ANNULAR NEBULÆ.
The model annular nebula is the beautiful filmy ellipse situated between
ß and γ Lyræ (N.G.C. 6720). Discovered by Darquier at Toulouse in 1779,
it appeared to Sir William Herschel a simple hoop of light, quite dark
within. The “gauze drawn over the hoop,” spoken of by Sir John Herschel,
had been perceived by Schröter in 1797, and with long exposures the
photographed ring fills up into a disc. Plate XXV. is taken from a
singularly perfect representation obtained by Mr. W. E. Wilson in twenty
minutes, while an hour’s exposure sufficed to blur beyond recognition
the characteristic annular aspect of the structure. Yet the havoc thus
wrought, in a pictorial sense, is compensated by the experimental
significance of a result proving the interior of the ring to be far from
vacuous of luminous or luminescent material.
[Illustration:
PLATE XXV.
The Ring Nebula in Lyra. Photographed by W. E. Wilson, F.R.S.
Exposure, 20^m.
]
The dimensions of the ring are about 87″ by 64″. It was shown in the
Lick negatives as an exceedingly complex structure. “It seems,”
Professor Keeler wrote,[1017] “to be made up of several narrower bright
rings, interlacing somewhat irregularly, the spaces between them being
filled with fainter nebulosity.” Many bright patches and condensations
diversify the main annulus, which is, nevertheless, fundamentally
continuous; it does not break up into detached knots of nebulosity. The
light is strongest near the extremities of the transverse axis, and its
failure at each vertex of the ellipse, conspicuously shown in our Plate,
was, already in 1785, noticed by the elder Herschel.[1018] It is
accompanied by symptoms of effusion, marking possibly equatorial
outflows due to rotative acceleration. Lord Rosse in 1863, and Schultz
in 1865, were alike struck with “nebulous radiations in the direction of
the longer axis, which seemed momentarily almost to destroy the annular
form.” They issued from the north-eastern side, while a similar
appearance on the south-westerly side of the nebula was observed by
Professor Holden at Washington in 1875.[1019] He perceived, too, with
surprise that the south end of the minor axis, which Lord Rosse had
represented as the best terminated, was unmistakably, after thirteen
years, less clearly finished than the north end. Similarly, the Lick
thirty-six-inch disclosed in 1888[1020] the whole southern margin as
filamentous, by a kind of alternation with the corresponding state of
the northern edge noticed at Parsonstown. This tufted or fringed effect
was caught, for the first time photographically, by Professor Keeler.
The oval imprinted on his plates was fringed on both sides, and his
measurement of eleven nebulous projections from it will give the means
of detecting any future variations in their luminosity or distribution.
The gauzy stuff in the interior is not a mere formless light-mist. A
drawing published by Lord Rosse in 1844[1021] represented it as divided
into longitudinal striæ, and their reality was confirmed by the Crossley
photographs after fifty-five years. “I have tried,” Professor Keeler
wrote, “to verify this band-structure by visual observation with the
thirty-six-inch refractor, and have fancied at times that I could catch
glimpses of it; but the observation is a most difficult one, the
contrast of the bright and dark bands, exaggerated by the photograph,
being almost too slight to affect the eye.” The surprising accuracy of
the delineations made with the Rosse speculum, while its burnish
continued unimpaired, was thus once more exemplified. It conveys a
warning against lightly setting aside any of the earlier records
concerning nebulæ obtained at Parsonstown, even when they imply
seemingly improbable changes. It is certainly by no accident that the
striæ within this nebula coincide in direction with its major axis; nor
can the termination of the transverse axis by maxima, of the
longitudinal axis by minima of brightness[1022] be regarded as casual
features. All the details of the edifice, in fact, are arranged with
obvious reference to its apparent shape, and this amounts to a
demonstration that the elongation is real. The nebula, then, is not
simply projected into an oval; it is not a circular formation viewed
obliquely. There seems no escape from the conclusion that it is an
ellipsoid of revolution—that the bands follow the line of the equator,
and originate under conditions prescribed by the rotation of the body;
while the partial interruption of luminosity at the ends of the oval
mark outflows of matter where centrifugal velocity overrides the holding
power of gravity. Everything, indeed, leads us to suppose that this
nebula, like the rest of its kind, is actually a hollow spheroid of
shining fluid, the marginal brightness resulting from the increased
thickness of the luminous shell penetrated by the visual ray. The “hoop”
and the “gauze” drawn round it are then two aspects of the same thing.
