THE NATURE OF NEBULÆ.
The relations of white to green nebulæ are obscure. Unitive links
between the two classes have yet to be established. In most respects
they stand at present widely apart. They present a superficial likeness,
but their dissimilarities seem to be radical. They are unconnected by
any marked spectral affinities; they differ organically in structure;
their distribution on the sphere is regulated by opposite principles.
Hence their genealogical precedence remains unsettled. It would be rash
to say that either family developed from the other, or even that they
are collateral offshoots from a common stock. That a line of continuity
will, sooner or later, become traceable is more than likely, but we must
wait for the guidance of facts with regard to it; premature divinatory
efforts are usually good for less than nothing.
As to the constitution of white nebulæ, we seem on the verge of knowing
something definite. Premonitions of their being a species of
fine-grained star-cluster have become audible. The subject, however, is
not ripe for discussion. A comparatively advanced stage has, on the
other hand, been reached by the problems connected with gaseous nebulæ,
since their spectra let us, to a certain extent, into the secret of
their composition. This, on the whole, seems to be remarkably uniform.
Individual differences, it is true, both physical and chemical,
distinguish the various members of the class; but they are of a
subordinate kind. We may then safely attempt to generalise as regards a
few of their more obvious properties. Three of these can be at once
enumerated:—
(1) Gaseous nebulæ are almost perfectly transparent.[1171]
(2) They shine with extreme feebleness.
(3) Their mass is vanishingly small in proportion to their bulk.
We will take each point separately. That nebulæ offer no appreciable
obstacle to the transmission of light is attested by the unaltered
radiance of stars shining through them. No absorption that can possibly
be due to the cosmic fog in which they are plunged to depths of many
millions of miles, is traceable in the spectra of such objects as Θ
Orionis, of ρ Ophiuchi, of Maia or Merope in the Pleiades. Similarly,
the central stars of planetary nebulæ shine through an interposed
medium, the extent of which is measured by the radius of each gaseous
globe; and this, by a rough minimum estimate, can rarely be less, and
must often be a great deal more than 50,000 to 60,000 million miles. Yet
from the heart of these extraordinary formations the light of their
nuclei comes to us, so far as it is possible to judge, absolutely
intact. The impotence of comets for light-stoppage is thus vastly
enhanced in nebulæ.
The feebleness of their luminosity is a matter of direct observation. A
shining superficies, unlike a shining point, loses none of its lustre
with increased distance. Its area of course diminishes according to the
law of inverse squares, but every minute element of that area continues
to radiate with the same intensity as before. The sun, for instance, is
no less _bright_ as viewed from Neptune than when it crosses the
meridian of Khartoum, but it is 900 times _smaller_. So with the nebulæ.
They are really as faint as they appear. Using the best available data,
Mr. Ranyard arrived at the conclusion that a planetary of the most vivid
kind emits per square mile less than (1)/(22,000) millionth of the light
sent abroad by the solar photosphere. This implies—adopting the result
of Langley’s experiment at Pittsburg—that white-hot iron glows at least
4,000,000 times more powerfully than the bluish disc of the “Saturn” or
the “Owl” nebula. Moreover, the differences in areal lustre between one
nebula and another represent actual varieties of emissive strength.
Remoteness has nothing to do with producing them. A _debilissima_—a
“breath-stain” on the sky—may be as near to us as the great hiatus in
the vault through which Huygens half-imagined the blazing of empyrean
fires.
Finally, nebulæ being prodigiously voluminous and of apparently
insignificant mass, must be of exceedingly low mean density. This
fundamental fact was realised with uncompromising distinctness by Mr.
