PLANETARY NEBULÆ.
Planetary nebulæ seem to be intermediate between nebulous stars and
annular nebulæ. Indefinite aureolas are replaced in them, as if through
the spreading outward of nebulous matter towards a limiting spherical
surface, by compact discs, and with the further advance of this process
of exterior condensation, the discs become rings. A stellar nucleus
persists throughout these phases. They may not be strictly phases of
development. To establish an actual sequence of growth, facts of various
orders should be considered. As a simple matter of fact, however,
objects are found which combine so closely the visual features of
planetary nebulæ and nebulous stars on the one side, and of planetary
and annular nebulæ on the other, that their classification in the above
order is prescribed inevitably, if only for mental convenience.
As a preliminary to the physical study of these objects, we must try to
attain a clear conception of their real forms in solid space. This is
not easy when their structural complexities are taken into account; but,
setting these for the moment aside, we can gather some indications
regarding the general plan of their fabrication. As a rule, planetary
nebulæ are markedly elliptical. They may then be concluded to be
spheroidal in shape; and even those sensibly circular are probably
spheroids viewed along their shorter axes. That their compression is due
to axial rotation is a fair inference, verifiable, possibly, by
spectroscopic measurements. These were indeed ineffectually tried by
Professor Keeler, 3rd April 1891, on a bright planetary in Hydra (N.G.C.
3242);[970] but he doubted whether the conditions of the experiment
permitted the detection of a difference in the velocities of the
advancing and retreating limbs of the nebula of less than seven or eight
miles a second, and the movement of such bodies is likely to be
excessively slow. Otherwise they could scarcely be supposed capable of
holding together; for they are obviously of tenuous composition, and
gravity at their equators can act very feebly in equilibrating
centrifugal impulse.[971] And here an untrodden field opens to
enterprising inquirers. A theoretical investigation might, to begin
with, be attempted of the figures which should belong to rotating globes
of the kind on certain probable assumptions as to their nature and modes
of movement; and the results might be tested by the application to a
number of promising objects of Keeler’s spectrographic method, in which
the linear images of an equatorial slit serve, through the tilt imparted
to them by the contrary motions of the opposite limbs, to measure, in
miles per second, the speed of gyration. Small, lucid, strongly
elliptical planetaries would be the most hopeful subjects for
experiment; nor would a negative upshot be without value. The Hydra
nebula, for instance, which is nearly as much flattened as the globe of
Saturn, is equal in light to a seventh-magnitude star; it is about 20″
across, and somewhat diffuse at the edges.[972] With it may be compared
a condensed planetary in Ophiuchus (N.G.C. 6572), sometimes called
Struve No. 6 (Σ 6). Its light, which is comparatively intense, emanates
from a small disc 8″ in diameter (exclusive of a hazy margin), with an
ellipticity of about ¹⁄₂₀₀. Professor Keeler’s drawing of the object
with the visible part of its spectrum is copied in Plate XXIII. Fig. 2.
The nucleus is not stellar, although it gives strongly continuous light.
The lines in its spectrum were found by Keeler to be displaced upward by
an amount corresponding to a motion towards the earth of 6·3 miles a
second;[973] but this motion should be considered to belong to the sun,
not to the nebula. Judging by its appearance, axial acceleration may
have advanced farther in it than in most members of the class, and might
record itself in an equatorial spectrograph. The trial, at any rate,
should be made. Anticipation of failure need not be allowed to paralyse
effort. In science the rush of a forlorn hope often carries a fortress
that has obstinately held out against a siege in regular form.