Nebulous rings, as such, probably do not exist. They would be subject to
perspective effects, no traces of which are to be found in the heavens.
Annularity in nebulæ may accordingly be considered as a purely optical
modification of a different structural plan.
A glittering point of light occupies the centre of the annulus in Lyra.
Many anomalies are connected with its visibility. The first heard of it
is in 1800, when Von Hahn of Remplin, in Mecklenburg, was surprised by
its disappearance.[1023] He attributed the change, not to loss of light
in the star, but to the nebulous clouding-over of the black background
upon which he was accustomed to see it relieved. Unperceived by the
Herschels, it was next seen by Lord Rosse in 1848,[1024] and attracted
Father Secchi’s attention at Rome in 1855.[1025] A ten-inch Steinheil
sufficed, in 1865 and 1867, even in unfavourable weather, to show Hahn’s
star to Hermann Schultz;[1026] yet it unaccountably evaded the
deliberate scrutiny, ten years later, of Professor Asaph Hall,[1027]
armed though he was with the twenty-six-inch Washington equatorial. The
same instrument, however, displayed its evasive sparkle to Mr. A. C.
Ranyard, 23rd August 1878;[1028] while to Dr. Vogel, with the Newall
telescope in 1875, and the Vienna twenty-seven-inch in 1883, it remained
consistently imperceptible.[1029] Very remarkable, too, is its
non-appearance to Dr. Spitaler at Vienna in 1885, when he carefully
delineated the nebula, as well as in 1886, during frequently repeated
verifying observations.[1030] The interior seemed then to contain only
dimly luminous floccules; nevertheless on 25th July 1887 the star caught
his eye at the first glance. It had, in the meantime, 1st September
1886, been photographed by Von Gothard, and has since abstained from
capricious disappearances. It is indeed of such exceptional actinic
power that the camera cannot easily lose sight of it. Fainter visually
than the fifteenth magnitude, it needed only an exposure of one minute
to come out distinctly on a Crossley negative, and it left a dim
impression in half that time.[1031] With all exposures the image was
clearly defined, although in photographs taken with other instruments it
had usually presented a hazy disc. Its light appears to be of normal
stellar quality. Its maximum of intensity, that is to say, falls in the
yellow part of the spectrum. Both Keeler and Barnard agreed that, with
the Lick and Yerkes refractors, the focus for the star was about
one-fifth of an inch shorter than for the encircling nebula. And a
similar disparity exists between the nuclei and discs of most
planetaries. This, however, leaves their special chemical effectiveness
unexplained; for Keeler’s suggestion of its being due to ultra-violet
emissions lacks the support of known facts.
In the field with the Lyra nebula, Barnard perceived, 2nd October 1893,
a second of about the fourteenth magnitude, 30″ in diameter, and
somewhat irregular in shape.[1032] Professor Keeler’s longest-exposed
negative showed the new object to be “a left-handed, two-branched
spiral.”
In 1891 Mr. Burnham measured the nucleus of the ring in Lyra with
reference to an external star of the twelfth magnitude which closely
follows it. Eight years later, Professor Barnard employed the forty-inch
Yerkes telescope to repeat his determinations,[1033] and was inclined to
attribute a slight discrepancy to real motion in the nebula. But the
Potsdam plates lent no confirmation[1034] to a suspicion which will
probably remain long unverified. His experience was entirely negative as
regards light-variability in any part of the object. Alleged changes in
the stellar kernel he translated into genuine changes in visual
facilities. “The fact,” he wrote, “that the nucleus is seen on a
nebulous background makes steadiness of the atmosphere a most important
factor in its distinctness—far more so than in the case of an ordinary
star in the open sky. When the seeing is exceptionally good, the nucleus
appears with a distinctness strikingly in contrast with its ordinary
condition, so much so that one has to guard against deception in
supposing a real change of light.”
