of the sun itself.
I give, in the adjoining figure (Fig. 10), portions of the photographs
of two spectra of celestial objects. They have been taken from the Atlas
of representative stellar spectra in which Sir William and Lady Huggins
have recorded the results of their great labours. Two spectra are
represented in this picture, the uppermost being the spectrum of the
sun, while the lower and broader one is the spectrum of the bright star
Capella. It has not been possible within the limits of this picture to
include the whole length of these two spectra, and it must therefore be
understood that the photographs given in the Atlas are each about five
times as long as the parts which are here reproduced.
[Illustration: Fig. 10.—SUN AND CAPELLA.
Sun above. Capella below.
(_Sir William and Lady Huggins_.)]
But the characteristic portions of the spectra selected are sufficient
for our present argument. It will be noted, first of all, that there is
a singular resemblance between the details of the spectrum of the sun
and those of the spectrum of the star. No doubt the breadth of the
stellar picture in the lower line is greater than that of the solar
picture in the upper line; but this point is not significant. The
breadth of the spectrum of the sun could easily have been made as wide
or wider if necessary. The breadth is immaterial, for the character of a
spectrum is determined not by its breadth, but by those lines which
cross it transversely. It will be seen that there are here a multitude
of lines, some being very dark, and some so faint as to be hardly
visible. Both spectra exhibit every variety of line, between the
delicate marks which can barely be seen and the two bold columns on the
right-hand side of the picture.
The characteristic of the spectrum is given by the number, the
arrangement, the breadth, the darkness, and the definiteness of the
lines by which it is crossed, and the first point that we note is the
remarkable resemblance in these different respects between the two
spectra. The lines are practically identical, at least so far as those
parts of the spectrum represented in this picture are concerned. We have
thus a striking illustration of the important fact, to which we have so
often to make allusion, of the general resemblance of the sun to the
stars. Not only do we know that if the sun were removed about a million
times as far as it is at present its light would be reduced to that of a
star, but that the star Capella transmits to us light consisting
essentially of the same waves as those which enter into a beam of
sunlight. No more striking illustration of the analogy between the sun
and a star can be found than that which is given in this photograph from
the famous Observatory at Tulse Hill.
But it must not be inferred that because the spectra of sun and star are
like each other, they are therefore absolutely identical. There are many
lines and details to be seen on the actual photographic plate which are
too delicate to be reproduced in such copies as it is possible to make.
When a close comparison is made on the actual plate itself of the lines
in the solar spectrum and the lines in the spectrum of Capella, it is
observed that, though they are the same so far as the more important
lines are concerned, yet that there are many lines found in the spectrum
of Capella which are not found in the spectrum of the sun.
[Illustration: Fig. 11.—SPECTRUM OF NEBULA IN ORION AND SPECTRUM OF A
WHITE STAR.
(_Sir William Huggins, K.C.B._)]
The contrast between the spectrum of a nebula properly so called and the
spectrum of a star is well illustrated by the accompanying picture (Fig.
11), in which Sir W. Huggins exhibits the photograph of the spectrum of
the Nebula in Orion in comparison with the spectrum of a star. The
uppermost of the two is the spectrum of the star. It will be noted that
this spectrum is very different from that which we have already seen in
Capella. Instead of a vast multitude of lines resembling the lines of
the solar spectrum, the spectrum of a star of the type here represented,
of which we may take Sirius as the most striking example, exhibits but a
few lines. We regard them as one system of lines, for we know they are
physically connected. They are all alike due to the presence of a single
element in the star, that element being in fact hydrogen. But though the
spectra of Capella and Sirius are so totally different, the differences
relate only to the distribution of the lines, and to their number,
darkness, and width. In both cases we observe the characteristic of the
light from an ordinary bright star, namely, that the spectrum is
composed of a bright band with dark lines across it. It ought, perhaps,
to be mentioned here that there are certain very special stars which do
exhibit some bright lines in addition to a more ordinary spectrum; this
is especially the case in the new stars which occasionally appear. Thus
in the case of the new star which appeared in Perseus, in 1901, there
were several remarkable bright lines. This most interesting object will
be referred to again in a later chapter.
