WHITE NEBULÆ.
“White nebulæ”—so called by Professor Young[876]—are those giving
continuous spectra. They are in an immense majority. They are reckoned
by thousands, or tens of thousands, gaseous nebulæ by the score. True,
very little progress has been made with their actual spectroscopic
examination, the faintness of their rays forming, in general, an
insuperable obstacle to their analysis; but their shape and aspect
supply indications, rarely misleading, as to the quality of their light.
That of elliptical and spiral nebulæ is, to the best of our knowledge,
always continuous; and with these may be classed the round,
centrally-condensed objects which abound in every nebular region of the
sky. Several other varieties of this great sidereal family might be
indicated, but they are by comparison scantily represented, and have
been but little investigated. The paragon of white nebulæ is the grand
ellipse in Andromeda. No other is visible to the naked eye; it should
be, judging by appearances, much the nearest to the earth of the whole
tribe; its structure is splendidly definite, and profoundly significant;
its spectrum shows peculiarities challenging inquiries which must be
long-continued and arduous, but promise results of far-reaching
importance. In January 1899 Dr. Scheiner,[877] employing a small
spectrograph in combination with a mirror of nearly thirteen inches
aperture, and only forty inches focus—an apparatus specially adapted for
dealing to advantage with objects of extended surface—obtained in seven
hours a legible spectrograph of the nebula. The indications gathered
from it were of a most surprising kind. Dark rays were perceived to
interrupt the continuous light, and they seemed to agree with the
Fraunhofer lines in the solar spectrum. The Andromeda nebula was
accordingly inferred to be a genuine cluster of solar stars; but this
conclusion is very far from being securely established. No bright lines
could be made out in the Potsdam photograph, but many have been _seen_
at Tulse Hill.[878] On 13th November and 11th December 1897, when they
were particularly distinct, approximate wave-lengths were assigned to
six or seven, all of which fall near lines in the Wolf-Rayet stars. The
reality of the coincidences cannot at present be pronounced upon; they
are hinted at rather than asserted; but their verification would enforce
an entire recasting of ideas as to the nature of white nebulæ.
A photograph of the Andromeda ellipse, taken by Dr. Roberts 10th October
1887, set the example, since extensively followed, of resolving into
spirals, with the help of the camera, all sorts and conditions of
nebulæ. It was, indeed, a memorable picture. The vast structure is shown
in it and its successors[879] to be furrowed through and through by dark
channels, or rather by a single continuous channel, winding in
symmetrical convolutions in a left-handed direction from the compact
nucleus outward to the dim, indefinite margin. Thus the nebula is not
simply a concatenation of flat rings separated by vacant intervals; if
it were, the problem of its construction would be less difficult; since
the annular gaps might represent spaces cleared of their contents by
exceptionally acute gravitational disturbance, while the ejection of
matter along a spiral track belongs to a totally different order of
phenomena, and implies the operation of laws scarcely yet brought within
our ken.
The Andromeda nebula is presumably a round disc viewed obliquely. If so,
the angle of its inclination is about 25°.[880] Remarkably enough, the
nucleus does not share the elongation of the surrounding spires, as it
should if it were no more than a flat condensation in their plane. Its
outline is, on the contrary, circular,[881] and its true shape must be
that of a globe. There is no probability that the innumerable stars
strewing the formation have any physical connection with it. Two small
nebulæ in its immediate neighbourhood, on the other hand, certainly
belong to its system. The closer and brighter (M 32) was discovered by
Le Gentil in 1749; the other, which is situated in a nearly opposite
direction, by Caroline Herschel in 1783. Both can be seen with powerful
telescopes to be included within the limits of the primary
agglomeration.[882] Le Gentil’s nebula, indeed, appeared on a Meudon
negative to lie as a condensed knot upon one of its external
spires,[883] and the companion object doubtless owns a similar origin.
The latter is an oval, apparently amorphous mass; its longer axis is
inclined 60° to that of the great nebula. The two satellites may
eventually yield signs of orbital revolution; or the whole disc perhaps
rotates as one piece, and they along with it; we cannot attempt to
decide which condition is the more likely to prevail. Perhaps neither to
the exclusion of the other. It is conceivable that the more remote
member of the system circulates independently, while the inner companion
is borne onward with the general swirl.