Nine bright lines have been seen or photographed in the spectrum of the
Lyra annulus, that of shortest wave-length at λ 3727 being, according to
Von Gothard, the most intense.[1035] Hydrogen shines feebly; no C has
been recorded, but the Rydberg ray at λ 4688 was detected by
Campbell.[1036] Helium is unrepresented; since a violet line at λ 3869,
which appears prominently and persistently in nebular spectra, cannot be
ascribed to that substance. It chances, indeed, to coincide
approximately with one belonging to a known series, but is itself of
“rare and strange” origin.
Several analogues of the Lyra nebula have been discovered; none so large
or so bright. The best imitation is situated in Cygnus (N.G.C. 6894). It
measures 47″ by 41″, the inner vacuity 20″, and was marked “resolvable”
at Parsonstown.[1037] Needless to say that the sparkling effect which
conveyed the impression of a stellar constitution was altogether
illusory. A fifteenth-magnitude star near the interior border of the
ring to the north-west was measured by Burnham in 1891.[1038] The true
nucleus is considerably fainter. First brought to view in a photograph
taken by Dr. Roberts 31st August 1897,[1039] it duly reappeared in one
of the Crossley pictures of 1899,[1040] and was visually discerned with
the great Paris siderostat by M. Antoniadi, 17th July 1900.[1041] The
elongation of the annulus from north-east to south-west was obvious to
him no less than the faint haze with which it is filled. Nothing is
known about the spectrum of the ring nebula in Cygnus. It doubtless
resembles that of its prototype in Lyra.
Even more scanty is the information at hand regarding “a beautiful
delicate ring” in Scorpio, about 40″ in diameter (N.G.C. 6337). It is in
a field crowded with stars, two of which are projected upon, but may not
belong to it. Neither occupies the proper position of a nucleus, and the
one seen by Lassell at Malta had to him somewhat the aspect of a
nebulous knot.[1042]
An annular nebula in Ophiuchus (N.G.C. 6369) was found by Mr. Burnham
“very like the well-known example in Lyra, except in brightness.”[1043]
A central star of 14·5 magnitude was probably then first noticed. The
longest diameter of the ring measures 31″; the edges seemed to Sir John
Herschel “a very little cottony,”[1044] and they are doubtless fringed
with dim appendages, like those attached to its model in Lyra. This is
again copied in an object discovered by Mr. Gale of New South Wales in
1897, or perhaps a little earlier. It is bright enough to make its late
detection somewhat surprising.[1045]
Immersed in a fine cluster in Argo (Messier 46), a nebula of planetary
aspect, about 60″ in diameter (N.G.C. 2438), drew the attention of Sir
William Herschel. The Rosse reflector showed a central star dominating a
vacuous interior, besides two stars sparkling on the condensed
border.[1046] Lassell perceived in the object a resemblance to a large,
dim, compound planetary in Eridanus (N.G.C. 1535), but with the
qualifying circumstance that in N.G.C. 2438 only one “stratum of
nebulosity” was discernible.[1047] This singleness of construction
appears characteristic of perfected nebular rings, and such the inmate
of the cluster in Argo has declared itself to be. In a photograph taken
by Dr. Roberts, 24th February 1894,[1048] it is definitely and
unmistakably annular. Three stars are projected upon, or contained
within it, we cannot tell which; although one by its nuclear position
gives some assurance of being there through organic relationship. Nor
can we venture to assert that the nebula is really in the cluster. It
may only be thrown accidentally into line with it. Still the fact that
Sir John Herschel recorded in two cases similar collocations of
planetary nebulæ with clustered stars[1049] inclines the balance of
probability towards the side of genuine association. One of these groups
(N.G.C. 5979) is situated in the constellation Circinus, the other
(N.G.C. 2818) near the mast of Argo.