Widely different from the spectrum of any star whatever is the lower of
the two spectra which are shown in the figure. This lower spectrum is
that of the Great Nebula in Orion. At once we see the fundamental
characteristic of a nebula; its spectrum exhibits five bright lines on a
dark field. I do not say that the Great Nebula in Orion has not more
than five lines; there are indeed many others, for Sir William Huggins
has himself pointed out a considerable number, and the labours of other
observers have added still more; but the five lines here set down are
the principal lines. They are those most easily seen; the others are
generally extremely delicate objects arranged in groups of five or six.
But the lines which this picture shows are quite sufficient to exhibit
that fundamental characteristic of the nebular spectrum, namely, a
system of bright lines on a dark field. I may further mention that
certain lines in the spectrum indicate the presence of the element
hydrogen in the Great Nebula in Orion, and we owe to Dr. Copeland the
interesting discovery that the remarkable element helium is also proved
to exist in the nebula.
The pictures, at which we have been looking, will suffice to make clear
the criterion, which astronomers now possess, for deciding whether an
object which looks nebulous is really a gaseous nebula, or ought rather
to be regarded as a star-cluster. If the object be a star-cluster, then
the spectrum that it gives will be the resultant of the spectra of the
stars, and this will be a continuous band of light. If the stars are
bright enough, it may be that dark lines can be detected crossing the
spectra, but in the case of the clusters it will be more usual to find
the continuous band of light so faint that the dark lines, even if they
are there, are not distinguishable.
If, on the other hand, the object at which we are looking, not being a
cluster of stars, is indeed a mass of glowing gas, or true nebula, then
the spectrum that it sends us is not the continuous spectrum such as we
expect from the stars. The spectrum which the nebula proper transmits to
the plate is said to be discontinuous. In some cases it is characterised
by only a single bright line, and in others there may be two, or three,
or four bright lines, or, as in the case shown in Fig. 11, the number of
bright lines may be as many as five. It may indeed happen, in the case
of some exquisite photographs, that the number of lines in the spectrum
of the nebula will be increased to a score or possibly more. There may
also be faint traces of a continuous spectrum present, this being due to
the stars scattered through the object, from which perhaps even the most
gaseous nebula is not entirely free. But the characteristic type of
nebular spectrum is that in which the bright lines, be they one, or few,
or many, are separated by intervals of perfect darkness. When it is
found that the spectrum of a nebula can be thus described, it is correct
to say that the nebula is truly a gaseous object.
In the lists given by Scheiner in his interesting book, “Astronomical
Photography,” the number of gaseous nebulæ is set down as seventy-three.
Of course no one pretends that this enumeration is exhaustive. It claims
to be no more than a statement of the number of nebulæ which have been
proved, by observations made up to the present, to be of a gaseous
description. Seeing that there are, as we have already stated, many
scores of thousands of nebulous-looking objects, it is probable that the
number above given is not more than a small fraction of the number of
gaseous nebulæ actually within reach of our instruments.
It may, however, be assumed that more than half the objects which are
called nebulæ are not of the gaseous type. This is a point of some
importance, which appears to follow from the facts stated by Professor
Keeler in connection with his memorable researches with the Crossley
Reflector. In a later chapter we discuss important questions connected
with what are called _spiral_ nebulæ. We may, however, here record that
no spiral nebulæ have as yet been pronounced gaseous. Professor Keeler
assures us that, of the one hundred and twenty thousand nebulæ which he
estimates to be within reach of the Crossley Reflector, far more than
half are of the spiral character. If, then, we assume that the spectra
of spiral nebulæ are always continuous, it seems to follow that less
than half the nebulous contents of the heavens possesses the
discontinuous spectrum which is characteristic of a gaseous object.
We are not entitled to assume that a nebula, or reputed nebula, which
shows a continuous spectrum, must necessarily be a cluster, not merely
of star-like bodies, but of bodies with masses comparable with those of
the ordinary stars. Our argument does most certainly suggest that the
body which yields a continuous spectrum is not a gaseous body; but it
may be going too far to assert that therefore it is a cluster of stars
in the ordinary sense. We do often find true nebulæ and star-clusters in
close association. The Nebula in the Pleiades (Fig. 13) is an example.