Ranyard in 1892.[1172] Taking, for illustrative purposes, the Orion
nebula to be a sphere 20′ in diameter, composed uniformly of materials
1,000,000 times rarer than atmospheric air at sea-level, he found that
its mass would be such as to impart, to a body at a distance from its
centre equal to that of α Centauri from ourselves, a circular velocity
of 180, or a parabolic velocity of 255 miles per second. In the
neighbourhood of the nebula, accordingly, there should be a marked
prevalence of large proper motions. A star, travelling across the line
of sight under the influence of its attraction at the rate of 100 miles
a second, would, it was shown, have an annual displacement on the sphere
of no less than 25·5″, and this independently of remoteness. For with
the same angular dimensions, the solid contents of the nebula would
increase as the cube of the distance assumed for it; while the seeming
velocities of bodies in gravitational dependence upon it would undergo
no change. This is rendered obvious by a moment’s consideration. For
take any given star circulating round its centre of gravity at the rate,
let us suppose, of 100 miles a second. And let us further suppose, to
begin with, that the distance of the system is such that light would
spend ten years on the journey thence to our eyes. Let us now double
that distance and follow out the consequences. First, the nebula is
eight times more massive than would have comported with the previous
arrangement. Next, the revolving star is twice as far from it as before,
since the apparent interval has not changed. Whence we easily gather, by
the application of Kepler’s third law, that it now moves with double its
previous speed. But its distance has, by hypothesis, also been doubled;
consequently, its proper, or apparent motion remains just what it was.
If, then, the stars about the Orion nebula were really in swift
circulation, they should appear to be conspicuously progressive. This,
however, is so far from being the case that the region is one of
exceptional fixedness. The spectroscopic information at command is to a
corresponding effect. The six brightest stars of the constellation,
measured at Potsdam in 1892, proved to be all affected, in varying
degrees, by the retreat of our system from that locality of the heavens,
but gave no signs of travelling rapidly on their own account. The
conclusion is inevitable that the Orion nebula—and it may be accepted as
typical—contains inestimably less matter than should be comprised by it
if its average density were that of a Crookes vacuum. Mr. Ranyard,
indeed, assigned to it a consistence not exceeding (1)/(10,000)
millionth that of air at standard pressure, which, he continued, “would
about correspond to the mean density of the solar nebulous mass,
supposing it to have been spherical when its radius was a little more
than 107 astronomical units, or when the sun occupied a sphere with a
radius of a little more than three and a half times the distance of
Neptune.” The potential solar system in those days lay muffled in the
haze of a small planetary nebula.
Yet it is impossible to conceive of nebulæ as formed simply of matter in
an aerial condition. They are no mere vague effusions. They possess
definite and characteristic structure. Lord Rosse,[1173] indeed, thought
sharpness of contours distinctive of the gaseous kind. Mr. Maunder[1174]
speaks of their “strange and complicate shapes, showing here and there
strongly-marked outlines”; and he adverts to the difficulty of
explaining this peculiarity in vast, uncontrolled extensions of rarefied
gas. The abolition of this incongruity was one of the strong points of
Sir Norman Lockyer’s “meteoritic hypothesis” of nebular constitution.
The spectroscope, it is true, pronounced against it; nebular chemistry
has very little in common with the chemistry of “uranoliths”; yet amid
much that was precarious or unsound, the valuable idea was introduced
that a proportion of solid matter must enter into the composition of
nebulæ. Its condition and distribution, however, remain unknown. It
would seem to be devoid of light, for the faint continuous spectrum
accompanying the nebular bright lines would be displayed even by a
homogeneous gaseous mass, unless its radiations were of purely
superficial origin.[1175] But here we meet the unresolved enigma of
nebular luminosity. How do they shine? Is it through the direct agency
of heat? Experimental evidence does not countenance this view. In the
laboratory, hydrogen and helium can be induced to give out their
characteristic rays only under the stress of electrical excitement. The
concomitant high temperature might—as Sir William Huggins pointed out in
1891[1176]—prevail only along the path of the discharge, while the
surrounding gases remained cool, producing inequalities in
heat-distribution similar to those believed to exist in vacuum tubes.
