VARIABLE NEBULÆ.
The occurrence of local changes of brightness is reasonably certain, as
we have seen, in some of the great irregular nebulæ, and may be
suspected in others. Variability affecting smaller objects in their
entirety must then be admitted as possible. No doubt the phenomenon
would introduce ideas difficult to adjust and unexpected; but the
heavens are full of surprises. The immediate question to be put
regarding it is, Does it really subsist? The answer must be given with
extreme circumspection. The visibility of nebulæ depends upon contrast;
the blackness of the sky has as much to do with it as the brightness of
the filmy masses projected against it. They are besides apt to disappear
with high magnification, and that for two reasons. First, because of the
diffusion over a larger area of the same quantity of light; secondly,
because of the restriction of the background in narrower fields of view.
Hence there are drawbacks to the employment of large telescopes in
nebular observation. The history of Tempel’s Merope nebula, marked by
vicissitudes ascribed again and again to intrinsic causes, now fully
recognised as non-existent, is a warning against hasty conclusions on so
delicate a point. The lesson has indeed been so thoroughly learned that
changes of the sort have of late been announced only with a certain
timidity, and under reserve. Caution in the matter can, indeed, hardly
be blamed for exaggeration, in view of Swift’s remark that, after the
Krakatão eruption, many faint nebulæ absolutely disappeared.[1146] Nor
need we go beyond Chacornac’s “temporary nebula” for an exemplification
of optical caprices. On 19th October 1855 the French observer noticed a
striated haze (N.G.C. 1988) attached to the star ζ Tauri, which seemed
to have gained brightness in the ensuing January. No one else, however,
saw it, and it had vanished by 20th November 1862. According to Tempel
it never shone in the sky, but was a telescopic creation—a false image
of an eleventh-magnitude star near ζ Tauri;[1147] and Burnham
unhesitatingly adopts this opinion.[1148] Thirty years later its reality
could have been tested by photographic means; but astronomers in those
days had to rely upon the fallible human retina.
Chacornac’s phantom formation emerged near the site of two genuine
nebular Novæ. On 11th October 1852, Hind detected, close to a star, then
of the tenth magnitude, but since registered as an irregular variable
under the designation T Tauri, a dim, round nebula (N.G.C. 1555) which
brightened steadily until 1856, when it was obvious to general
observation. A comparatively rapid decline ensued. Auwers[1149] could
barely discern the object with the Königsberg heliometer in January
1858; to d’Arrest,[1150] using the eleven-inch Copenhagen refractor, it
was wholly invisible 3rd October 1861; in 1862 it was vainly sought at
Paris and at Malta with Foucault’s and Lassell’s great mirrors; Hind
himself was unable to find it; Secchi, under the pure Roman sky, was
equally unsuccessful; only at Pulkowa it continued to glimmer just
perceptibly for a few months longer. From 1863 the sky in its place
seemed a dead blank. At last, 15th October 1890, Mr. Burnham requested
his colleague, Professor Barnard, to examine the region with the Lick
thirty-six-inch, whereupon a nebulosity about 50″ in diameter, and so
faint as to be at the limit of vision, was detected[1151] (see Fig. 49).
Burnham too saw it, but believed that he could not have done so
independently, his splendid powers of sight being better adapted to the
discernment of concentrated than of diffused light-rays.
The nebula was again observed by Barnard—and with somewhat increased
facility—in February 1895.[1152] Seven months later he was amazed to
find it utterly gone! His search was repeated on three nights, under
supremely good conditions, with the same negative result. And the
object, so far from evading the grasp of large apertures, is peculiarly
fitted for observation with them, owing to its small size and compact
shape. Nevertheless, the forty-inch Yerkes refractor failed to show it
at all in 1897, and barely enabled Barnard to catch a glimpse of it,
28th September 1898.[1153] Finally, Professor Keeler obtained faint
images of it on two Crossley plates, exposed during four hours each, in
December 1899.[1154] A copy of his drawing from them is given in Fig.
50. It exhibits the nebula as composed of three vaguely defined patches,
united by a dim haze, the camera having, as usual, descried structural
complexities inappreciable by the eye. On 20th January 1900, the great
refractor just availed to bring it into view, and it has not since been
heard of. To Professor Keeler it appeared inconceivable that in its
present obscure state it could ever have been seen with small
telescopes; and indeed the evidence of variability is conclusive. Will
it ever recover any of its lost brightness? It may be doubted. The
changes so far undergone by it have been, though comparatively slow,
strictly analogous in character to those of “new” stars; a presumption
hence arises that it will share their fate of permanent extinction.
