NEBULOUS STARS.
“There is a vast difference,” Professor Swift wrote in 1897, “between a
nebulous star and a star in a nebula.”[937] Vast indeed, since a star
and nebula in reality billions of miles apart may be thrown by
perspective into the same visual line, while a true nebulous star claims
the ownership of its luminous appendage on the evidence of obviously
adapted form. The nimbus is not more unmistakably fitted to the head of
a saint in a picture than nebulous halos are, very often, to the stars
they encircle. The relationship is patent. That the glow emanates from
the star, and is no casual adjunct to it, the instinctive logic of the
eye suffices at once to decide.
Sir William Herschel was the first to give express attention to “stars
with burrs.” They struck him as remarkable, not only in themselves, but
for what they implied. They served as the basis of a memorable train of
reasoning.[938] Their atmospheres, he argued, being plainly “not of a
starry nature,” must be composed of a “shining fluid,” the same which is
seen to be diffused through space in milky tracts, or curdled into
fantastic shapes of chaotic irregularity. He was, in a word, led by them
to the capital discovery of _nebulæ_ as a distinct sidereal order. On
13th November 1790, he came across the “singular phenomenon” which
determined his abandonment of the view that the universe is constituted
exclusively of stars variously aggregated. This was “a star of about the
eighth magnitude, with a faint luminous atmosphere of a circular form,
and of about 3′ in diameter. The star is perfectly in the centre,” he
continued, “and the atmosphere is so diluted, faint, and equal
throughout that there can be no surmise of its consisting of stars, nor
can there be a doubt of the connection between the atmosphere and the
star.” This exemplar object, situated in Taurus (N.G.C. 1514), he
regarded as “decisive in every particular”; yet its nature is still to
some extent dubious. It has even been classed of late as a planetary
nebula, and certainly shares not a few characteristics of that family.
The uncertainty of its status renders its study especially instructive.
A planetary nebula is definitely terminated; a nebulous star fades off
into space. One shows a disc; the other is surrounded by an
“atmosphere.” Moreover, a star, or stellar nucleus, is subordinate in
the one formation, while it dominates the other. These distinctions,
however, cannot always be unhesitatingly drawn, since the relative
strength of the stellar and nebular elements varies widely in different
objects. Hence the doubt as to the category in which Herschel’s typical
specimen should be ranked. For the glow round it is uncommonly bright;
d’Arrest found it to strike the eye with a four-and-a-half-inch
refractor.[939] Nor is it equably diffused. The Parsonstown telescope
showed it as spotted and patchy, and very curiously “ragged” at the
edges;[940] and its aspect to Mr. Burnham in 1891 was essentially the
same. A “broken and mottled” surface, about 126″ across, emerged in the
field of the Lick thirty-six-inch.[941] He was inclined, though under
reserve, to agree with Barnard in considering the object planetary. Its
affinities might be settled by spectroscopic means; the attempt to do so
could, at any rate, hardly fail to have an interesting result. So far,
little has been ascertained about the spectra of true nebulous stars.
Their halos, certainly in some cases, presumably in all, emit bright
lines, and it might be expected that the same lines would show by
absorption in the spectrum of the central star. The presumption has not,
indeed, been fully verified; while the expectation founded on that
presumption is disallowed by the facts scantily at our disposal. Stars
with nebulous appendages are usually, if not invariably, distinguished
by “early Orion” spectra—a combination already noted as significant in
connection with the course of sidereal growth; but they show no special
lines that could be attributed to light-stoppage by the immense bulk of
rarefied incandescent matter interposed between our eyes and their
shining photospheres. This is one of the many perplexities involving the
luminous relations of nebular stuff, which, setting “Kirchhoff’s law” at
defiance, exercises no absorption correlative to its emission.
One of the few nebulous stars bright enough for easy spectroscopic
investigation is situated in Scutum Sobieski. Of 5·5 magnitude, it is
enrolled in the Bonn Durchmusterung under the heading S.D. −10°4713. On
a plate exposed by Professor Barnard with the Willard lens, 29th June
1892,[942] a large diffused nebulosity was seen to encircle it. The
appendage must be visually very faint to have escaped notice so long.