Far inferior to this “Ajax” among the nebulæ, although eminent among the
“other Argives,” is a large lenticular object in Cetus (N.G.C. 252),
noticed by Caroline Herschel in 1783. Sir John Herschel[884] considered
its “streaky and knotty” texture to denote resolvability into stars; but
it came out instead as a fine spiral in a photograph taken by Dr.
Roberts 25th December 1899, a reproduction of which is given, by his
kind permission, in Plate XX. The whorls are evidently much
foreshortened. They are studded, as Dr. Roberts remarks,[885] “with
numerous condensations of a stellar character,” while six ordinary stars
are probably seen in projection upon them. Measures of their positions
relative to each other and to exterior stars might serve, he adds, for
the detection of any movements, rotational or translational, by which
the nebula may be affected. Its considerable south latitude brings it
within the spectrographic domain of the Cape Observatory, and the
McClean apparatus might be competent to obtain an impression of a
spectrum sure to prove interesting, if only it can be made distinctly
visible.
A nebulous “ray” in Ursa Major (M 82) was described by Lord Rosse as “a
most extraordinary object, at least ten minutes of arc in length, and
crossed by several dark bands.”[886] These run obliquely to the axis,
and give the nebula—as Mr. Ingall said—“a twisted appearance, like a
distaff of flax.[887] It appears to possess two centres of condensation,
which must lend no slight complexity to its internal economy. Each is
perhaps the starting-point of a separate arrangement of luminous coils,
but no fair view can be got of them; they are foreshortened into mere
broken lines.” The nebula, Dr. Roberts explains,[888] is presented to us
“in section, and the upper and lower surfaces are very rugged.” The
divisions between the rings hence took shape in a negative, to which he
gave three and a half hours’ exposure, 31st March 1889, as “rifts and
attenuated places” not obviously fitting together into a harmonious
plan. The profile of a corrugated disc is not an easily intelligible
object, and that is all that can be seen of M 82. Perspective has done
its utmost to disguise its true aspect. Turned edgewise towards the
earth, it betrays only by its indentations and rugosities the effects of
the ploughing action to which for ages it has been subjected. Swift’s
“hairline nebulæ” belong to the same category. They show as bare streaks
of nebulosity, bulging a little where the nuclei protrude. Presumably
they are flat, circular surfaces, the planes of which coincide with the
line of sight.
The “ray” in Ursa Major is not solitary. It is placed at a distance of
only 42′ from a larger structure (M 81), evidently of the same general
character. The two were photographed together by Dr. Roberts in 1889,
and cannot be wholly disconnected. The primary—if we may call it
so—resembles the great Andromeda nebula, and, like it, was resolved into
a fine spiral. The spectra of both objects were found by Sir William
Huggins to be continuous; but the significant details disguised by
apparent continuity have still to be revealed.
[Illustration:
PLATE XX.
Photograph of a Spiral Nebula in Cetus. Taken by Dr. Roberts, 25th
December 1899.
]
The essential formative law of white nebulæ is unmistakably that of
spirality. This conviction, strongly upheld by the long series of the
Crowborough pictures, was irrefutably established by Professor Keeler’s
photographic survey with the Crossley reflector.[889] Owing to the
strong light-collecting power of the instrument, the harvest of nebulæ
garnered was so plentiful that the number within its reach over the
whole heavens was estimated at no less than 120,000, and nearly all of
these can be inferred, from the preliminary results obtained, to have a
spiral shape. On the Lick plates, in fact, a small compact nebula, _not_
disposed in luminous coils, stood out as a rarity. All spindle-nebulæ
were resolved into spirals viewed aslant, but into spirals of various
degrees of complexity. Some consist merely of two curved branches,
shaped like the letter S, and diverging oppositely from a nuclear
condensation. An object of the kind situated in Pegasus (N.G.C. 7479) is
reproduced from Professor Keeler’s photograph in Plate XXI. Subjoined
are the drawings by J. Herschel, d’Arrest, Lord Rosse, and Tempel, with
which Keeler compared the autograph picture. They make an instructive
study. Herschel saw the object as a narrow spindle “extended between two
stars,” d’Arrest as a lozenge; Lord Rosse perceived, in addition, a mass
of spiral convolutions surrounding a faint star, while Tempel caught the
double effect of a round attached to an elongated patch of luminosity,
but failed to discern their true connection. At last on the Lick plates
the object disclosed itself under an intelligible aspect. “A glance at
the photograph,” Professor Keeler wrote,[890] “shows that the nebula is
a two-branched, left-handed spiral, with a nucleus or condensation near
the point of inflection. The preceding branch is strong and single, but
the following branch is split into two, which cross where their
curvature is greatest, at some distance from the centre of the spiral,
and unite again at their extremities. This appearance in the components
of the following branch, and the fact that the ends of both branches
curve around so as to approach the centre more closely than do the
intermediate parts, are doubtless effects of projection, the plane of
the spiral lying obliquely to the line of sight.”