The uniformity of aspect at first supposed to characterise planetary
nebulæ disappears before the searching scrutiny of the powerful
telescopes now in use. Their surfaces prove to be full of suggestive
detail. They are broken up by irregular condensations, or furrowed by
the operation of antagonistic forces; they betray, here possibly the
action of repulsive, there of attractive influences, and bear
inscriptions of no less profound historical import than the contortions
and faultings of terrestrial strata. They are quite commonly multiplex
formations. One glimmering disc is superposed upon another, intimating
the analogy of the successive filmy envelopes flung round the heads of
active comets. With the twenty-seven-inch Vienna refractor Vogel
succeeded, in 1883, in resolving the above-mentioned planetary in
Ophiuchus (N.G.C. 6572) into three distinct layers,[974] doubtless
representing concentric ellipsoids of unequal antiquity. A triple
outpouring of matter at age-long intervals seems recorded. And the case
is typical, although the nebulous shells are not often so clearly
defined. This, however, is, on the one hand, a matter of telescopic
seeing; on the other, perhaps of evolutionary progress. By way of
illustration, let us take as the first term of a sequence an average
nebulous star, such as that detected by Auwers in Auriga (N.G.C. 2175);
next in order we might place an object near 16 Cygni (N.G.C. 6826), in
which the “glow” has become compacted into a large, round, seemingly
uniform disc.[975] Sir William Herschel described it as “a beautiful
phenomenon, of a middle species between the planetary nebulæ and the
nebulous stars.”[976] The following term of the series may be found in a
planetary not far from γ Eridani (N.G.C. 1535), consisting of an
eleventh-magnitude star, embossed upon two concentric nebulous shields;
or rather, presumably, enclosed within a pair of nebulous globes. If we
suppose these to have been produced by successive outflows, checked at a
limiting surface, we could easily see that the occurrence of a third
access of ejective energy would complete the model of the triple
specimen in Ophiuchus. It may be added that the difficulty appears
almost insuperable of explaining the growth of multifold planetaries on
the hypothesis of simple contraction. A repulsive agency in a manner
asserts its past activity.
Let us now imagine one of these spherical envelopes to be hollow. The
effect to the eye would be that of a luminous ring. And, in fact, an
appreciable proportion of planetary nebulæ show an interior circlet of
dim radiance, which can hardly be otherwise interpreted than as the
projection on a plane of a vast nebulous bubble. This mode of
construction is fully carried out in “annular nebulæ”; it is partially
realised in not a few specimens in which a ring within a disc is
perceptible or conspicuous. All will be considered together in the next
chapter; in this we are concerned only with examples devoid of—so to
speak—symptoms of inflation.
[Illustration:
PLATE XXIV.
1. Photograph of the Owl Nebula (Roberts).
2. Photograph of the Orion Trapezium (W. H. Pickering).
]
The largest planetary in the heavens lies south-east of the second
Pointer, β Ursæ Majoris. Discovered by Méchain in 1781, it was numbered
97 on Messier’s list (N.G.C. 3587), and has been extensively and
carefully observed. Its dimensions were given by Lord Rosse in 1874 as
163″ by 147″;[977] but the major axis measures 203″ on a photograph
taken by Dr. Roberts, with an exposure of four hours, 20th April
1895,[978] and reproduced, by his kind permission, in Plate XXIV. Fig.
1. The observational history of the object is extremely curious. Sir
John Herschel saw “a large, uniform, nebulous disc, quite round, very
bright, not sharply defined, but yet very suddenly fading away to
darkness.”[979] On 2nd March 1848, however, Lord Rosse perceived a
doubly perforated surface, with a star in each cavity,[980] and his
observations were confirmed by Dr. Robinson. The drawing representative
of them is a record of permanent interest. The resemblance to an owl’s
face, given to it by the symmetrically placed _oculi_, is unmistakable;
and the great planetary in Ursa Major became known, from the middle of
the last century, as the “Owl Nebula.” The name has survived the
similarity. On 9th March 1850 the two stars were noted at Parsonstown
shining as usual in their respective excavations; five weeks later the
fainter one had vanished;[981] nor could it ever again be found, though
looked for about forty times during the ensuing quarter of a century. To
Professor Keeler in 1891[982] the nebula wore indeed an entirely
different aspect from that previously attributed to it. “There is but
one nucleus,” he stated, “which is by estimation almost exactly central,
at a place which in Lord Rosse’s drawing is occupied by a bridge of
light between two dark openings. There is also but one central dark
space.” Nevertheless, two appear in the Crowborough picture, like
lagoons separated by an isthmus, and on the isthmus there is planted, as
it were, a lighthouse, diffusing a brilliant illumination. Thus the
interior vacancies remain in _statu quo_, while the stars that formerly
occupied them have both faded out of sight, leaving the prodominance to
a third, not identical with either. The change, in short—admitting that
there has been change—relates, not to the structure of the nebula
itself, but to the relative brightness of three connected stars. The
question whether those at present extinct will ever become revivified,
can be answered only by prolonged experience. Dr. Roberts thought that
his photograph indicated for the nebula a combined ring and disc
formation;[983] but this is not manifest. The picture affords no
verification of the marginal inequalities recorded at Parsonstown; yet
there is reason to believe that they were not illusory. The torn and
jagged contour, which Professor Alexander expounded as the effect of
“disruption and dispersion outward,”[984] may come into view in
representations on a larger scale, taken with special precautions for
the definition of minute details.
