THE ORION NEBULA.
The first place among irregular nebulæ is, by universal consent,
accorded to the gleaming formation in the Sword handle of Orion.
Although incidentally referred to by Cysatus of Lucerne in 1618, it
received little attention until Huygens, in 1656, affixed his note of
admiration, and executed a drawing of the bright central part, still
known as the “Huygenian region.” Here is situated the trapezium, the
hub, as it might be called, in which all the spokes of the great wheel
are inserted. A photograph of the group is reproduced in Plate XXIV.
Fig. 2. It was taken by Professor W. H. Pickering from Mount Wilson,
California, 29th September 1889, with the thirteen-inch Boyden
telescope. The exposure allowed was only ten minutes, and already the
enveloping haze was beginning to cloud the images of the stellar
sextett. The companions (which seem to vary in light[1071]) of the two
brighter stars are distinct in the original negative, and can be made
out in the figure, one as a tiny blotch, the other as a mere deformation
of the lowest and largest disc. The physical association of these six
stars may be assumed without much risk of error; but there is nothing to
show that any real tie exists between them and four adjacent
star-points, at the limit of vision with the Lick thirty-six-inch,
detected by Barnard and Alvan Clark in 1889.
The nebula, as it developed on a plate exposed, under the same
circumstances, during 2^h 36^m, is shown in Plate XXVI. The stars of the
trapezium are here completely submerged; only their influence can be
traced, or suspected, in the symmetrical arrangement of the expansive
wings of light stretching away from their place. These are by no means
vague or indefinite outflows. Some of the long streamers have sharp
inner edges, peculiarly curved and notched. And the texture is generally
filamentous, like that of solar prominences, the characteristic forms of
which—as Mr. Ranyard effectively pointed out[1072] are faithfully
imitated in some of the minor features of the nebula. An outlying mass
to the north (N.G.C. 1977) not only claims affinity by some degree of
structural resemblance, but is seen, on Professor Pickering’s plate, to
be linked on to it by a faint intermediate extension. The gap between
them is absolutely black to telescopic vision.
[Illustration:
PLATE XXVII.
Nebulous Formation in Orion. Photographed, 14th January 1890, by W. H.
Pickering.
]
Improvements and modifications in photographic methods have led, by
successive steps, to a remarkable increase in the compass assignable to
this extraordinary formation. Portrait-lenses have played a leading part
in the process. Professor Wadsworth’s reasonings[1073] have made it
clear that small apertures, owing to the greater contrast afforded by
them with the sub-sensible illumination of the sky, are positively
advantageous for obtaining impressions of faint, extended objects. They
have the further prerogative of a wide field of view, giving room for
the grasp and delineation of large contours; so that, in certain
branches of celestial portraiture, they render indispensable and
invaluable services. By their aid the nebulous stars ι and c Orionis
were proved to be dimly connected with the trapezium nebula; a sharply
indented streamer became apparent, issuing towards it from the belt
star, ζ Orionis, long recognised as a separate focus of nebulosity;
while Max Wolf derived evidence of a luminous union between the nebulæ
surrounding ζ and ε Orionis.[1074] Finally, the combined picture, shown
in Plate XXVII., of all the nebulous patches in the constellation was
obtained by W. H. Pickering, 14th January 1890.[1075] The instrument
employed was a Voigtländer lens, 2·6 inches in aperture, and of 8·6
inches focal length; the time of exposure, 6^h 22^m. As the upshot, the
fragmentary condensations previously known became united into a vast
spiral formation, 15° across. Starting from near Bellatrix (which lies
outside the field), it sweeps round by κ Orionis and Rigel to the south,
then bends upward to η Orionis, and most probably effects a junction
with the nuclear mass in the Sword handle, although the further course
of the stream is rendered indistinct by the fogging of the plate. After
an interval of nearly five years, Professor Barnard independently
repeated and confirmed the Mount Wilson experiment.[1076] His small
“lantern lens” showed, with exposures of 2^h and 1^h 15^m, “an enormous
curved nebulosity encircling the belt and the great nebula, and covering
a large portion of the body of the giant.” In comparison with it, he
remarked, the old trapezium nebula “is but a pygmy.” The astonishing
extension given to the luminous structure in Orion was, however, of less
importance than the disclosure of its architectonic plan. Nor can it be
supposed that we are, even now, fully acquainted with it. Further
developments may be looked for. The nebulous connections of the various
parts will doubtless be more clearly expounded in future photographs.