Lord Rosse’s star occupies the centre of the space fenced round by the
preceding branch (that to the left). “It would be of great interest,”
the Lick astronomer continued, “to know whether this singular position
of the star is accidental, or whether the star and the nebula are
physically connected, and if so, in what way the star was left in its
present position during the process of contraction. On the first of
these questions an investigation of the spectrum, which will be made in
due time” (the time, alas! never came), “may throw light. Assuming for
the present that the star is physically connected with the nebula, it
seems to me possible that the proximity of this star may account for the
unsymmetrical appearance of the spiral, which may be due to an actual
difference in the dimensions of the two branches, or to their lying in
differently inclined planes.”
The first nebula in which a spiral conformation was recognised is still
unsurpassed as a specimen of its class. We are enabled, by Mr. W. E.
Wilson’s kindness, to reproduce in Plate XXII. his fine picture of this
stupendous object. The coils are left-handed; they follow, as they issue
from the nucleus, the line of movement taken by the hands of a watch.
Our view of them is straight and square; they can be little, if at all,
foreshortened. Yet they do not wind symmetrically round their origin.
Their flow is broken and distorted, like the current of a river by
jutting rocks. The spiral is fundamentally double. Two main streams
leave the nucleus at diametrically opposite points, and preserve their
separate individuality until they melt away into the outer darkness.
Their course seems to be prescribed essentially by the combination of an
ejective with a rotatory velocity; disturbances, however, manifestly
supervene. The branches divide and reunite; they are cloven and bossy;
they swerve widely from the circular track. This is especially
remarkable in the case of the longest and brightest arm, which stretches
irregularly outward to join a secondary exterior nucleus. This
circumstance alone suffices to prove that the diffusion of matter in
this formation has been outward. The perturbing mass was undeniably
there before the luminous stream which it diverted began to flow; and
its flow was quite plainly towards it from within. Other indications of
centrifugal action are visible. Mr. Wilson’s photograph shows “cometary
tails curved like a plume away from the central nucleus,” attached to
some of the denser knots on the convolutions of the spiral;[891] and
these effects of apparent repulsion are likewise clearly legible on Lick
and Crowborough plates of the same object.
[Illustration:
PLATE XXI.
1. Photograph of a Spiral Nebula in Pegasus. Taken by the late
Professor Keeler.
2. Drawings of the same Nebula by Herschel, d’Arrest, Rosse, and
Tempel.
]
Indraughts or infalls from space are not here concerned, whereas, in Mr.
T. C. Chamberlin’s words,[892] “the effects of explosive projection,
combined with concurrent rotation, must obviously give rise to a spiral
form.” Each such nebula (and there are tens of thousands of them)
results, in his view, from the “approach without collision” of a roving
star to a compact gaseous mass. Strained to the point of disruption by
tidal influences, this embryo vortex would, at a given moment, project
from both extremities of the ellipsoid into which it had become
elongated, a stream of material curved into whorls through the continual
slackening of its angular rate of rotation; and the double catastrophic
outrush served to constitute a great system of shining spires,
subsequently diversified by the supervening phenomena of minor
outbreaks. This rationale has much to recommend it, and probably rests
upon a substratum of truth; yet the events contemplated in it are on a
small scale by comparison with the grandiose dimensions which we must
ascribe to spiral nebulæ.
