Chapter VII: 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.

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.