It may be desirable to add a few words here as to the physical
difference between a continuous spectrum and a discontinuous spectrum.
The light from a body, known to be gaseous, shows through the prism the
discontinuous spectrum of bright lines upon a dark background. If, on
the other hand, a solid be raised to incandescence, such, for instance,
as a platinum wire heated white-hot by an electric current, or a
cylinder of lime submitted to an oxyhydrogen blowpipe, then the spectrum
that it yields is continuous. All the colours of the rainbow, red,
orange, yellow, green, blue, indigo, violet, are shown in such a
spectrum as a continuous band of light, though the band is not crossed
by dark lines. It would therefore appear that the continuous spectrum is
characteristic of an incandescent solid, and the discontinuous spectrum
of a glowing gas. But here it may be urged that the sun presents a
difficulty. We so often refer to the spectrum of the sun as continuous,
that it might at first appear as if the spectrum of the sun resembled
that produced by radiation from a solid body. But, as is well known, the
sun is not a solid body. Even if the sun be solid at the centre, it is
certainly far from being solid in those superficial regions called the
photosphere, from which alone its copious radiation is emitted. If the
sun is not a solid body, how comes it to emit a radiation characterised
in the same way as the radiation from a white-hot solid? Why does the
solar spectrum not exhibit features characteristic of radiation from an
incandescent gas? The point is well worthy of attention; it finds an
explanation in the nature of the photosphere from which the sun’s
radiation proceeds.
The photosphere, though not, of course, to be described as a solid body,
does not most certainly, so far as its radiation is concerned, behave
like a gaseous body. In the glowing clouds of the photosphere the
carbon, of which they are composed, is not in the gaseous form; it has
passed into solid particles, and it is these particles, in the highest
condition of incandescence, which emit the solar radiation. Although
these particles are sustained by the gases of the sun, and are
associated in aggregations which form the dazzling clouds of the
photosphere, yet each one of them, in so far as its individual radiation
is concerned, ought to be regarded as a solid body. The radiation from
the sun is, therefore, essentially not the radiation from an
incandescent gas; it is the radiation from a glowing solid. This is the
reason why the solar spectrum is of the continuous type.
[Illustration: Fig. 12.—SOLAR SPECTRA WITH BRIGHT LINES AND DARK LINES
DURING ECLIPSE.
(_Photographed by Captain Hills, R.E._)]
By the kindness of Captain Hills, R.E., I am able to show a photograph
(Fig. 12) containing two spectra taken during a recent eclipse, which
will serve as an excellent illustration of the different points which we
have been discussing. It is, indeed, true that neither of the spectra,
here referred to, belongs to nebulæ, whether genuine gaseous objects or
not. Both of the spectra in Captain Hills’ picture are actually taken
from the sun. The conditions under which these spectra were obtained
make them, however, serve as excellent illustrations of the different
types of spectra. We are to notice that the upper band, which contains
what is called the “flash” spectrum, exhibits bright lines on a dark
background. See, for instance, the two lines so very distinctly marked,
which are indicated by the letters H and K. These lines are very
characteristic of the solar spectrum, and it may be mentioned that they
are indications of the presence of a well-known element. These lines
prove that the sun contains calcium, the metal of which common lime is
the oxide. It is, indeed, the presence of this substance in the sun
which gives rise to these lines. We shall refer again to this subject in
a later chapter.
As the upper of the two spectra exhibit H and K as white lines on a dark
background, so the lower represents the same lines as dark objects on a
white background. These photographs give illustrations of spectra of the
two different classes which provide means of discriminating between a
genuine nebula and an object which, though it looks like a nebula, is
not itself gaseous.
[Illustration: Fig. 13.—THE NEBULA IN THE PLEIADES (Exposure 10 hours).