Lord Rosse described a nebula situated near the star 83 Ursæ Majoris (M
101 = N.G.C. 5457, 5458), as a large faintish spiral, with several arms
and knots, at least 14′ across.[893] A four hours’ exposure at Lick
brought into view a surprising wealth of intricate details. The
groundwork of the structure agrees closely with that of the great spiral
in the Hunting Dogs. It is composed of two main effusions, sweeping
round from left to right. But they spread, and split, and ramify, drawn
hither and thither by multiple attractions, while preserving in their
complex interlacings, the whirling impress of their origin.
“Three-branched spirals” still survive here and there in catalogues.
Such were supposed to be the delicate objects, M 99 in Virgo (N.G.C.
4254), and M 83 in the head of the Centaur. But the triplicate form
ascribed to them was most likely of optical creation. There is no
satisfactory evidence that it exists in nature. So far as we can judge,
the spiral type originated, by fundamental necessity, through a double
outflow, in contrary directions, from the parent mass. A mode of genesis
is intimated which recalls, though distantly, the diametrically opposed
eruptions not uncommonly witnessed on the sun. It may be added that no
genuine spiral appears to be a simple watch-spring coil. This, to be
sure, is, to some extent, a matter of definition. It depends upon what
we agree to call a spiral nebula. Yet the difference will most likely
prove to be radical between stars with curving trains, like Maia in the
Pleiades, and those cosmic “whirlpools,” every trait of which testifies
to the counterplay of multiple activities.
“Cometary nebulæ” are not very rare, and they present aspects of
considerable variety. The nuclei are not always stellar, nor are the
appendages attached to them in all cases inflected. A few have been
photographed. Thus an object (N.G.C. 1999) 50′ south of ι Orionis was
noticed by Lord Rosse as resembling “a comet coiled into a ring
nebula,”[894] and appeared under the same form on a plate exposed by Dr.
Common with his three-foot (now the Crossley) reflector in February
1883.[895] Its spectrum has not, that we are aware of, been examined.
“Reaping-hook” shapes also occur. West of the Argo nebula, a falcated
and forked tail, 10′ long, was observed by Sir John Herschel to issue
from a granulated, perhaps a double nucleus[896] (N.G.C. 3199). The
inner edge is sharp, but it fades gradually outward. Cometary, too, is
N.G.C. 520. It has an indistinct nucleus and a bifid train.[897] A
nebulous hyperbola with a star near the vertex (N.G.C. 2366) is met with
in Camelopardalis;[898] and fan-shaped appendages to stellar
condensations are a recognised variety of the species. A pair of these
singular objects were photographed by Professor Barnard, 2nd February
1894,[899] in the immediate neighbourhood of the bright-line star γ
Cassiopeiæ. His sketch, showing their positions with regard to it, is
copied in Plate XXIII. Fig. 1. These do not seem to be casual. The
opening out of the two fans straight away from the star suggests an
express plan of orientation. Each nebula is about 15′ in diameter. They
are “excessively faint and dilute,” and almost elude visual observation.
In Professor Barnard’s opinion, they would never have been detected
otherwise than by chemical means.[900] Yet they photograph easily
enough; and if this actinic quality denotes, as seems probable, a
gaseous constitution, mention of them should, properly speaking, be
postponed to a later chapter. Their spectral classification, however, is
likely, for some time to come, to remain matter of conjecture. Dr.
Roberts obtained, with ninety minutes’ exposure on 25th October 1895, an
excellent photograph of the twin fan nebulæ, in which traces of a
luminous connection are apparent.[901]
[Illustration:
PLATE XXII.
Photograph of Whirlpool Nebula (M 51). Taken by Mr. W. E. Wilson, 6th
March 1897.
]
“Rifted” nebulæ must be classed as a variety of the elliptical sort.
They appear either as cloven discs—N.G.C. 5128 in Centaur is an
example—or as parallel rays, such as a bifid streak in Leo (N.G.C.
