THE FORMS OF NEBULÆ
Sir William Herschel's celestial surveys first made the classification
of nebulæ practicable. Until he began grinding specula at Bath very
few such objects were known, and those too imperfectly for the
effectual discrimination of their differences. Arrangement presupposes
comparison, and comparison some variety of specimens to be compared,
which became available only through Herschel's scrutiny. The rapidity
and penetrative power of his observations in this field almost passes
belief. He detected with discernment. Discovery and enrolment did not
satisfy him; he was, besides, keen to note analogies and contrasts,
likenesses and dissimilitudes. He could not see without at the same
time setting in order what he saw; and the law of order that commended
itself to him was founded on an evolutionary principle. The contents
of the heavens seemed to fall spontaneously, as he regarded them,
into genetic sequences; and the nebulæ with particular facility. The
criterion adopted was that of progressive condensation. Development
must clearly, he judged, be attended by contraction and local
brightening. Diffused milky tracts represented cosmic formations in
their most rudimentary form; they assumed, through the unremitting
action of gravity in drawing their particles together, a more compact
texture, more definite shapes, and a heightened lustre.
But things have changed somewhat in aspect during the last hundred
years. Herschel's simple regulative plan, although of unquestioned
validity, needs to be supplemented and controlled. Much auxiliary
knowledge has been acquired since it was formulated. In attempting
to estimate the comparative antiquity of nebulæ, we no longer depend
exclusively upon one set of indications. The conclusions drawn from
their immediate inspection can at least be checked by the study of
their spectra and distribution.
The Milky Way might be figuratively described as the nursery-garden
from which the parterres of the universe are stocked. A primitive
condition is usually assigned, not without good reason, to any class
of objects markedly tending to collect in its plane. And this is the
case with gaseous or 'green' nebulæ. Moreover, their materials appear
to be in a highly elementary state (if it be permissible to speak of
one kind of matter as more elementary than another); their spectra
including no rays due to metallic incandescence, but mainly those
of nebulium, hydrogen, and helium. These substances, inconceivably
attenuated, constitute the vast irregular formations placed by
Herschel at, or near, the start of cosmical development. And so far
he has been justified by the outcome of modern research. But he has
not been justified in his description of planetary nebulæ as 'very
aged, and drawing on towards a period of change or dissolution.' For,
despite their determinate shape and definite boundaries, they do not
appreciably differ in composition from nebulæ of the irregular class,
and must be reckoned as, in a manner, coeval with them.
There is, on the whole, a concurrence of evidence that gaseous nebulæ
are at a very early stage of growth. They are the least elaborated of
sidereal objects; they seem, many of them, barely to have crossed the
threshold of creation. Yet their mutual relations in time are by no
means obvious. They cannot easily be disposed in any kind of rational
sequence. Each of the great nebulæ, at any rate, exhibits features and
occupies a position shared by none of its fellows. The most discerning
cosmologist cannot pretend to say that the Argo nebula, say, is of
greater or less antiquity than the Orion or the 'America' nebula.
They are individual growths, simultaneous, not successive. The line
of development suggested by their relationships is rather towards the
formation of star-clusters than of diverse nebular species. Thus, the
Pleiades illustrate not improbably the future condition of the Orion
nebula, the contained stars having gained predominance over their misty
envelopments, though fragmentary swaddling-bands, later, presumably, to
be shaken off, still adhere to many of them.
Planetary nebulæ have much more in common than irregular nebulæ, and
their minor varieties might, with some plausibility, be associated with
differences in relative age. They are marked chiefly by the character
of the nuclear star which, in nearly all such objects, appears to act
as the pivot of the surrounding vaporous structure. The supposition
lies close at hand that it is designed as a provision for the
nourishment of the star--that the star gains in mass and light at the
expense of the nebula, which it is eventually destined wholly to absorb
and supersede. On this view, planetaries like the green glow-lamp at
the pole of the ecliptic (N.G.C. 6,543) should be regarded as the
most advanced, while Webb's planetary in Cygnus (N.G.C. 7,027) would
exemplify an inchoate condition. In the former the central star is of
9·6 magnitude, and sharply stellar; in the latter it is double and
diffuse,[85] perhaps a wide binary system in embryo.