Possibly an effective delineation might be obtained in sections with
instruments, like the Crossley or the Meudon reflector, too restricted
in angular grasp to embrace the whole at one view. The study of details
would thus be made feasible, their subordination to the general design
being at the same time kept well in sight. The relations of the members
to the main body of this nebula offer a problem of extreme complexity.
The stars of the trapezium, for instance, have an appreciable proper
motion of about 5″ a century. The nebulous plumage attached to them must
evidently partake in their displacement; but this cannot be
unhesitatingly asserted of the appendages to the belt stars, still less
of the ocean-river of nebulosity flowing outside them. Professor W. H.
Pickering discussed the movements of nineteen stars involved in the
spiral, though not of course necessarily connected with it.[1077] And so
far as any common character could be ascribed to them, it was that of
recession from the trapezium. It would be ridiculous to found any
conclusion upon so flimsy a basis; yet the indication harmonises with
hints, or nascent symptoms of a centrifugal tendency among clustered
stars such as the Pleiades. The determination and comparison of their
radial movements will be the best means of promoting knowledge on the
subject; and since Pickering’s nineteen stars are all, except one,
brighter than 6·2 magnitude, their spectroscopic measurement should
present no difficulty.
Meanwhile the original “Fish-mouth” nebula raises, even when treated
apart, questions of extreme intricacy. Seventy of the stars scattered
through it were photographically determined by Scheiner, and compared in
1898[1078] with Gould’s similarly deduced places for them. The close
agreement of the results showed them to have remained sensibly
stationary during about a score of years, not sharing the drift of the
central group. They must accordingly become, in course of time, detached
from it, and from the encircling nebula. They constitute then no
physical part of the structure, but are scattered casually over its
surface. The trapezium-stars, on the contrary, are really embedded in
the nebula. And the grounds for this assertion are twofold. Visual logic
by itself certifies its truth. The wings of a bird do not start more
definitely from the breastbone than the nebulous plumes from the stellar
group at their base. The relation is emphasised by the circumstance that
two of its members serve as the abutments of a luminous arch,
photographed at Potsdam,[1079] which spans the interval between them. A
similar nebulous bridge was remarked by Dr. Scheiner to connect two
other stars in the outskirts of the formation. Then, as we have already
seen, the spectrum of θ Orionis presents peculiarities indicative of a
close affinity with the nebulous masses around, and indeed suggests,
albeit doubtfully, an actual commingling of luminous substance. It
follows that the nebula shares the proper motion of the stars, the small
secular value of which (5″), at the probably vast distance of the
involved objects, corresponds to a very considerable real velocity.
Admitting for a moment the correctness of Professor Pickering’s estimate
of one thousand years for the duration of the light-journey from the
Orion nebula hither,[1080] we should have to ascribe to the trapezium
and its belongings a lateral speed of forty-nine miles a second, the
greater part of which would be inherent, since its direction precludes
the supposition that it is a perspective effect of the solar journey.
The receding radial motion of eleven miles per second, determined for
the nebula by Keeler, seems, on the other hand, to belong almost
entirely to the sun. Yet the line-displacements by which it is indicated
vary, in some degree, from tract to tract of the formation. Recent
spectrographic measures executed at Potsdam[1081] imply the progress of
interstitial movements, the nature and laws of which it will be of the
highest interest to ascertain.