3628). A probably analogous structure in Andromeda was photographed by
Dr. Roberts in 1891.[902] It came out immensely elongated, and with just
such a “chink in the middle” as had been seen by Sir John Herschel,
whose opinion that the nebula was the foreshortened representative of a
thin, flat ring of enormous dimensions thus received strong
confirmation. Yet the added light-power of the Parsonstown mirror had
extended Herschel’s “chink” into a channel, running from end to end of
the formation. Or rather the retinal impression afforded by it had been
misinterpreted in this sense; for that it was a misinterpretation the
camera incontrovertibly asserts. The error may serve as an illustration
of Professor Keeler’s remark that “the most obvious tendency of the
draughtsman is to prolong a line or curve beyond the point at which it
actually stops.”[903] A suspicion even arises that other telescopic
presentments of rays split throughout their length are similarly
misleading, and that they are, in fact, like the glimmering oval in
Andromeda, rings thrown into perspective. Otherwise why should the dark
rifts always coincide in direction with the major axes of such
formations? If these are really circular discs, they might as well run
across as along them; but they never do. We must then choose between two
inferences. Either the nebulæ are in fact, and not merely by optical
projection, elliptical, in which case a longitudinal line of cleavage
would be intelligible, or they are luminous rings viewed very obliquely.
It must indeed be admitted that the strong development of nuclear
condensations in some rifted nebulæ appears almost to exclude the latter
alternative. Dr. Roberts’s photographs, for instance, of N.G.C. 4565 in
Coma Berenices, and of N.G.C. 4594 in Virgo,[904] exhibit an arrangement
of parts insistently demanding a different explanation. What seems
certain is that no single principle is valid all round. Modifications
must be introduced to meet the exigencies of nature’s endless variety.
The great majority of white nebulæ might be called globular clusters in
disguise. They present a round surface, condensed centrally by
gradations testifying to their true spherical form. The only obvious
distinction between them and “balls of stars” is that they are
irresolvable by any telescopic powers that can be brought to bear upon
them. And the suggestion lies close at hand that this quality depends
wholly upon distance—that round nebulæ are neither more nor less than
remote globular clusters. Yet it cannot be adopted without hesitation.
The space-relations of the two classes of object are very different.
Clusters frequent the Milky Way; white nebulæ avoid it. The discrepancy,
it is true, may be capable of reconcilement, but by a somewhat elaborate
artifice of speculation. Nor is there any immediate prospect of solving
the difficulty by the aid of the spectroscope. We are unacquainted at
present with any criterion for distinguishing continuous nebular light
from that of compressed clusters. One may eventually be found, but its
application must always be a matter of extreme delicacy.
[Illustration:
PLATE XXIII.
1. Fan Nebulæ near γ Cassiopeiæ (Barnard).
2. Drawing of Struve’s Planetary with Spectrum (Keeler).
3. Drawing of Webb’s Planetary with Spectrum (Keeler).
4. Drawing of Annulated Planetary in Andromeda with Spectrum (Keeler).
]
Professor Max Wolf’s explorations of the heavens show them to be strewn
with an incredible number of small faint nebulæ.[905] Directly visible
only by elusive glimpses, they come out individually distinct and
measurable on sensitive plates; and the Heidelberg observer has already
laid his plans for the construction of a photographic catalogue of
nebulæ, likely to be at least twenty times more voluminous than the most
exhaustive visual enumeration. In certain regions he indeed found that
only two per cent of the dim objects delineated on his plates had been
previously recorded. The newly discovered crowd vary greatly in shape.
Some are round and compact; many more are round and diffuse; there are
spirals among them, and spindles, and draped or arched formations. A
surprising number are marked “planetary,” and are hence, presumably,
gaseous. The physical nature of the rest is inferable only when the
definiteness of their shapes prescribe their arrangement in some
established category.
Thus we are able to assert confidently that those disposed along coiling
lines or projected into rays and ellipses, give continuous light; but if
we attempt to go further, and obtain a clear conception as to how the
light originated, embarrassments beset our path. Let us confront them
fairly. The only white nebula of which the spectrum has been observed to
any purpose is the great elliptical spiral in Andromeda. It almost
certainly includes lines or bands of absorption; it is probably marked
by traits of emission as well. Plausibility is thus lent to the opinion
that the nebula is a genuine cluster of stars amalgamated by distance
into a soft haze. The haze, however, shines very dimly; its lustre is
almost evanescent comparatively to that of the sun. If, then, its
component particles are true suns, they must be inordinately far apart.