Its proper spectrum may then be nearly evanescent. The star it belongs
to is No. 8198 of the Draper Catalogue, where it is set down, although
doubtfully, as of the second type (Spectrum E). Now this is a point of
crucial importance to theories of stellar development. The pronounced
nebulous condition of a star near the solar stage would have a bearing
on such inquiries that could not be ignored. Should it be established,
current ideas will need revision. The spectral character of Barnard’s
_nebulosa_ in the Shield promises, indeed, to afford a test by which to
try the validity of reasonings on sidereal evolution. The test ought,
with the least possible delay, to be applied.[943]
A seventh-magnitude star in Eridanus was perceived by Swift in 1859 to
be almost centrally placed in a shining corona.[944] It is perhaps
identical with N.G.C. 5315. Its spectral classification should present
no difficulty. A similar object, equally adapted for spectroscopic
inquiry, was detected by the elder Herschel in Cepheus (N.G.C. 7023).
The nebulosity is particularly strong north and south of the star.
Irregularities of a more marked kind are apparent in other instances. A
tenth-magnitude star in Monoceros was found by Barnard visually nebulous
in 1888, photographically in 1894.[945] A “small dark space,” however,
interrupts the encircling halo. A subsequent exposure with the same
instrument disclosed as “closely nebulous” the 9·5 magnitude star, D.M.
+ 23° 1313.[946] And here again the illumination is unevenly
distributed, the “fuzzy” border to the star-disc being denser south and
east than elsewhere. This object lies almost midway between η Geminorum
and χ^2 Orionis. One in most respects analogous (N.G.C. 2247), detected
by Swift, 24th November 1883,[947] came out noticeably “blurred” on the
same plate with Barnard’s nebulous star in Monoceros. Two further
specimens of the class were photographed by Barnard in Sagittarius.
One—D.M. −19° 4948—is fainter than the ninth magnitude. It has a narrow
fringe of light.[948] The second—D.M. −19° 4953—is of 7·6 magnitude, and
is encompassed by a far-spreading halo, 15′ in diameter,[949]
conspicuous with the camera, although nearly invisible to the eye.
Nebulous stars are frequently compound—perhaps more frequently than
stars clear of cosmic fog. Sir John Herschel recorded at the Cape a
close pair (N.G.C. 5367) as involved in a bright glow two minutes of arc
in extent; and a faint star with an aureola, discovered by Tempel in
Cetus (N.G.C. 707), proved, when scrutinised by Burnham in 1891, to have
a minute attendant at an interval of 10″.[950] A still more interesting
detection concerned a nebulous triplet in Auriga (N.G.C. 1931).
Discovered by Sir William, and described by Sir John Herschel as “one of
the most curious objects in the heavens,” it consists of three stars,
the brightest of 9·5 magnitude, forming an equilateral triangle with a
side of about 8″, placed precisely at the centre of a small circular
nebula. Mr. Burnham had repeatedly inspected it with minor
instruments,[951] but it needed all the power of the Lick refractor to
bring into view a fifteenth-magnitude satellite at a distance of little
more than 2″ from one of the stars of the triangle. As a test for
“seeing” facilities, the pair is of unsurpassed delicacy. Again, a wide
double star occupies the middle point of a pretty large faint nebula in
Monoceros (N.G.C. 2182). The attendant may be only optically such; the
circumstances are on this point indecisive. The chief star, however, was
found by Mr. Burnham to be double in a perfectly unequivocal sense.[952]
A companion of nearly its own magnitude (8·6) is separated from it by a
spatial gap of less than half a second, and the two must assuredly
revolve round their common centre of gravity. Here, indeed, we are
confronted by a profoundly embarrassing question. The couple are
evidently plunged in nebulous matter; their movements must then,
according to received ideas, be impeded, with the result of an eventual
collapse of the system. We can see no escape from the dilemma except by
adopting the startling hypothesis that the nebulous fluid does not
constitute a resisting medium. The difficulty greatly enhances the
interest of spectroscopically determining the velocities of bodies
nebulously connected.