The question is, however, still open as to the real nature of the
connection between planetaries and their central stars. The pabulum
theory is a promising conjecture; but no facts with which we are
acquainted stringently enforce it. Ideas on the subject will need
complete revision if the traces of spirality noted from time to time in
some of these peculiar objects prove to be of radical significance. The
_oculi_, distinctive of the 'Owl nebula' (N.G.C. 3,587), as originally
shown by the Parsonstown reflector, consisted of luminous traceries
coiled round _two_ interior stars,[86] but the appearance was either
due to illusion, or became effaced by change, since the camera has
refused to endorse it as genuine. The 'helical' planetary in Draco[87]
is doubtless essentially a spiral conformation;[88] and Professor
Schaeberle, by means of exposures with a 13-inch reflector of 20
inches focus, has compelled, not only the Ring nebula in Lyra,[89] but
the Dumb-bell in Vulpecula, to betray the surprising secret of their
whorled structure. Both these nebulæ give a spectrum of bright lines,
and inventiveness is at a loss to devise means for building up gaseous
materials into edifices of strongly characterized architectural forms.
The materials, indeed, may not be wholly gaseous;[90] or we possibly
see (as Professor Darwin long ago suggested) merely illuminated
stream-lines of motion furrowing an obscure mass. But if this be indeed
so, there is the further question to be asked: What direction does the
motion take? Do the tides set inward or outward?
Our spontaneous impressions are all in favour of concentrative
tendencies. We cannot easily shake off centripetal prejudices. Our
lives are passed under a regimen of central attraction, and we
naturally incline to universalize our experience. Herschel's scheme
of sidereal evolution invites accordingly at first sight ready
acceptance. Stars seem as if they could not act otherwise than as
foci of condensation in nebulæ; the lucid stuff involving them must,
apparently, with the lapse of ages, settle down towards their surfaces,
and become absorbed into their substance. Such processes, indeed,
belong, unless counteracted by different modes of action, to the
inevitable order of nature; but these may, and probably do, exist.
From sundry quarters the conviction is pressed upon us that cosmic
bodies can drive out matter as well as draw it in. Repulsive forces
insist upon recognition, and their effects become more palpable the
more attentively they are considered. Under certain conditions they get
the better of gravity; and stars may possibly, like cocoon-spinning
insects, expend their organic energies in weaving themselves faintly
lucent envelopes, the products of subtle and unaccountable activities.
The example of Nova Persei is fresh in every mind, but we make
no pretension to decide the controversy it raised. A dogmatic
pronouncement is unadvisable where the unknown elements of the question
obscure and outweigh those that are known. A less slippery foundation
for reasoning is afforded by the permanently visible spiral nebulæ, and
features charged with an emphatic meaning have been revealed in them by
photographic means.
Looking at the entire contents of the nebular heavens, we find the
spiral type very largely predominant. It claims more specimens, and
emerges more distinctly with each development of delineative power. Its
chief prevalence is among 'white' nebulæ, showing continuous spectra.
They are vastly numerous. Gaseous nebulæ are reckoned by the score,
white nebulæ by tens of thousands. Moreover, they collect near the
poles of the Milky Way,[91] while the gaseous variety crowd towards
its plane, both branches of the family thus manifesting galactic
relationships, though of an opposite character. Now, these facts of
distribution have some bearing on the question of relative age. There
is, as already remarked, a consensus of opinion that objects showing
a marked preference for the Milky Way are in a more primitive state
than those withdrawn from it, and the inference is supported by the
circumstance that nebulæ situated in high galactic latitudes shine with
continuous light, those near the galactic equator with vivid lines. Yet
it would be rash to assume that any individual nebula traverses these
successive stages. The series would be satisfactorily established only
if we could point to a number of intermediate instances, which seem
to be almost wholly lacking. We cannot trace in nebular as we can in
stellar growth the insensible gradations of progressive change. They
are perhaps complicated in nebulæ by influences of a different kind
from those which have gained the ascendancy in stars. Diffusive effects
may in them be more conspicuous than concentrative effects;[92] or a
balance may be temporarily struck between antagonistic tendencies.
Spiral conformation is the real crux of nebular cosmogony. The
conditions from which it arises are met with only in the sidereal
heavens, but are there widely prevalent. Though remote from our
experience, they are fundamental in the realms of space. If we could
define and comprehend them, we should be in a better position for
determining the cosmical status of nebulæ.
