CARBON STARS.
Stars of Secchi’s fourth type, also known as “carbon stars,” are the
most exclusive of stellar families. They hold remarkably aloof from
every other. They have indeed traceable relationships; but the genealogy
obscurely indicated by them needs authentication.
Mr. Espin published in 1898 a catalogue of 237 carbon stars,[396] and
about a dozen objects of the kind have since been detected. None are as
bright as the fifth, and only seven exceed the sixth magnitude.[397]
Their inconspicuousness probably arises, not from any deficiency of
intrinsic light-power, but from the overwhelming absorptive action of
their atmospheric envelopes. Thus only a small part of their original
radiations attain to outer space; none of the shorter wave-lengths
escape; the spectra are cut off short a little below the place of the
blue calcium line. Intense visual redness is a consequence. These stars
glow sanguine in the field of the telescope; they are variously compared
to “drops of blood,” to carbuncles, garnets, or rubies. By a rough
estimate, 12 per cent are strikingly variable, the proportion being
nearly the same as for third-type stars. Scarcely any, however, shine
steadily; if attentively watched, they can be perceived to flicker and
fluctuate more or less extensively.[398] Some of their changes are
indeed so lasting as to suggest a permanent drop or rise (as the case
may be) in the photometric scale. The circumstance that instability of
light ordinarily accompanies redness of colour in stars is most curious
and significant.
The fundamental characteristic of fourth-type spectra is the presence of
three deep bands, degraded towards the violet, sharp towards the red.
They are a negative copy of the emission-bands displayed by comets. Five
or six additional dusky stripes, so far unidentified, are of less
distinctive construction. The general effect of these spectra differs
from that of the fluted sort chiefly in two ways. First, the _columns_
of absorption are broader and more massive; they are of Doric rather
than Ionic proportions. Secondly, they are illuminated from the opposite
direction; the chiaroscuro is inverted. Their variations of relative
intensity in different objects have been proposed as subsidiary
classification-marks, but cannot be much insisted upon. Such
individualities—as Professor Dunér pointed out—do not imply radical
distinctions; and they are so prevalent and so various that, by closely
attending to them, “one might easily get as many subdivisions as there
are stars.”[399]
The linear spectrum in carbon stars is seen with difficulty through the
cloak of the bands. It is, however, none the less important. Sodium and
iron contribute to it, but most of its constituents still lack
interpretation. Hydrogen and helium alike fail to appear. No calcium
lines are visible; indeed those in the blue and violet, which would most
naturally be looked for, could not show through the dense veil of
absorption shrouding the upper spectral reaches; so that their seeming
absence is consistent with the presence of a calcium ingredient in the
stellar atmospheres. Their carbonaceous strata nevertheless give them
their special character. No other sidereal objects, except an
imperfectly observed variable star, show a trace of the cometary analogy
prominent in the fourth spectral type.
The carbon bands, which constitute its leading feature, were identified
by Father Secchi in 1868, and he noticed besides certain bright lines,
the reality of which, long discredited, has quite recently been
confirmed. He noticed them, however, somewhat confusedly, for he at
times failed to keep them apart from the illusory effect of vivid
emission caused by the prismatic gleaming of the intercolumnar zones.
Professor Hale in 1898, effectively aided by Mr. Ellerman, applied the
photographic method with remarkable success to the investigation of
these spectra. They offer no facilities to the camera. The use of
ordinary plates is of course precluded by their deficiency in blue rays;
only those rendered “orthochromatic” by suitable dyes avail for their
delineation; and these are found practically inconvenient owing to
irregularities in sensitiveness. A series of splendid pictures was
nevertheless obtained with the aid of the Yerkes forty-inch refractor;
but the arduousness of the undertaking can be estimated from the fact
that, with a train of three prisms, exposures of nine hours were
required to secure impressions comparable with those given by the
spectrum of Betelgeux in twenty seconds.