Certain complex formations, intermediate between planetary and annular
nebulæ, have now to be considered. A striking specimen of the kind is
met with in Andromeda (N.G.C. 7662). The disc, which includes perhaps
more than one ring, measures 32″ by 28″. Alexander, about the middle of
last century, and Lassell subsequently, thought the structure
bi-annular. Vogel[1050] and Holden remarked its warped and twisted
appearance, denoting possibly a multiple combination of rings thrown off
in various planes as the outcome of long-past crises in a slow process
of development. A central star surrounded by close spirals of nebulosity
was seen at Parsonstown,[1051] but evaded the scrutiny of O. Struve in
1847, of Searle in 1866, and of Vogel in 1883. Lassell perceived it
under the guise of a minute, bluish disc, Burnham as an ordinary
fifteenth-magnitude star.[1052] Fig. 4 in Plate XXIII. reproduces
Professor Keeler’s drawing of the Andromeda planetary with its visual
spectrum.[1053] “This nebula,” he wrote, “is annular, with a bright
inner ring and a very small nucleus. It is somewhat elongated north and
south.” The fourth line in the spectrum is the “fundamental” of
Rydberg’s hydrogen series; its unusual strength makes it the equal of
the ordinary hydrogen lines on either side of it. The spectrum of the
nucleus appears to be perfectly continuous, save for a possible bright
knot about the place of D_{3}. Campbell, however, caught no glimpse of
this radiation, although he determined, visually and photographically,
eighteen bright lines in the spectrum of the nebula.[1054] We subjoin
his list, with notes and comments between brackets.
BRIGHT LINES RECORDED IN N.G.C. 7662.
┌────────────┬────────────────────────────────────────────────────────┐
│Wave-length.│ Remarks. │
├────────────┼────────────────────────────────────────────────────────┤
│ 540│Very faint, difficult (Wolf-Rayet line; Pickering │
│ │ series). │
│ 532│Very faint, difficult (possibly the chromospheric K │
│ │ 1474). │
│ 5007│First nebular line, very bright. │
│ 4959│Second nebular line, very bright. │
│ 4861│Hβ, very bright. │
│ 4744│Faint (origin unknown). │
│ 4715│Faint (unknown). │
│ 4688│Very bright (Rydberg hydrogen line). │
│ 4663│Very faint (unknown). │
│ 4643│Faint (nitrogen ?) │
│ 4472│Very faint (helium; prominent in Orion stars). │
│ 4364│Bright (unknown). │
│ 4341│Hγ, very bright. │
│ 4102│Ηδ, very bright. │
│ 4067│Very faint (unknown). │
│ 4026│Very faint (helium). │
│ 3969│Hε, very bright. │
│ 3869│Very bright (unknown). │
└────────────┴────────────────────────────────────────────────────────┘
The brilliancy of the last line, in view of the faintness and fewness of
other helium emanations, confirms the inference that its association
with that substance is inadmissible. Experiments in the laboratory would
nevertheless be valuable on the behaviour, for instance, of the adjacent
violet line of helium in a mixture of that gas with hydrogen.
Campbell perceived this nebula to consist “of two nearly concentric
rings more or less broken up, with a fourteenth-magnitude stellar
nucleus near its centre.” Ingall described it, 18th December 1885,[1055]
as a “superb planetary,” of a bright greenish-blue tint, the centre not
quite dark, and thus fitly to be called annular. On applying a power of
500, “an extraordinary structure appeared to unfold itself. The bright
ring seemed very jagged and fringed at the edges, and the centre was
mottled with unequal shades, often as if of _two_ dark centres.” The
note made on it at Harvard College was: “Somewhat annular; edges
hazy.”[1056] The blue colour of the object faithfully corresponds to the
actinic energy of its rays. In two seconds of exposure to them the
Crossley reflector gave a weak image, including a barely visible central
star,[1057] and Professor Keeler obtained finished pictures in 20, 30,
and 60 seconds. At Potsdam, in 1892–93,[1058] thirty-three impressions
were taken for the purpose of parallax-determinations, but they yielded
no positive result. They afforded only the information that the nebula
could not have an annual parallax so large as one-fifth of a second;
and, indeed, the true value of the quantity, judging from other
indications, may very well fall short of one-tenth the assigned maximum.