There is much reason to suppose that Hind noted in 1852 an early stage
of its kindling; that its maximum in 1855–56 was solitary, its
declension irretrievable.
[Illustration:
FIG. 49.—Sketch of T Tauri and Hind’s Nebula, 15th October 1890
(Barnard).
]
Strange to say, the phenomenon was duplicated. While looking fruitlessly
for Hind’s nebula, Otto Struve came upon another unfamiliar object
(N.G.C. 1554) 4′ east of its predecessor’s empty place. This was early
in 1868; and the Nova—for d’Arrest was sure of its previous
non-existence[1155]—was kept in view until 1877, when absolute
obliteration covered it. Even Barnard’s quest for it in 1890, 1895, and
1899 was ineffectual. Its former position is marked in Keeler’s drawing
(Fig. 50) by the thirteenth-magnitude star _b_, but no nebulous
impression was there made upon the plates. Dr. Roberts had indeed
already, in 1890,[1156] vouched for its photographic disappearance. When
last seen by Tempel,[1157] 8th November 1877, the nebula was 90″ across,
and d’Arrest had expressly recorded the presence in it of an
eccentrically situated, though definite nucleus.
[Illustration:
FIG. 50.—Region of T Tauri and Hind’s Variable Nebula. Drawn from
Photographs by Professor Keeler.
]
Hind’s and Struve’s nebulæ were bright enough, as Professor Barnard
recalled wonderingly,[1158] to be ranked, soon after the middle of the
last century, in Herschel’s first and second classes respectively. Both
were watched “and measured by the best observers then living,” and “were
easily visible in ordinary telescopes.” Yet one survives only as a
fitful shimmer; the other is utterly, and probably for ever, quenched.
They are perhaps obscurely connected, and the system—if such it can be
termed—may include the nebulous variable T Tauri; although nebulæ and
star alike seem to fluctuate in complete independence one of the others.
These are the only authentic instances of temporary nebular
developments; but allegations of nebular light-change are common. Some
have proved groundless; not a few, however, rest on a solid substratum
of fact. The following may serve as specimens.
On 17th October 1785 William Herschel discovered, not far from Algol, “a
pretty bright star with two faint branches” (N.G.C. 1186). Sir John
verified the observation, while estimating the star at only fourteenth
magnitude (about twelfth on the modern scale). Yet neither it nor its
appendages could be seen with the Parsonstown reflector, and d’Arrest,
after diligent and repeated search, affirmed decisively, _Nostra ætate
in hac regione tale quid non exstat in cœlo._[1159] The lost object,
nevertheless, came again into view in 1891. On 31st January and 26th
February of that year, Bigourdan, using the thirteen-inch Paris
equatorial, perceived it as a twelfth-magnitude star, with an
unmistakable “fan” of nebulosity spreading from it over 1′ of arc. Only
a fortnight later, Spitaler made a drawing of the object with the Vienna
twenty-seven-inch, and described it as an elongated nebula, not regular
enough to be called elliptical, 2′ in extent, and including a focal star
of the eleventh magnitude.[1160] And Burnham, in the following August,
“readily found in the proper place a _tenth_-magnitude star involved in
a faint elongated nebula” measuring at least 2′ or 3′.[1161] During the
course of 1891, accordingly, the star seems to have been progressively
gaining light and the nebula compass. But if the star only were
variable, the attached nebula would have appeared to shrink and become
effaced as the bright point within it acquired intensity. Its
simultaneous increase could not have been counterfeited. Proof was
afforded by it that the growth, like the previous failures of
luminosity, were due to influences diffused throughout every part of the
formation.
The next variable nebula was a “find” of Barnard’s. It was conspicuous
to him 30th November 1888. He judged it equal to a ninth or
tenth-magnitude star, and remained convinced that its lucidity was of
recent origin. Three years later it had parted with quite four-fifths of
its lustre, and had faded down nearly to evanescence.[1162] It is
situated in Cetus. No information is at hand as to whether its decline
has continued since 1891. Unless arrested, it must, before the century
closed, have carried it out of sight even of the chemical retina, and
the object should then probably be relegated to the class of
“temporaries.” The question is of great interest, and might be answered
by taking one long-exposed photograph with a portrait-lens or a large
reflector.
This is not the only case in which an accession of brightness has been
thought to be demonstrated by the lateness of discovery. A nebula was
encountered by Tuttle in Draco, 1st September 1859 (N.G.C. 6643), which,
in d’Arrest’s opinion, should certainly have been caught in the meshes
of the Herschelian nets unless in their time comparative obscurity had
enveloped it. Similarly, a nebula in Camelopardalis detected by Barnard
in 1889,[1163] and again independently by Denning in 1890,[1164] could
not long, Barnard considered, have been thus readily apparent. He
recommended its being kept under surveillance as a probable variable,
but as yet it has shown no sign of being so;[1165] unless, indeed,
Swift’s earlier observation of the same object, recorded without date by
a simple entry on a star-map,[1166] indicated a previous maximum.