The research embraced, to begin with, twenty-two stars, ranging from 5·4
to 8·2 magnitude.[400] Most of the spectrographs were limited to the
region D to _b_; but a few extended to λ 4450, where dark blue merges
into indigo, and one ranged far down in the crimson. This was derived
from the brightest specimen of the type, numbered 152 in Schjellerup’s
Catalogue of Red Stars. Situated in Canes Venatici, it shows a spectrum
of such rare beauty as to justify the title of “La Superba,” bestowed
upon it by Secchi in 1868. Its yellow section, photographed by Hale, is
shown in Plate XI. Fig. 2. A surprising amount of detail is imprinted in
it. On the original negatives over a hundred lines were measured where
no more than three or four had been previously recorded.[401] Most of
them are dark, but some are bright, among which, apparently, may be
reckoned two yellow rays, compared by Secchi to “exquisite threads of
gold.” They are prominent in our figure at wave-lengths λ 5593 and λ
5693, which, as Professor Hale was careful to point out, “agree very
closely with” those of “two bright lines in the spectra of the
Wolf-Rayet stars.”[402] He adds the caution that, several other similar
approximate coincidences notwithstanding, “it is too soon to conclude
that these classes of stars are related.” A fine group of vivid green
lines was also photographed, and some blue rays were suspected. Although
intensely red, 152 Schjellerup has not been observed to vary from 5·5
magnitude.
The typical star of Miss Maury’s “Group xxi.” is 19 Piscium. It shows a
splendid four-zoned spectrum, vivified by the twinkling of emission rays
(see Plate XII. Fig. 2). Secchi noted in it the shining of the same
“threads of gold” previously seen in 152 Schjellerup,[403] and they may
be considered as a feature common to all spectra of this class. The
opinion to this effect expressed by M. Dunér carries great weight. It
was in 1884 entirely adverse to the reality of the bright lines recorded
by the Roman astronomer;[404] but he changed his view on improving his
instrument. The materials for his invaluable Memoir of 1884 were
collected with the Lund ten-inch refractor; in 1893 a Steinheil of
fourteen inches aperture became available to him at Upsala. With it he
at once undertook a revision of his former work,[405] which, although
hampered by serious interruptions, progressed steadily down to 1898. M.
Dunér was not disappointed in his hopes of seeing more and better with
the larger instrument; and he chronicles as of primary importance “the
fact that he was able to detect without difficulty bright lines in
various spectra, which at Lund were either invisible, or at least could
not be discovered.” Professor Hale’s photographic registration of them
was thus visually authenticated by an observer of unrivalled experience,
and was further verified with the great Lick telescope by Professors
Keeler and Campbell under conditions so admirable as to leave little or
no room for surviving doubts.[406]
It may then be regarded as an established fact that spectra of the
fourth type include elements of direct emission. They are subsidiary,
yet distinct, and seem to be unfailingly present in all members of the
class. Three characteristics may provisionally be ascribed to these
curious bright lines. In the first place, they are of entirely unknown
origin. Hydrogen and helium are equally (so far as published measures
enable us to judge) alien to their production. Some of them may coincide
with Wolf-Rayet lines, but if so, it is with Wolf-Rayet lines which
themselves lie outside the range of terrestrial acquaintanceship. Not
even the exotic light of nebulium or coronium can be seen to glimmer in
carbon stars. Secondly, they are independent of luminous change. They do
not betoken variability. They occur indifferently in objects of steady
lustre and in those subject to wide vicissitudes. Nor has the slightest
sign of inconstancy been detected in the rays themselves. They do not
fade and flash capriciously or periodically. They shine equably—to all
appearance—from year to year, and from decade to decade. Thirdly, the
locus of their development is above the region of carbon absorption. The
tinted rays evidently overlie the dark bands; they are seen projected
upon them. The substances to which they are due must then be found at a
higher level in the stellar atmospheres than the carbon vapour. As
Professor Hale remarks,[407] the case is paralleled in the sun, where
hydrogen and calcium rise to great heights, while a shallow layer of
carbon-gas lies low at the base of the chromosphere. This arrangement of
emissive and absorptive strata does not prevail—as we shall see later—in
stars of all spectral classes. They are, on the contrary, markedly
distinguished in this respect, and the distinction implies profound
physical differences.