Thirty bright lines, from λ 5007 up to the seventeenth member of the
hydrogen series, have been photographed in the spectrum of the Orion
nebula, besides the baffling groups faintly apparent on the Tulse Hill
plate of 1888. The yellow helium ray (D_{3}), visually detected by Dr.
Copeland in 1886, has at least three more refrangible associates; but
several lines met with in planetary nebulæ are missing, notably those
distinctive of the Wolf-Rayet class of stars. Thus the Pickering and the
Rydberg series are alike unrepresented in this grand object. Nor does it
glow with the light of any metallic vapour. Its hydrogen emissions are
in some respects peculiarly conditioned. The red line is invisible; the
green and blue lines are especially bright; beyond them there is a
progressive decrease of intensity, though the complete series, up to Hρ,
was photographed by Mr. S. A. Mitchell, with a concave grating of 11½
square inches, mounted on the twelve-inch refractor of the Yerkes
Observatory.[1082] Thus the state of the nebula is such as to favour,
apparently, the development of the quicker luminous vibrations. The
inference has, indeed, been controverted. Dr. Scheiner maintained that
the suppression of C in this spectrum is “due to purely physiological
causes, and warrants no conclusions as to the physical conditions of
luminosity,” in nebulæ.[1083] He regarded it, in fact, as an
illustration of the “Purkinje effect,” by which the eye, owing to its
differential colour-sensitiveness, loses sight of a red sooner than a
green ray, when the light-source from which both proceed is gradually
enfeebled. In the laboratory, accordingly, F survives alone in the
spectrum of a hydrogen-tube at minimum illumination. The plausible
deduction that there is no objective reality in the anomalous variations
of comparative strength affecting the hydrogen lines derived from
celestial objects is, nevertheless, untenable. It collapsed hopelessly,
on being confronted with the simple fact, noted by Keeler,[1084] that
the third hydrogen line (Hγ) always vanishes experimentally before the
first, while in nebulæ it shines unfailingly, although F be
imperceptible. The order of brightness, then, of the members of the
series is not, in nebulæ, prescribed by physiological causes alone; the
faintness of Hα is intrinsic. This is further proved by Professor
Keeler’s observation of “Hβ, and several of the hydrogen series above
it, glowing brilliantly in the spectrum of R Andromedæ, while not a
trace of Hα could be found.” So that “in some of the variable stars we
seem to have hydrogen in the same condition as in the nebulæ.”
Finally, Professor Campbell made the decisive experiment of directly
comparing the nebulæ with an artificial spectrum.[1085] The two kinds of
light being admitted through the upper and lower halves respectively of
the same slit, their spectra were seen side by side in the eye-piece,
and the corresponding lines in them could be at once equalised by merely
altering the distance of the hydrogen-tube from the slit. Combined
experiments with this apparatus by Keeler, Wright, and Campbell proved
that (1) when the F lines from the nebula and tube were matched in
brightness, the blue line above it (Hγ) was markedly stronger in the
nebular than in the tube-spectrum; (2) the equalisation of the blue
lines left the green line from the nebula conspicuously fainter than the
same line from the tube. “The relative intensities,” it was concluded,
“of the hydrogen lines from the nebula and from the tube are, therefore,
not the same; the nebular lines are relatively the stronger toward the
violet, the lines from the tube are relatively the stronger toward the
red end of the spectrum.” Absolute measures showed the three principal
lines in the spectrum of the Orion nebula to be of very low intensity.