The suspected alteration of the Owl planetary accentuates the need for
keeping watch over nuclear stars, more especially since they present
unexplained peculiarities. Their light is of purely stellar quality, but
of remarkable actinic power. They are very much brighter chemically than
visually. Further, they often give nebulous images on the sensitive
plate, while appearing sharp with the telescope. In some rare cases (as
in N.G.C. 6781), they are eccentrically situated. Burnham is probably
justified in regarding the possession of a central star as an essential
feature of planetary nebulæ,[985] although it is occasionally
undiscernible with the telescope. N.G.C. 6563 in Sagittarius, and N.G.C.
7354 in Cepheus, are examples of nebulous discs unrelieved to the eye by
the sparkle of any stellar points; and a beautiful little planetary in
Perseus, discovered by Barnard 11th December 1890,[986] is of similar
aspect. The missing nuclei, however, of all such objects would, it is
tolerably certain, become manifest in long-exposed photographs, which
should accordingly be taken for the purpose of deciding a point of
fundamental importance in the economy of planetary nebulæ.
Perhaps the most noted member of the class, from the numerous
experiments of which it has been made the subject, is located in Draco
(N.G.C. 6543), quite close to the pole of the ecliptic. To ordinary
observation it presents a greenish-blue surface, 22″ by 18″, centred on
a vividly white star of the tenth magnitude; but Professors Holden and
Schaeberle recognised, with the Lick refractor, its “helical”
conformation.[987] Two brighter intersecting hoops, perceived at a
glance to diversify the disc, fell into position on closer scrutiny as
the thread of a screw, uniting their curves into one continuous
tri-dimensional spiral. The discernment of this novel form—believed to
be typical—was a suggestive contribution towards what might be termed
the solid geometry of nebulæ. Speculations as to its mode of origin
would, however, be premature until, by the unerring testimony of the
camera, it has been definitively proved to subsist.
The Draco planetary was observed on the meridian by Lalande 26th July
1790, and there are no good grounds for holding it to have since shifted
appreciably from the place then assigned to it. Burnham, it is true, has
found a progressive diminution of the distance between the central star
of the nebula and one external to it, amounting to 0·033″ annually;[988]
but the movement, if real, may confidently be ascribed to the
disconnected star. The nebula is indeed very far from being stationary
in space. Professor Keeler determined for it spectroscopically in 1891 a
velocity of approach towards the solar system of forty miles a
second;[989] and its apparent fixity on the sphere is doubtless only an
effect of extreme remoteness. Some attempts to determine its annual
parallax[990] have resulted only in showing it to be small “below
compute.” From the prismatic examination of this object, 1st August
1864, Sir William Huggins learned the existence of gaseous nebulæ.
A nebula in Cygnus (N.G.C. 6826) was described by Mr. Burnham as “almost
an exact duplicate” of the planetary in Draco. It is nearly circular,
the longer diameter measuring nearly 27″, the shorter 24″. The nucleus
is very bright (8·8 magnitude), and gives a strong continuous spectrum.