A 6·5 magnitude star in Cepheus (D.M. +57° 2309) appeared in a
photograph taken by Barnard in 1893 “surrounded by a rather
unsymmetrical dense nebulosity.”[953] A “hazy glow” could be seen with
the Lick thirty-six-inch, which, in Burnham’s employment, had already
revealed the star to be very unequally double at 4″.[954] A first-type
spectrum is dubiously ascribed to it in the Draper Catalogue.
The nebulous triplet, ι Orionis, has been more completely observed than
perhaps any of its congeners. It consists of a third and an
eighth-magnitude star 11″ apart, with an eleventh-magnitude satellite at
49″, described by Admiral Smyth as “grape-red” in colour.[955] Sir John
Herschel perceived the group to be “involved in a feeble nebula 3′ in
diameter,”[956] and in the nebula (N.G.C. 1980) there was apparent with
the Rosse reflector a central cavity containing the bright
star-couple.[957] Possibly the effect was an illusion due to their
effacing radiance; but this cannot be taken for granted, since “holes”
in nebulæ are an attested phenomenon. And the early observations at
Parsonstown approve themselves as singularly accurate through the
confirmatory evidence of the best recent photographs. The spectrum of ι
Orionis is of the helium type, and Dr. McClean identified in it three
members of the Pickering series of hydrogen, besides many oxygen
lines.[958] The surrounding glow emits the ordinary nebular rays, but
they make no show, either directly or by reversal, in the dispersed
stellar light. Yet before reaching outer space, that light has to
traverse enormous volumes of incandescent or luminescent nebulium. The
anomaly presented by the absence from the Fraunhofer spectrum of the
solar coronal green line is here repeated in an emphasised form. A long
nebulous streak, visible only on sensitive plates, links the hazy
appendage of ι Orionis with the great formation in the Swordhandle.[959]
Far away in the northern part of the constellation, there is found in λ
Orionis a combination very similar to that presented by the nebulous
trio just considered. A yellowish and purple pair (Σ 738), of 3·7 and
5·6 magnitudes, at 4·2″, are immersed, with a comparatively remote
twelfth-magnitude attendant at 29″, in a nebulous haze, photographed by
Barnard in three hours, 17th September 1893.[960] The discovery was at
once telescopically verified.
Five nebulous stars occur together in a narrow region of Sagittarius,
and three of them are double. The two apparently single are Barnard’s
stars, already mentioned. The pairs are N.G.C. 6589 and N.G.C. 6590,
both first noticed by Swift,[961] and N.G.C. 6595, delineated seventy
years ago by Sir John Herschel. The character of an object photographed
by Dr. Roberts[962] near the spiral nebula M 81 in Ursa Major, needs to
be more satisfactorily determined. Known to Herschel and d’Arrest as a
condensed nebula (N.G.C. 3077), it appeared on the sensitive plate with
a sharp, stellar nucleus in lieu of the woolly disc visually
perceptible.[963] The spectroscope may perhaps help towards its rightful
classification. Nebulous stars merge insensibly into stars with nebular
appendages, such as ω Orionis and σ Scorpii. The former has a dimly
luminous, curved spur running out from it, besides a larger mass hanging
like a cloud above it to the north; the latter is winged with
nebulosity, two pointed projections issuing from it in divergent
directions.[964] The whole of these appurtenances were detected
photographically by Professor Barnard. Both stars afford spectra marked
by helium absorption. The relations of stars and nebulæ are manifold.
Misty trains and tails of all sorts and sizes have stellar foci; they
emanate from stars, or condense into stars; but nebulous stars are,
properly speaking, what Herschel called “stars with burrs”; they give
the usual sharply defined diffraction-discs, although a dim halo spreads
more or less symmetrically round each. The nebular element in such a
combination is entirely subordinate to the stellar; while in stars with
appendages the disparity gradually becomes reversed.