Their faintness completely neutralised, to Professor Campbell’s eye,
their differences of tint. Hence the “Purkinje phenomenon,” which
depends upon the perception of colour, cannot in any degree affect their
comparative visibility. There is, however, another aspect to the
question. The spectrum of the nebula varies, not alone from the standard
of comparison supplied by the vacuum-tube, but also locally, within the
formation itself. The relative strength of the constituent rays is
different for its different sections. Dr. Runge of Hanover,[1086] who
devoted special attention to the point during a visit to the Lick
Observatory in September 1897, satisfied himself that the F line of
hydrogen, which, near the trapezium, had only one-third (or possibly
two-fifths) the intensity of the nebulium line, was ten times brighter
in the faint outlying sections of the nebula. The relative gain was then
twenty-five to thirtyfold; while the alleged physiological cause,
admitting that it had full scope and play, could at the utmost have
produced a gain of 1·8 times. The reality of the change was further
certified by observing the second nebular line (λ 4959), which in the
“Huygenian region” just equals F, to disappear as the slit was moved
outward, while F continued to shine with a very sensible lustre. And
since, in this case, the Purkinje effect was null (the lines being
almost indistinguishable in colour), a demonstration was afforded of a
genuine modification in the curve of emissive energy in passing from one
part of the great nebula to another.
Professor Keeler devised a completely novel method of demonstrating its
non-homogeneous character.[1087] Pictures of the nebula taken on
orthochromatic plates protected by colour-screens from the blue
radiations, were compared with impressions on ordinary unscreened
plates, all being exposed with the Crossley reflector, though during
very unequal times. The result was to show that, for equal intensity of
the Huygenian region, that of the remote parts and outlying streamers
fell greatly below its normal value in the screened photographs.
“Conversely,” to quote the words of the ingenious operator,[1088] “where
photographs made by the two methods, on the same night, show an equal
extent of nebulosity, the Huygenian region is very much more intense on
the orthochromatic plate. We infer, therefore, that in the remote parts
of the nebula the two lowest nebular lines are weak, or the hydrogen
lines strong, as compared with the Huygenian region. Thus the results of
spectroscopic researches are confirmed, and are extended to parts of the
nebula too faint for visual observation.” That is to say, the hydrogen
image of the Orion nebula is larger than its image in nebulium. And this
corresponds precisely with Campbell’s spectroscopic discovery about the
planetary nebula S.D. −12° 1172,[1089] already adverted to. When
nebulium comes to be examined in the laboratory—if that shadowy
possibility be ever realised—the cause of the discrepancy may be laid
bare. The most obvious is a difference of density between the two
substances; yet we cannot unreservedly assume its validity, considering
the noted effects of what we may call electrical preferences in
modifying the spectra of attenuated gaseous structures.
Two facts, then, have been definitely ascertained regarding the hydrogen
spectrum in the “Fish-mouth” nebula. The first is that its more
refrangible constituents are preferentially developed as compared with
the standard set by the vacuum-tube. The second is that of its
persistence in regions of the nebula too dim to glow with any other
species of light. Both peculiarities urgently demand explanations, which
can probably be afforded only by arduous experimental work. Dr. Scheiner
has not neglected this side of the inquiry; and although the outcome of
his efforts is negative, it serves none the less to answer a fundamental
question. He postulates in nebulæ extreme rarefaction and excessively
low temperature; and to test the effect of the latter condition he
plunged hydrogen-tubes into liquid air and examined the spectrum. He
found it entirely unchanged by cooling to −200° centigrade.[1090] This,
he pointed out, harmonises with the view that the luminosity of gases
originates solely through internal movements of the individual
molecules, and is hence independent of external temperature. Moreover,
one of the few means available for terrestrially altering the relative
strength of the red and green hydrogen lines is that of electrical
differentiation. Professor J. J. Thomson’s observation[1091] of C
bright, F invisible near the positive, F bright, and C invisible near
the negative electrode, offers a clue which has not yet been followed
up, for the threading of the labyrinth.
In another of its elements besides hydrogen, the spectrum of the Orion
nebula is suspected to vary regionally. The ultra-violet ray at λ 3727
is second to none in importance when photographed with suitable
apparatus. Nevertheless, it was missing from a plate exposed by Sir
William and Lady Huggins 28th February 1889,[1092] although the
impressions upon it of two much weaker lines near its place were clearly
to be seen. The anomaly of its total absence has not recurred. Professor
Keeler vainly went over the ground in 1892–93,[1093] groping with his
slit for the blank district. Professor W. H. Pickering, it is true,
derived indications of local diversities in the intensity of the line
from a photograph taken without a slit, 10th July 1888; but they were
rather suggestive than conclusive.