The light from the disc, too, includes an unusually large _white_
ingredient.[991] The radial motion of this nebula is very small; it
approaches the sun by 3·3 miles a second. But the sun is travelling on
its own account towards its place at a much higher rate of speed, so
that the apparent sluggishness of the nebula indicates that it is really
moving away from us, its measured pace representing only the velocity
with which our system gains upon it.
The spectra of planetary nebulæ are fundamentally alike; they differ
only in details. About forty bright lines have been determined in them,
visually and photographically, and they are invariably fine and sharp,
as if emitted by materials of great tenuity. They seem hazy only just
where they cross certain diffuse nuclei; and this feature seems to imply
a gradual condensation of the nebulous stuff towards the central mass,
which, in such cases, cannot properly be called a star. The essential
characteristic, however, of nebular light is the presence in it of the
ray, or rays, of “nebulium.” This is the one sure criterion by which
gaseous nebulæ can be distinguished from stars.[992] The chief nebulium
line has a wave-length of λ 5007, and is of a clear green colour. With
it is constantly associated a ray about one-third as bright at λ 4959,
and the invariability of their relation lends strong probability to the
opinion that both emanate from the same substance.[993] A strong
ultra-violet line at λ 3727, photographed for the first time by Sir
William and Lady Huggins in 1882, perhaps claims an identical origin.
This, however, remains doubtful in the absence of decisive evidence that
it is an unfailing constituent of the nebular spectrum. The green gas
designated “nebulium” is unknown, so far, terrestrially; nor has it been
observed to shine in any of the heavenly bodies except nebulæ. There is
reason to believe it denser, or at any rate less diffusive than
hydrogen. An object catalogued in the Southern Durchmusterung as a
ninth-magnitude star under the title S.D.M. −12° 1172, was found by Mrs.
Fleming in 1891 to give the spectrum of a planetary nebula. But hydrogen
glows in it with unusual intensity. The relative brightness of the three
green lines is estimated by Campbell[994] to be ordinarily 10 : 3 : 1, F
(Ηβ) being the faintest and most refrangible. But the proportion in the
planetary near Rigel is 10 : 3 : 7. In other words, hydrogen is of seven
times its normal lustre comparatively to nebulium. Further, the three
lines, when viewed through an open slit, form discs of severally 11″,
9″, and 14″ diameter. Here then, apparently, a hydrogen-envelope
constitutes an outer shell to the nebulium-sphere; and a gas that rises
higher than another is presumably specifically lighter, although, in
view of the enormous altitudes attained by calcium vapour near the sun,
the inference must be regarded as subject to qualification. Certainty on
the point, and on many others connected with the physics and chemistry
of nebulæ, may be said to be unattainable until nebulium is captured in
the laboratory. And the prospect of this achievement, although not
hopeless, is remote.
The following table gives the wave-lengths and origins, when they are
known or can be conjectured, of forty bright lines in the spectra of
planetary nebulæ. A few are common to all, notably the trio in the
green, with about half a dozen of the blue and ultra-blue hydrogen
lines, while others are more individual in their occurrence; but, on the
whole, bodies of this class seem to be of remarkably uniform
constitution.
LINES OBSERVED IN THE SPECTRA OF
PLANETARY NEBULÆ.
Wave-Length. Origin.
6563 Hydrogen (Hα).
5876 Helium (D_{3}).
5751 Oxygen?
5680 Unknown.
5540 Unknown.
5412 Hydrogen; Pickering series?
5313 Unknown.
5183 Unknown.
5007 Nebulium.
4959 Nebulium?
4861 Hydrogen (Hβ).
4790 Unknown.
4743 Unknown.
4715 Unknown.
4688 Hydrogen; Rydberg series?
4662 Unknown.
4643 Nitrogen?
4610 Nitrogen?
4597 Nitrogen?
4574 Silicon?
4472 Helium.
4390 Helium.
4363[995] Unknown.