“Rejected” nebulous stars are still worth attention in view of the
possibility that they may be subject to genuine change. The case of 55
Andromedæ is particularly instructive, if only as illustrating the
propagation of error. This is a 5·6 magnitude star, qualified as
_nebulosa_ by Flamsteed and Piazzi, and regarded by Sir John Herschel as
a typical specimen of a hazy star. It figures as No. 428 in his _General
Catalogue of Nebulæ_ (1863), but was omitted by Dreyer from the revised
edition of that work. Sir William Huggins, nevertheless, observed it in
1864 to be “a fine nebulous star with a strong atmosphere;”[965] and
since he used a very perfect refractor, his confirmation of what
Herschel had seen with a reflector had an independent value which might
seem to exclude the hypothesis of association in optical illusion. Yet
neither Lord Rosse in 1848 nor d’Arrest in 1856 had perceived any trace
of nebulosity, and Schjellerup, during some years previous to 1866,
always found the star sharp.[966] So again it appeared to Mr. Burnham in
1879–80,[967] and so it seems likely to remain. We can, however,
scarcely persuade ourselves that several eminent observers conspired to
blunder; and Schjellerup’s theory that Piazzi merely repeated
Flamsteed’s note, which crept, he supposes, into the British Catalogue
by a transference from the great Andromeda nebula, is rendered
unacceptable by the circumstance that 55 Andromedæ follows the nebulous
structure held to have been confused with it at an interval of
considerably more than one hour of right ascension. Nor even if so
extraordinary a mistake had been made, was Piazzi capable, one would
think, of copying it unawares. His high astronomical reputation suffices
in itself to clear him from the charge of such astounding carelessness.
The spectrum of the star resembles that of the sun, a type never yet
unequivocally associated with nebulous attachments. On the other hand,
the normal quality of the light renders their optical creation more
difficult of explanation. Thus the nebulous aspect of 55 Andromedæ must
stand over as one of the unsolved problems of astronomical history.
A similar, but less convincing case is that of 8 Canum Venaticorum. On
four separate occasions Sir John Herschel noticed this fourth-magnitude
star to be surrounded by a “considerable atmosphere.” Yet since no one
before or after him has vouched for its presence, he was presumably
deceived. The spectrum of 8 Canum is of the solar class.
Finally, a 7·5 magnitude star in Cetus, discovered as nebulous by
Stephan at Marseilles in 1880[968] (N.G.C. 988), appeared to Burnham and
Barnard in 1891 devoid of any such peculiarity.[969] A photograph taken
with suitable exposure would serve decisively to test its present
condition. The criterion might indeed fail with stars so bright as 55
Andromedæ and 8 Canum Venaticorum; for their imprinted discs would
become, through chemical irradiation in the time needed to bring out
faint glows, sufficiently distended for their obliteration.
In connection with nebulous stars two lines of inquiry open out. First,
the spectroscopic. The scanty evidence at our disposal is to the effect
that the stellar rays of such objects are of the “Orion” kind; that they
show the quality believed to characterise suns in a primitive stage,
while their aureolas shine like gaseous nebulæ. But these
generalisations rest on a very narrow basis of fact, and probably admit
of interesting and significant exceptions. Indeed, each nebulous star
should be treated as a separate spectroscopic problem, destined to
afford in the course of its solution insight into many obscure secrets.
A second branch of research relates to the structural peculiarities of
stellar halos. Their luminosity is seldom, perhaps never, equably
distributed. Its irregularity sometimes goes so far as to produce the
effect of dark vacuities, photographically attested to be no mere visual
deceptions. What their true nature and origin may be, is a subject for
inquiries likely to be long and arduous. It is scarcely credible that
they are what they appear to be, obscure tunnels, striking, in the
direction of the earth, right through the heart of immense spheres of
shining tenuous matter. The alternative view is preferable that the
so-called “atmospheres” of stars are really effluences—that they consist
essentially of spiral coils wound closely enough to merge ordinarily
into an approximately uniform surface, while leaving in certain
circumstances conspicuous gaps between their luminous folds. If this be
so, nebulous stars fall into line with cometary nebulæ, the trains of
which take a more or less completely annular shape; but their nearest
allies are unquestionably to be found in the planetary family; and this
brings us to the subject of our next chapter.