A “sky-blue likeness of Saturn” in Aquarius (N.G.C. 7009) is built very
much on the lines of the Andromeda planetary, but with the addition of
“ansæ.” These, in August 1888, were resolved by Holden and Schaeberle,
with the aid of the Lick refractor, into a pair of attendant
nebulosities, situated in line with the major axis of a strongly
elliptical body, and subsensibly united to it by evasive gleams of
illumination.[1059] Yet the likelihood is small of their being really
satellite-globes revolving in the same track at an invariable interval
of two right angles. The probabilities of the case oblige us to believe
rather that the original interpretation of them as the extremities of an
annular appendage came nearer the truth. An analogy indeed suggests
itself between them and the nebulous effusions from the vertices of the
ring formation in Lyra.[1060] And here again we are assured that they
mark an equator—that the disc they seem attached to must be the
projection of a rotating spheroid. Intricacies of interior arrangement
are, however, visible, showing the progress of manifold activities. Two
dark cavities, extended parallel to the major axis, and helical wisps of
nebulosity, were observed by Vogel at Vienna in 1883.[1061] Keeler saw
and photographed a somewhat distorted condensed ring measuring 26″ by
16″,[1062] and Scheiner’s plates recorded curious spoke-like projections
from an intensely actinic central star.[1063] The spectrum differs from
that of the Andromeda planetary only by the inclusion of the enigmatic
“last line” at λ 3727. From the displacement of the green ray of
nebulium Keeler determined for the Saturn nebula a movement of approach
towards the sun at the rate of thirty-one miles a second, only a small
proportion of which can be due to our own journey through space.
Sir William Herschel observed in the constellation Gemini in 1787 “a
star of the ninth magnitude, with a pretty bright nebulosity equally
dispersed all around” (N.G.C. 2392). Lord Rosse found in it a dark hole
close to a slightly eccentric nucleus.[1064] D’Arrest thought the object
might be called annular;[1065] Lassell perceived a ring surrounding a
bluish disc;[1066] Secchi described it as a star with an annular
aureola.[1067] H. C. Key, using an eighteen-inch silver-on-glass
reflector, noticed about 1868[1068] a concatenation of bright and dark
rings besides the patch of interior obscurity detected at Parsonstown.
To Burnham the nebula in Gemini appeared “one of the most beautiful
objects of the kind in the heavens.”[1069] He assigned to it a diameter
of 45.″ Barnard, finally, was impressed by its “magnificent and
beautiful” effect in the Yerkes telescope.[1070] It disclosed to him a
ninth-magnitude star encircled not quite symmetrically by a brightish
oval ring, partially incomplete towards the south. “This ring,” he
continued, “which is well defined inside and out, is surrounded by a
vacuity, and this in turn by an almost circular broad ring of light less
intense than the inner ring, and with a distinct break in it
north-preceding. It breaks up into a clouded or unequal surface, and is
very irregular on its inner edge, but fairly uniformly circular on its
outside edge. The inner ring is filled with a nebulous light which has a
black spot in it, south-preceding the nucleus.”
That the perforated effect is no illusion may be taken as proved. Nor is
it subject to change. It has obtruded itself now for half a century upon
one observer after another, under divers conditions, both instrumental
and climatic. This nebula then, and a few others like it, betray the
action of some force strange to our experience, by which the matter
contained in an extensive region is either expelled thence, or its
light-giving faculty suppressed.
The annular nebula in Lyra is the only member of the class that has been
satisfactorily investigated. Until spectrographic possibilities are
further developed there seems little chance of dissipating the
perplexities that still envelop its nature. That is, by direct means;
for indirectly much may be done. Comparisons, for instance, of detailed
results for sister-objects ought to prove highly instructive as to the
laws governing the construction of all alike. Most of them have been
singularly neglected, considering the interest attaching to their
peculiarities. The hooped nebula in Cygnus, the “ghost” in Scorpio,
Gale’s annulus, the ring in Ophiuchus, should be photographed with long
exposures, on a scale sufficiently large to bring into view specialties
of texture and build. Their agreement in certain fundamental relations
would thus be tested, and its importance as a guide to theories of their
mode of origin cannot be overrated. Their self-delineation would,
however, doubtless accentuate besides that variety in similarity which,
throughout the whole created world, illustrates the wealth of the
resources disposed of by Nature, and the inexhaustible inventiveness of
the Mind revealed in Nature.
[Illustration:
PLATE XXVI.
Photograph of the Orion Nebula (W. H. Pickering)
]