4341 Hydrogen (Hγ).
4265 Unknown.
4145 Unknown.
4122 Helium?
4102 Hydrogen (Hδ).
4067 Unknown.
4026 Helium.
3970 Hydrogen (Hε).
3968[995] Unknown.
3889 Hydrogen (Hζ).
3869[995] Unknown.
3836 Hydrogen (Hη).
3795 Hydrogen (Hθ).
3768 Hydrogen (Hι).
3727 Unknown.
3460 Unknown.
3390 Unknown.
Several of the Wolf-Rayet lines, it will be observed, are comprised in
this list, notably those at λ 541, λ 469, and λ 464. And the absence of
recognisable metallic rays strengthens the analogy with stars of that
peculiar description. As a rule, the hydrogen spectrum in planetaries
begins with the green line. The red line has been distinguished in only
a few specimens, which appear more condensed than the rest. Yet it would
be rash to assume that this is really the case.[996] The relative
intensity of the hydrogen lines in stars and nebulæ is an intricate
subject, the ramifications of which have yet to be tracked out. The
coincident appearance of D_{3} with C is worth notice as a hint that the
conditions favourable to the development of the slower light-vibrations
are the same for helium as for hydrogen. They are markedly present in
Struve’s planetary in Ophiuchus (N.G.C. 6572 = G.C. 4390), which gives a
complex spectrum of at least thirty lines,[997] accompanied by faint
continuous radiance. Three classes of fact regarding it are recorded in
Professor Keeler’s sketch of the portion of it accessible to
eye-observations (see Plate XXIII. Fig. 2): first, the positions of the
component rays in the scale of wave-lengths; next, their relative
lustre; thirdly, the extent of the nebula from which they are derived.
And it is of interest to perceive that only those associated with
nebulium, together with the green and blue lines of hydrogen (Hβ and
Ηγ), seem to reach the limits of the disc, while the others are radiated
only by its central parts. Still we have to remember that the length of
the lines must depend to some extent upon their intensity; and that they
may be short only because the sections of them given out by dim regions
of the nebula are of evanescent faintness. Professor Keeler suspected
the presence of dark bands interrupting its continuous light between
D_{3} and λ 5007,[998] and they will, if verified, supply the only
extant proof of absorption in gaseous nebulæ. The nucleus of Σ 6
apparently reinforces the emissions from the disc; but it is plainly not
a genuine star. Further spectrographic investigation of the object is
most desirable.
In the Draco planetary (N.G.C. 6543), on the contrary, the stellar and
nebular elements of the spectrum are perfectly distinct. Some of the
bright lines can indeed be seen only when the central star is outside
the slit, and are therefore due, in Professor Campbell’s words, “to the
nebula proper, as indeed are all the lines observed, and there is no
evidence to show that they exist at all in the central star.”[999] A few
of those registered in Struve’s planetary are missing here, particularly
the hydrogen line (Pickering series) at λ 541; but C, D_{3}, and both
the Wolf-Rayet blue radiations are perceptible, while the leading
ultra-violet line at λ 3727 appeared conspicuously in Von Gothard’s
photographs.[1000] The relative strength of the green lines in this
nebula, as determined by Campbell, is 10 : 3 : 2. It is of a
verd-antique hue, and, indeed, the nearly total suppression of red rays
in their light gives to all planetaries a blue or greenish tinge.