(_Photographed by Dr. Isaac Roberts, F.R.S._)]
But, it will be asked, how can the spectra of the two distinct types
both be obtained from the sun? The explanation of this point is an
interesting one. The lower of the two is the ordinary solar spectrum; it
is a continuous spectrum showing dark lines on a bright field. The upper
spectrum, which shows bright lines on a dark field, is produced by a
small part of the sun just at the moment when the eclipse is total. The
circumstances in which that picture was secured will explain its
character. The moon had completely covered that dazzling part of the sun
which we ordinarily see, but a region of intensely glowing gaseous
material in the sun’s atmosphere was too high above the surface to be
completely hidden by the moon. The spectrum of this region, consisting
of the characteristic bright gaseous lines, is here represented. The
ordinary light of the sun being cut off, opportunity was thus afforded
for the production of the spectrum of the light from the glowing gas,
and we see this spectrum to be of the nebular type.
And now we may bring this chapter to a close by calling attention to the
very important bearing which its facts have on the Nebular Theory. It is
essential for us to see how far modern investigation and discovery have
tended either to substantiate or refute that famous doctrine which
traces the development of the solar system from a nebula. To do this it
is necessary to contrast the knowledge of nebulæ, as it exists at
present, with the knowledge of nebulæ as it existed in the days of Kant
and Laplace and Herschel.
We assuredly do no injustice to Kant or to Laplace if we say that their
actual knowledge of the nebulous contents of the heavens was vastly
inferior to that possessed by Herschel. There is not a single
astronomical observation of nebulæ recorded by either Kant or Laplace;
it may be doubted whether either of them ever even saw a nebula. Their
splendid contributions to science were made in directions far removed
from those of the practical observer, who passes long hours of darkness
in the scrutiny of the celestial bodies. Herschel, on the other hand,
was pre-eminently an observer. His nights were spent in the most
diligent practical observation of the heavens, and at all times the
nebulæ were the objects which received the largest measure of his
attention, with the result that the knowledge of nebulæ received the
most extraordinary development from his labours. Earlier astronomers had
no doubt observed nebulæ occasionally, but with their imperfect
appliances only the brighter of these objects were discernible by them.
The astonishing advance made by the observations of Herschel is only
paralleled by the advance made a hundred years later by the photographs
of Keeler.
But it must be remembered that though Herschel observed nebulæ, and
discovered nebulæ, and discoursed on nebulæ in papers which to this day
are classics in this important subject, yet not to the last day of his
life could he have felt sure that he had ever seen a genuine nebula. He
might have surmised, and he did surmise, that many of the objects he set
down as nebulæ were actually gaseous objects, but he knew that many
apparent nebulæ were in truth clusters of stars, and he had no means of
knowing whether all so-called nebulæ might not belong to the same
category.
It was not till nearly half a century after Sir William Herschel’s
unrivalled career had closed that the spectroscope was invoked to decide
finally on the nature of these mysterious objects. That decision, which
has been of such transcendent importance in the study of the heavens,
was not pronounced till 1864. In that year Sir William Huggins
established the fundamental truth that the so-called nebulæ are not all
star-clusters, but that the universe does contain objects which are most
certainly gigantic volumes of incandescent gases.
This great achievement provided a complete answer to those who urged an
objection, which seemed once very weighty, against the Nebular Theory.
It must be admitted that before 1864 no one could have affirmed with
confidence that any genuine nebula really existed. It was, therefore,
impossible for the authors of the Nebular Theory to point to any object
in the heavens which might have illustrated the great principles
involved in the theory. The Nebular Theory required that in the
beginning there should have been a gaseous nebula from which the solar
system has been evolved. But the objector, who was pleased to contend
that the gaseous nebula was a figment of the imagination, could never
have been effectively silenced by Kant or Laplace or Herschel. It would
have been useless for them to point to the Nebula in Orion, for the
objector might say that it was only a cluster of stars, and at that time
there would have been no way of confuting him.
The authors of the Nebular Theory had, in respect to this class of
objector, a much more difficult task than falls to its modern advocate.
The latter is able to deny in the most emphatic manner that a gaseous
nebula is no more than an imaginary conception.
The famous discovery of Sir W. Huggins has removed the first great
objection to the Nebular Theory.
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