The choice is open between two rival theories of nebulous spirals.
The first is the more obvious, and readily falls in with admitted
mechanical principles. Sir Robert Ball has adopted and ingeniously
advocated this view.
A globular collection of promiscuously revolving particles inclines,
if left to itself, to flatten down into a disc. The reason is this: In
a system of the kind moment of momentum is invariable, while energy
constantly diminishes. To render the contrast intelligible we have
only to consider that moment of momentum is the algebraic sum of all
the products of mass and motion in the aggregation, reduced to, or
projected upon, its 'principal plane,' while energy is independent of
the varied directions of velocity. Collisions consequently involve no
diminution of moment of momentum, but combine with radiative waste to
produce a steady loss of energy. Inevitably, then, the system will
assume the form in which it possesses the minimum of energy that is
consistent with the maintenance of its original momentum; and it is
that of a disc extended in the principal plane. Retrograde movements
will by the time this shape is definitively arrived at have become
eliminated; the constituent particles circulate unanimously in one
direction; and Sir Robert Ball adds that their circulation, owing to
the more rapid rotation of the central mass, is along spiral paths.[93]
They would accordingly present the twisted conformation so commonly
observed in the heavens, and might even include subordinate centres of
attraction, fitted to ripen and strengthen into a full-blown retinue
of planets. Such are spiral nebulæ regarded in their direct mechanical
aspect. Spherical nebulæ are their immediate progenitors; suns, with or
without trains of dependent worlds, their lineal descendants.
Let us, however, consult some autographic records and weigh attentively
what these peculiar objects tell us about themselves. We see at once
that their curving lines, far from being laid down at the dictate of
chance, follow a strictly defined plan. Spiral nebulæ are not formed
like watch-springs by the windings of a single thread. They are always
two-branched. From opposite extremities of an elongated nucleus issue
a pair of nebulous arms, which enfold it in double convolutions.
Their apparent superposition and interlacements occasion, in the Lyra
nebula, the noted effect of a fringed and ruptured annulus, and it is
of profound interest to perceive that even in gaseous masses the same
constructive rule prevails as in the great Whirlpool in Canes Venatici.
Yet this circumstance is well-nigh irreconcilable with the hypothesis
that an influx of material is in progress.[94] Falls due to gravity
could not be limited to two narrow areas on the central body. Matter
ejected from it might, on the other hand, quite conceivably follow this
course. Interior strain could easily be supposed to cause yielding
along a given diameter, and nowhere else. Solar disturbances partially
and dimly illustrate such a kind of activity. Diametrically opposite
prominences are not unknown. They indicate the action of an explosive
force right across the solar globe. Similarly, the formation of a
spiral nebula cannot be rightly apprehended otherwise than as the
outcome of long-continued, oppositely-directed eruptions.
The history of the heavens involves the law of spirality. The scope
of its dominion continually widens as research becomes intensified.
The Huygenian 'portent' in the Sword of Orion now figures as merely
the nucleus of the 'great winding Nebula' photographed by Professor
W. H. Pickering in 1889. That the vast nebulosity encompassing the
Pleiades is an analogous structure seems eminently probable, though
the brilliancy of the enclosed stellar group obliterates most traces
of its ground-plan. The magnitude of the mixed system, we are told by
Professor Barnard,[95] who detected it in 1893 by means of a ten hours'
exposure with the Willard lens, transcends our powers of realization.
It covers 100 square degrees of the sky with intricate details. Again,
some four minutes of arc to the north-west of the Ring in Lyra lies
a small nebula discovered visually by Professor Barnard in 1893, and
photographically resolved by Keeler into a delicate spiral. It is a
two-branched, left-handed spiral, as the large adjacent object has also
proved to be. One is, in fact, the miniature of the other, and they
are now shown, by Professor Schaeberle's short-focus reflector, to be
linked together by curving folds of nebulosity into a compound spiral
system. The Dumb-bell is held, on the same authority, to be similarly
conditioned, and the analogy frequently noted between its aspect and
that of the Ring nebula has thus become incalculably widened in scale.
The galactic relations of the Magellanic Clouds are not easily defined.