One of the Durchmusterung stars in Cygnus (D.M. + 41° 4004) was noticed
by the late Prebendary Webb, 14th November 1879, to have a hazy disc,
some 10″ in diameter.[1001] Stephan, at Marseilles, independently
detected its nebular character, and Winnecke compared it to a small
comet with a tenth-magnitude nucleus at its preceding end. Moreover, the
nucleus is double. Professor Keeler described the object (N.G.C. 7027)
as follows:[1002]—“This is the brightest nebula that I have examined,
and its spectrum is exceedingly interesting. The nebula is irregular in
outline, and contains two central condensations, one of which has an
oval and fairly well-defined outline. The other is much fainter and more
diffuse.” His drawing of it, made at the great telescope, is copied in
Plate XXIII. Fig. 3, together with a representation of its spectrum. The
nuclei are obviously non-stellar. The continuous spectrum of even the
more conspicuous member of the pair is not incomparably brighter than
that derived from the disc, and it claims all the emission rays as
properly, though not exclusively belonging to it. Professor Keeler
accordingly regarded it as “in a much less condensed state than the
nuclei of Σ 6 and many other nebulæ of its kind,” its exceptional
brilliancy notwithstanding. The spectrum of Webb’s planetary is
remarkable for the intensity of some usually quite subordinate lines,
especially of the Rydberg hydrogen ray (λ 4688), and of the unknown
lines at λ 4743 and λ 4363. Campbell could just identify the red glint
of C,[1003] which by its faintness evaded Keeler’s survey; D_{3} was
made out by both observers, and an unidentified line at λ 3869 was
photographed by Von Gothard in 1892, although his plates were blank at
the place where the significant λ 3727 was expected to appear.[1004]
A companion to Webb’s nebula, both in physical aspect and by vicinity in
the sky (N.G.C. 7026), was discovered spectroscopically by Dr. Copeland
in 1880.[1005] It had, however, been observed telescopically by Mr.
Burnham seven years earlier.[1006] It is small, bright, duplex, a pair
of diffuse nuclei 6″ apart sustaining a filmy structure which, viewed
with the Lick refractor, suggested a comparison to two sheaves of corn
laid side by side.[1007] Burnham refuses to admit the planetary nature
of either of the objects in Cygnus,[1008] yet their spectra scarcely
allow them to be relegated to a different class. The “Rydberg line” is
equally prominent, relatively to their brightness, in both objects. From
the cosmogonic point of view they are of high illustrative importance.
We seem to have before our eyes double stars in slow course of
formation, and preparing to break loose by the development of systemic
revolutions from the trammels of a joint rotation.
Bi-nuclear planetaries are not uncommon. One such (N.G.C. 3195) was
observed by Sir John Herschel in the south polar constellation of the
Chameleon, and is depicted in his volume of _Cape Results_. The nuclei
are fairly well matched in lustre, and will perhaps grow into a double
star like γ Virginis. Their spectrum is still unrecorded; but it will be
of considerable interest to determine whether it exhibits the
peculiarities visible in those of the double planetaries in Cygnus. Two
bright patches near opposite margins of the circumference give the
nebula in Chameleon somewhat the air of a reduced copy of the celebrated
“Dumb-Bell” in Vulpecula.
Of a planetary in the Poop of Argo, originally discovered by the elder
Herschel, Lassell wrote at Malta about 1851:[1009]—“No description can
do justice to this singular object,” which is “not beautiful, for it has
no symmetry, but wonderful.” His drawing shows the disc, which had
appeared to d’Arrest perfectly round, as pear-shaped, with multiple
condensations. So that a quadruple or quintuple star in embryo may here
be offered for our contemplation.
“Stellar” nebulæ have been mostly discovered by Pickering’s method of
sweeping with a direct-vision spectroscope. There is probably no radical
difference between them and planetaries, for their comparative
minuteness may be a simple effect of distance. Or they may be
constructed on a reduced scale. We should naturally expect to meet in
nebulæ a variety of dimensions not inferior to that existing among
stars. Moreover, they are all alike gaseous, and give—so far as is yet
known—perfectly similar spectra. Nevertheless, Mr. Burnham writes of the
stellar kind as “very small, bright, round nebulæ, which in a small
instrument would resemble stars slightly out of focus, but do not appear
to come within the planetary class.”[1010] An admirable specimen was
detected spectroscopically by Pickering 16th July 1882. Previously
registered as a 9·4 magnitude star, it took rank, on the strength of its
bright lines, as a stellar nebula (N.G.C. 6790). The Lick
thirty-six-inch showed it to be round and lucent, with a minute nuclear
point.[1011] A miniature of Struve’s planetary in Ophiuchus seemed to
float in the field of the telescope. Without a slit, the spectrum,
examined by Keeler, resembled three tiny greenish _stars_, that formed
on Hβ being much the faintest. This stellar nebula accordingly is
analogous to Webb’s planetary in the feeble glow of its hydrogen
constituent. Another pseudo-star in Aquila (N.G.C. 6891) disclosed
itself prismatically to Dr. Copeland in 1884. It has a disc just 4″
across, and its spectrum, photographed by Von Gothard 27th October
1892,[1012] includes the usual range of nebular lines up to λ 3727,
besides a fair admixture of continuous light. A nebula of the fourteenth
magnitude, visually a finished star, was noticed for the quality of its
light by Pickering, 25th November 1881, near the star _b_^2 Cygni.[1013]
More conspicuous members of the class have been identified in
considerable numbers on the Draper Memorial plates.