For the sake of giving some precision to our ideas on the subject, we
will attempt to illustrate this numerically. If we assume the central
parts of the nebula to possess ¹⁄₂₀₀th the intrinsic lustre of the full
moon, or (what comes to the same) ¹⁄₁₂₄ millionth that of the sun, while
consisting of scattered globes of solar brilliancy, it follows that this
also is the proportion between the total bright area covered by their
discs and the dark area of vacancy, the dimness of the nebula measuring
the spread of the interspace. Hence the component stars, taking each to
be half a million of miles in diameter, should be separated from its
next neighbour by an interval of more than 5000 millions of miles, as
seen projected upon a plane perpendicular to our line of vision. Their
real distances, since they are presented to us slantwise, would of
course be very much greater; but with them we are not just at present
concerned. We must next try to form an estimate of how close together
these sun-like bodies should appear to be in order to produce the
observed effect of a smooth luminous surface. If the gaps amounted to
one-tenth of a second, the nebula would certainly, with the powerful and
perfect telescopes now in use, show symptoms of resolvability. Yet none
appear. The fog does not even tend to condense into droplets, and the
temporary star of 1885 stood out, to the last hour of its visibility, by
contrasted light-quality from the soft surrounding glow. Allowing, then,
that the linear intervals of 5000 millions of miles between the
constituent bodies of the Andromeda nebula are represented by optical
intervals of ¹⁄₂₀th of a second, we arrive at a parallax for that vast
structure of less than ¹⁄₁₀₀₀″. In other words, its rays spend about
3300 years in travelling to the earth. At this distance, the stars we
have supposed aggregated in it would appear of fifteenth magnitude. Now
they should in the fainter outlying parts of the nebula be more sparsely
distributed than near the centre, and as they thinned off they would
inevitably appear in their proper guise as fifteenth-magnitude stars.
But the texture of the glimmering haze remains the same in every stage
of attenuation.
It may then be taken as certain that, if this object be of stellar
constitution, it is made up of stars smaller and closer together than we
have supposed; unless we are prepared to lengthen still further, and
very materially, a light-journey already protracted to the verge of the
incredible. It is not, however, easy to conceive that bodies much less
than half a million miles in diameter can be truly sun-like. An
outpouring of light and heat in the profuse measure exemplified by the
sun, implies storage-accommodation on a colossal scale; and the spectrum
of the Andromeda nebula, so far as it can be deciphered, seems to
correspond to a high standard of temperature.
Undoubtedly the path “of least resistance” is to accept the stellar
origin of nebular radiance. It is not entirely practicable, but every
other is impassable. The solar corona presents no real analogy to white
nebulæ, since it is kept incandescent by the potent agency of the sun,
while their glow is self-sustaining. This it can only be—setting aside
the vague possibility of electrical discharges—by the sacrifice of
motion in some form. According to Sir Norman Lockyer’s well-known
hypothesis, the collisions of swarming meteorites supply the evolved
energy; but there is little or no evidence that the cause acts, or would
be adequate if it did act. We, at least, have no experience of its
operation. The only meteoric collisions we know of are with the earth,
which spreads a wide net for the capture of flying cosmic particles.
As an alternative suggestion it may be worth considering whether the
shining of nebulæ might proceed from a very slow loss of circulatory
speed through the resistance of a gaseous medium. A pulverulent
constitution, resembling that of Saturn’s rings, should then be
attributed to them; they would consist of relatively small masses
interfused with some highly subtle aerial remnant, the distinctive
bright lines of which add complexity to the nebular spectrum. But the
velocity of circulation in such structures should increase outward.
Other things being equal, they should accordingly, if arrested motion
were the source of their luminosity, gain brightness with increasing
distance from the centre. The reverse is very markedly the case; but the
attendant conditions are so intricate that the contradiction need not be
fatal to the speculation. It cannot, however, be usefully discussed
apart from a profound study of the dynamical condition of such a
peculiar system as that just indicated; and this we must leave to more
competent authorities.