Another instance of a possible rise in the scale of luminosity is
afforded by a small, fairly bright nebula in the Camelopard, first
observed by Denning 30th September 1891, and casually again four times
in the ensuing month. Yet his many previous reviews of the sky-contents
in that neighbourhood had failed to elicit any trace of its existence.
Like Barnard’s and Tuttle’s “new” nebulæ, however, it has apparently
come to stay; and since variability is in sidereal bodies usually an
ineradicable property, the hypothesis of an ascent from invisibility
cannot safely be accepted until a corresponding descent has been entered
upon.
The irregular variability, on the other hand, of two nebulæ adverted to
by Winnecke in 1877–78[1167] is almost incontestable. The first (N.G.C.
3666) is in Leo. Elliptic in shape, in size 90″ by 40″, it was marked
“very bright” by the elder Herschel 15th March 1785, but “extremely
faint,” 23rd March 1830, by the younger, who added the comment, “This
nebula must have changed greatly if ever it belonged really to the first
class.” But its waning was not definitive. Boguslawski inscribed it as a
_bright object_ in 1840 on the Berlin Academy star-map of that region;
and Winnecke found it, 10th April 1878, of unquestionable primary rank.
Yet meantime, in 1863, d’Arrest had described it as _subobscura_, and
manifestly of third-class lustre; while again, on 24th May 1887, Dr.
Dreyer perceived its diminished radiance only with the utmost
difficulty. Its further history remains untold.
Winnecke’s second variable (N.G.C. 955) is an inmate of the crowded
nebular district in Cetus. It consists, Burnham says,[1168] of long,
narrow “nebulous wings on either side of a bright central condensation.”
“On the whole,” he continues, “it is rather a curious object, and should
be easily found and seen.” This was in 1891, and agrees quite well with
Dreyer’s notice of the object in November 1887 as “fully of the second
class.”[1169] The case for change rests upon its invisibility to
Schönfeld in December 1861, and to Vogel in November 1865; although in
1856, 1863, and 1868 it had been seen at a glance by Schönfeld himself,
no less than by d’Arrest and Winnecke.
Winnecke’s nebulæ were at first held by him to be periodical; but this
they certainly are not. No fixed relation to time has so far been shown
to govern nebular fluctuations. They either consist—according to the
best evidence at command—of a solitary maximum, analogous to the
outburst of a new star, or of irregular accessions and losses of light.
No case of cyclical recurrence is on record. Photography is clearly
destined to play an important part in the investigation of this
difficult subject; its aid will be peculiarly welcome where visual
faculties are often baffled, embarrassed, and deceived.
Nebular variability is indeed a phenomenon not only evasive to the
senses, but startling to thought. It cannot be even remotely assimilated
to the light-changes that progress in certain globular clusters; it is
independent of geometrical conditions, of orbital movements, of planes
and periods. Its cause defies conjecture; we can only be sure that it
acts upon a prodigious scale. Thus Hind’s nebula in Taurus measured at
least 2′ across. Its parallax was almost certainly less than one-tenth,
and may not have exceeded one-hundredth of a second. The larger value
would give, for the smallest admissible linear diameter of the object,
1200 astronomical units (radii of the earth’s orbit) or 111,000 million
miles. Centred on the sun, it would extend on every side to twenty times
the distance of Neptune, the equation of light within the vast formation
being six and a half hours. Yet it kindled as a whole, through the
pervading influence of some far-reaching event. Did another dark nebula
sweep through it? We dare not pronounce. Its mysterious brightening,
however, hints at the existence of an indefinite multitude of similar
bodies lurking in the obscurity from which, by some rare chance, it
emerged. It introduces us, in fact, to a realm of invisible nebulæ,
impenetrable by observation, and hence pre-eminently adapted for the
sports of scientific fancy.
Hind’s and Struve’s vanished nebulæ were presumably of gaseous
composition, like the adjacent glow round T Tauri;[1170] Winnecke’s
variable pair doubtless shine with the white light characteristic of the
elliptic family to which they structurally belong. Their remoteness from
the Milky Way points to the same conclusion. Luminous instability does
not then appear to be associated in nebulæ with any special radiative
quality. Those giving continuous, and those giving discontinuous spectra
may equally be affected by it.