The crowd of small nebulæ photographed by Dr. Max Wolf in 1901 comprised
a remarkable proportion of seeming planetaries. They were collected on
his plates into pairs and groups in a manner recalling the distribution
of Wolf-Rayet stars, but not previously observed to characterise that of
planetary nebulæ. The question indeed arises whether they are really
such? Or do they rather belong to “that much less interesting class of
objects” designated by Mr. Burnham as “small circular patches of
nebulosity”? The spectroscope alone can decide, and its verdict should
be elicited without delay. Upon it will largely depend the conclusions
to be drawn respecting the affinities of the planetary family, their
mutual relations, and the mode of their scattering in space.
There is reason to believe them enormously remote. Four have been
directly measured for parallax, namely, the helical nebula in Draco, the
bi-nuclear planetary in Argo (N.G.C. 2440),[1014] Webb’s in Cygnus, and
a structure with a ring and disc in Andromeda (N.G.C. 7663).[1015] None
showed the least sign of perspective shifting when viewed from opposite
sides of the earth’s orbit; and the demonstration thus afforded of their
immense distance is confirmed by the insensibility of nebular proper
motion. This, since it is continually progressive, must eventually prove
determinable, and comparisons of its amount with the mean _radial_
velocity of these bodies will supply a criterion for their absolute
localisation. Professor Keeler derived from the line-displacements in
the spectra of eleven nebulæ a value for this quantity of sixteen miles
a second; and by sixteen miles a second they should, accordingly, on an
average, progress along each of the other co-ordinates fixing their
position in space. When corresponding angular advances have been
established, their average distance can then at once be estimated.[1016]
Let us assume, for instance, that the eleven nebulæ in question, taken
one with the others, have a secular proper motion of three seconds of
arc in declination, and as much in right ascension. This, although the
outside of what is probable, implies that their mean distance
corresponds to a light-journey of 580 years. A planetary not more than
10″ in diameter would, if thus remote, fill a globe about 600 times
wider in girth than one circled by the orbit of Neptune, and 216 million
times more capacious. The data supplied by Professor Keeler are indeed
an obviously insufficient groundwork for extensive generalisations; but
more of the same kind, and it may be hoped of not inferior quality,
cannot fail to be forthcoming shortly; while the precise visual
measurements executed by Burnham, Spitaler, Javelle, and others will
surely serve, after some decades, for the detection of genuine nebular
journeyings across the sky. A beginning of definite knowledge will then
have been made regarding the true magnitudes and place in the sidereal
scheme of these singular objects. It must not, however, be forgotten
that many of the faint stars taken as fiducial points for their
micrometrical determination may prove to be satellites of the
neighbouring gaseous globes drifting and shifting in their company. This
disquieting possibility, foreseen by Sir John Herschel and d’Arrest,
would, of course, if realised, vitiate conclusions as to parallax or
proper motion. Yet the work done for these purposes need not be looked
upon as wasted. By its aid, should the slow revolutionary movements of
stars round planetary nebulæ ultimately be brought to light, a new
department of sidereal mechanics may be founded. Exact determinations in
astronomy, made on a judicious plan, are rarely useless. If unprofitable
for their designed aim, they are pretty sure to turn to account for some
other, and perhaps a higher one.