If, on the other hand, nebular emissions were of simply thermal
production, the whole radiating mass should be at nearly the same pitch
of incandescence; for if the temperature were lower in some than in
other of its parts, absorption lines or reversals would betray the fact,
and the nebular spectrum bears no legible marks of selective
light-stoppage. Yet it is eminently improbable that formations so
circumstanced are, in their entirety, excessively hot. Their solid
inclusions should, if they were, glow powerfully, and strong continuous
radiance would replace the dim band, grey through faintness, actually
seen. On the supposition, however, that electrical discharges cause the
glow of nebulæ, their average temperature, judging by Dr. Scheiner’s
experimental results, might approximate to absolute zero.
The word “temperature,” indeed, when applied to matter in the last stage
of attenuation, has an extremely dubious meaning. Taking the kinetic
theory of gases to represent the literal truth, we find the effects of
heating upon them to be twofold, namely, increase (1) of translatory,
(2) of internal energy, the latter being perhaps a consequence of the
former. Since, then, their constituent particles travel faster when heat
is applied, they come into mutual collision more frequently and more
violently. That is to say, if their number per unit volume remains the
same. Let, however, the density of the gas be reduced, other things
being unchanged, what will ensue? There will be fewer encounters in
equal times, but they will _not_ be less violent, since the mean rate of
the molecules has undergone no alteration. Hence, apart from one
qualifying circumstance, the same species of vibration should be set
going as when the gas was denser and equally hot. The qualifying
circumstance is this. During the lengthened intervals between the
collisions, the molecules are radiating—that is, imparting energy to the
ether. If their free paths were sufficiently protracted, they would—if
that be possible—lose all they possess, and need to be fully restocked
at each encounter. The comparative isolation of the molecules would
accordingly result in their being mostly dark, and what we must call
cold; so that the extreme subtlety of matter seems incompatible with a
high temperature in any intelligible sense of the term. But there is
more. Reduction of pressure would have for its immediate result a
general diminution of luminosity. In all probability, it would also
produce a selective effect, for if, as seems likely, certain modes of
molecular vibration are more persistent than others, they would, with
the progress of cooling after each impact, tend to predominate, and the
balance of intensity among the spectral lines of a glowing gas would
thus be sensibly altered. Here, then, an explanation might be sought of
the spectral anomalies of hydrogen in nebulæ. Nevertheless, it does not
run quite smoothly, since up to the limit of possible rarefaction in
vacuum tubes the crimson line of hydrogen gains consistently in
strength.[1177] A corresponding difficulty presents itself in connection
with the helium spectrum.[1178] “Green tubes” are at a higher degree of
exhaustion than “yellow.” Yet in nebulæ the green ray is invisible;
nebular helium radiates only D_{3} with a few members of the same “set.”
By means of artificial exhaustion, accordingly, the nebular spectrum
cannot apparently be imitated. The only experimental clue to its origin,
in fact, is in Professor J. J. Thomson’s observation (already mentioned)
that F and C are of interchangeable intensity at the negative and
positive poles of a hydrogen tube.
To recapitulate. Two fundamental problems regarding gaseous nebulæ press
for solution—one connected with their structural forms, the other with
the nature of their light. Do they continuously fill the spaces they
appear to occupy, or should we figure them to ourselves as collections
of discrete bodies comparatively wide apart? In the former case the dark
gaps and chasms which form one of their leading features should be
ascribed, not to the absence of matter, but to defect of shining power,
illuminative, not architectural contours being disclosed by them. In the
latter, chiaroscuro effects would be reliable indices to the
distribution of material.
Secondly, we are confronted with the mystery of their shining. Why and
how are they lucent? Are they bright as a consequence of thermal or
electrical stimulation, or is their radiance some undefined species of
luminescence or phosphorescence? The answer must be given in view of the
three following peculiarities of their spectra: (1) The leading hydrogen
line in them is F, not C. (2) Helium shines in them as if in a “yellow”
tube. (3) Reversals are absent; lines of absorption make no assured
appearance, either as superimposed upon, or as subjacent to lines of
emission. None of these points are easy to decide. Their consideration
involves doubts and queries of an abstruse nature; but it will not
therefore be neglected. Indeed, such a region of inquiry as is here
presented, where we feel that at every step the Unknown may merge into
the Unknowable, has a particular and an illimitable fascination.