Suspected light-variations in the Orion nebula have not been confirmed
by modern research. There is absolutely no photographic evidence of
change, and visual discrepancies have not been attested with sufficient
precision for the support of any positive inferences. Professor Holden
was indeed disposed, after an exhaustive comparison of his own with
numerous recorded observations, to believe in luminous instability of a
partial kind;[1094] but the effects considered may have been only
apparent. Their production is at once rendered intelligible by Professor
Ormond Stone’s pertinent remarks:—“The general appearance of the
Huygenian region,” he wrote in 1896,[1095] “is very much like that of a
so-called ‘mackerel sky.’ Many of the condensations have pretty
well-defined nuclei, whose light diffuses, blending with the surrounding
nebulosity when the seeing grows poor. I have frequently been surprised
to find how greatly the definition changes the relative brightness of
the different condensations. This explains the many apparently
contradictory estimates.” So far as it is possible to judge, then, the
brightness of this nebula is not subject to change, either general or
local. Moreover, its component parts seem absolutely fixed in outline
and position. Internal movements, if in progress, will need the lapse of
ages to become sensible to the eye. No relative shiftings of knots or
nuclei have been detected; no alteration in shape of outgrowths and
effusions. Yet they are of an eminently unstable aspect, and might be
supposed no less essentially transitory than the appurtenances of
comets. And so most probably they are, although the unit of time by
which their duration is measured be long, and the scale of their
construction unimaginably vast.
Some of the stars, however, scattered near the trapezium preserve
anything but a constant lustre. One, catalogued as T Orionis, fluctuates
irregularly from 9·7 to 13·0 magnitude; and others vary as unmistakably,
though to a less extent. None are periodical, so that they belong to a
different category from the flash-lights of globular clusters. An
attentive study, photometric, photographic, and spectroscopic, of the
Orion variables could not fail to be fruitful and instructive. Their
instability is the more noteworthy from the whiteness of their light. T
Orionis (also known as “Bond 822”) is perhaps a unique example of a star
untinged with red losing and regaining nineteen-twentieths of its
visible radiance. Among its obviously variable neighbours are the
objects numbered by Bond in his survey of the nebula, 641, 647, 654, and
679. Scarcely any sustained attention has yet been paid to the group,
notwithstanding the many questions of interest connected with it. Enough
only is known to make it certain that its members exhibit no community
of character in their vicissitudes save that of exemption from any
traceable law of order. Thus Bond 654 was noticed by Holden, as it had
been noticed by Otto Struve twenty years earlier, to rise occasionally,
from habitual quasi-extinction, to brief maxima of about twelfth
magnitude. But Ormond Stone recorded none of these sudden brightenings,
although he observed the star, 30th September 1886, to rank higher than
the twelfth magnitude. Possibly the manner of its variability is itself
variable. The star Bond 647, on the other hand, has gained largely in
average lustre since 1837, when Sir John Herschel’s measures, reduced to
Struve’s scale, made it of 13·1 magnitude. Otto Struve chronicled its
disappearance in 1863, after a prolonged maximum at 12·5
magnitude;[1096] Bond found it to be of 11·6 magnitude in 1867; Ormond
Stone, of 10·9 magnitude, 1886 to 1894;[1097] and there is nothing to
show that this steady rise has reached its culminating point. A
compensatory decline may be anticipated, though not with entire
confidence. The situation of these stars lends a special meaning to
their variations. That they are physically connected with the nebula
they are seen projected upon, cannot indeed be proved, but it may
legitimately be assumed. Hence every advance in knowledge of their
vicissitudes cannot but help to elucidate the still obscure relations of
nebulous environment to stellar light-change.