They are within the Milky Way, yet not of it. Enigmatical excrescences
upon the universe, they suggest an origin from gigantic eddies in
the onflowing current of sidereal arrangement. Their miscellaneous
contents are, to all appearance, disposed along eddying lines. Mr. H.
C. Russell's photographs[96] rendered this, in 1890, to some extent
manifest, and their indications were ratified by the Arequipa plates,
from the study of which Professor Pickering gained the conviction that
the great Looped Nebula, 30 Doradûs, is the structural nucleus of the
Nubecula Major. 'It seems,' he wrote,[97] 'to be the centre of a great
spiral, and to bear the relation to the entire system that the nebula
in Orion bears to the great spiral nebula which covers a large part of
that constellation.'
On all sides, in the sidereal heavens, we can discern the signs of the
working of a law of convolution. Sometimes they are patent to view;
sometimes half submerged; but they can generally, with attention,
be disentangled from overlying appearances. They are exhibited by
stars no less than by nebulæ, as the late Dr. Roberts pointed out
from convincing photographic evidence; the 'hairy' appendages of
globular clusters betray them by their curvilinear forms; they meet
us in every corner of the wide nebular realm. Many investigators
recognise in the Milky Way itself the stamp of spirality. Stephen
Alexander, of New Jersey,[98] regarded the majestic galactic arch as
a four-branched spiral, resulting from catastrophic breaches in a
primitive, equatorially loaded spheroid, the currents of matter ejected
by which should, owing to their lower angular rotation, lag behind as
they retreated from the nucleus, and thus flow along helicoidal lines.
R. A. Proctor subsequently devised convoluted galactic streams, which,
however, corresponded imperfectly with what the sky showed. And Dr.
Easton[99] has designed by way of simple illustration an elaborate
series of spires, originating possibly from a central galactic
condensation, the projection of which upon the sphere may, he thinks,
account for the known peculiarities of the Milky Way.
Our interior situation, nevertheless, makes it extremely difficult to
determine the real relations in space of the star-streams circling
around it. The observed facts are, perhaps, equally compatible with
many other structural schemes besides those based on the idea of
spirality; and it will be prudent to adopt none, for the present,
with settled conviction. We can, however, gather one sufficiently
definite piece of information regarding the history of the Cosmos.
All the inmates of the heavens, stellar and nebular, represent quite
evidently the débris of a primitive rotating spheroid. Its equator is
still marked by the galactic annulus, its poles by a double canopy
of white nebulæ. The gyrating movement which it once possessed as a
whole doubtless survives in its parts, but ages must elapse before the
fundamental sidereal drift can be elicited.
FOOTNOTES:
[Footnote 85: Keeler, _Lick Publications_, vol. iii., p. 214.]
[Footnote 86: Rosse, _Transactions Royal Dublin Society_, vol. ii., p.
93.]
[Footnote 87: First detected as such by Holden and Schaeberle in 1888,
_Monthly Notices_, vol. xlviii., p. 388.]
[Footnote 88: Deslandres, _Bulletin Astronomique_, February, 1900.]
[Footnote 89: _Astronomical Journal_, Nos. 539, 547.]
[Footnote 90: Maunder, _Knowledge_, vol. xix., p. 39.]
[Footnote 91: Dr. Max Wolf places the point of nebular concentration in
R.A. 12h 53m, D. +61° 20´, that assigned to the galactic pole being
in R.A. 12h 49m, D. +62°. _Königstuhl Publ._, Bd. I., p. 174.]
[Footnote 92: T. J. J. See, 'Repulsive Forces in Nature,' _Popular
Astronomy_, No. 100, December, 1902.]
[Footnote 93: _The Earth's Beginnings_, pp. 243-247.]
[Footnote 94: _Cf._ Moulton, _Astrophysical Journal_, vol. xxii., p.
165.]
[Footnote 95: _Monthly Notices_, vol. lx., p. 259.]
[Footnote 96: See _Knowledge_, vol. xiv., p. 50.]
[Footnote 97: _Harvard Annals_, vol. xxvi., p. 206.]
[Footnote 98: _Astronomical Journal_, vol. ii., p. 100, 1852.]
[Footnote 99: _Astrophysical Journal_, vol. xii., p. 158.]