It was found possible at the Yerkes Observatory to form a sequence of
eleven stars,[408] in the order of growing depth of carbon absorption
(see Plate XII.). The transition from one of these objects to the next
was so gradual as to suggest that they represented actual phases of
development. This, however, is merely a convenient hypothesis. One of
the earliest of the series is unique, according to Dunér, in the
relative strength of its spectral bands. Those due to carbon are quite
feeble, while one of untraced origin in the red is broad and black. This
star, known as 280 Schjellerup, is scarcely brighter than the eighth
magnitude, so that it can be dealt with to advantage only by the aid of
powerful instruments. Passing on to 19 Piscium, we find _one_ of the
three carbon bands dim, the others—in the green and blue
respectively—very wide and dark. Their unequal prominence constitutes a
striking anomaly. It recalls the variations in relative brightness of
the hydrogen lines recorded with surprise in nebulæ and sundry species
of stars.
A spectrum intermediate between those of 19 Piscium and of 152
Schjellerup is shown by the variable star U Hydræ (132 Schjellerup.)
Dunér noticed long ago the wonderful chromatic effect of its four
brilliant zones,[409] set off by deep bays of absorption; and Secchi
perceived in it a green, as well as a yellow pair of fine rays, the
genuineness of which is more than probable. The star fluctuates
irregularly from 4·5 to 6·3 magnitude, but is very rarely seen at its
maximum brightness. Professor Hale’s spectrographs afforded evidence of
a partial but very interesting resemblance between its spectrum and that
of μ Geminorum, a fine example of the fluted description, and the
agreement—as can be seen by inspecting Plate XV.—extends to the sun.
“Further toward the red,” he tells us,[410] speaking of the banded
varieties, “the spectra become very unlike, though even here there are
certain important points of resemblance which must be carefully
investigated.” Only their linear elements are naturally in question; the
shadowing bands are totally unlike in the two classes. The coincidences
detected, however, are of real importance as forging a link, even if a
slight one, between stellar families that stood previously entirely
apart.
The invisibility (up to the present) of hydrogen in carbon stars is not
easily accounted for. The substance must enter into their composition;
its diffusion is seemingly universal and profuse; why, then, is its
manifestation, whether by emission or by absorption, suppressed in this
particular class of objects? The same query may be put in regard to
comets, and the same obvious, although perhaps insufficient answer
presents itself, namely, that their stock of hydrogen has been consumed
in the fabrication of hydrocarbons. It is worth noting besides that the
only metals yet identified in these stars—sodium and iron—are precisely
those perceived to glow in one or two exceptional comets. But this may
be only a chance concurrence.
[Illustration:
PLATE XV.
Spectra of Stars of Types II., III., and IV. (Hale and Ellerman).
1. The Sun (Type II.). 2. μ Geminorum (Type III.). 3. 132 Schjellerup
(Type IV.).
]
The rule of colour in carbon stars long seemed inviolable; yet there are
exceptions to it. Two spectra of the kind well extended in the blue were
photographed at Harvard College in 1891, and they belong, in fact, to
white stars.[411] They are situated, one in Aquila, the other about
three degrees north of ε Ceti, and are of the seventh and eighth
magnitudes respectively. Their investigation ought to prove peculiarly
instructive, for in them the type has developed under most unusual
conditions. It is besides more completely exhibited. Sections of these
spectra can be registered and examined which in other analogous objects
are concealed by dense general absorption. It should, for instance, be
possible to determine whether calcium lines are really or only
apparently absent from fourth-type spectra.
Carbon stars, there is little doubt, are inordinately distant from the
earth. None, we believe, have any measurable proper motion, and
experiments on their annual parallaxes would certainly prove a waste of
time and trouble. We have, then, no means of estimating their real
brilliancy, but it _must_ in some cases, and it _may_ in all cases, be
exceedingly great. These objects show a marked preference for the Milky
Way.[412] They occur, however, in other parts of the sky as well. They
are condensed towards the galactic plane, but not limited to it. They
are unmistakably, yet far from exclusively, swayed by its attraction.