Chapter VI: Stars with Fluted Spectra

The blue light of these stars is powerfully absorbed by an intensification of the screening effect observed in the sun. They are accordingly rufous, or red. Their spectra are profoundly scored besides with metallic rays, generally agreeing in position with, although differing in relative intensity from, the Fraunhofer lines. Thus the calcium line in the blue has gained still further upon the great pair in the violet than in transition stars of the solar type. Finally, banded absorption has come in. A complete system of ten or eleven flutings, sharp towards the violet, graduated insensibly towards the red, shadows nearly the entire visible spectrum. It is printed in stereotype. The bands are variously impressed, but similarly located, in all members of the class. This gives strong assurance of an identical origin. We do not yet know how they are produced, or by what substances, but there can be no doubt that their explanation in one star will apply to all.

With high dispersion the bands can be resolved into fine lines set very close together.[386] The fluted effect is due to the crowding of these lines towards a limiting wave-length prescribed, beyond question, by a rhythmical law. For that each band represents, as it were, a condensed series there need be no hesitation in admitting. Indeed, by mere reduction in scale the hydrogen procession in a white star assumes the aspect of a genuine fluting. A promising start has even been made in the research of laws regulating the distribution of lines in bands.[387] Each of the stellar stripes is then, so far, a separate entity, while all may be linked into harmony by subordination to some higher unknown principle. Whether they originate from one or many forms of matter has still to be determined; nor is there any certainty as to whether elements or compounds are concerned in their production.

Stars of Secchi’s third type, forming the class just now under consideration, are divided by Miss Maury into three groups. Antares and β Andromedæ belong to the first of these (“Group xvii.”). They show all the characteristic bands, slightly marked, and transparent enough to allow every detail of the linear spectrum to be clearly visible. Betelgeux (α Orionis) is the pattern of the next group. Its prismatic light makes a beautiful and wonderful effect. The usual multiple absorption is exerted upon it, but with a delicately balanced power. The blue rays retain appreciable vivacity; the flutings are not so deep as to obscure the underlying rays; they are finely shaded, yet exquisitely distinct. Ten were measured by Vogel and Dunér, the strongest of which are the fifth and seventh, with their _steep_ sides at λ 5453 and λ 5169 respectively. Battalions of dark lines show through them. On the Harvard plates Miss Maury counted 463, mostly composite, between the sodium D and the calcium H. The latter and its associate, K, have shrunken somewhat from their giant dimensions in Arcturus.[388] Iron absorption predominates. The rays significant of it are more prominent than in the sun, and some have unilateral shadings—a feature also visible in the spectra of sun-spots and of metallic oxides, and indicative probably of a decline in heat.[389] Professor Keeler remarked that the lines in Antarian stars “are essentially those of the solar spectrum, but the relative intensities are not the same, and the general aspect of the spectrum is quite different from that of the spectrum of the sun. The strong lines are mostly those of iron—apparently the low temperature lines. Their relatively greater strength in the star spectrum gives to some well-known solar groups (notably the _b_ group) quite an unfamiliar aspect.”[390]

Hydrogen-absorption is much more effective in Betelgeux than in any other spectrum of the fluted kind. It is, however, distinctly, though feebly represented in all by the four lowest members of the Huggins series. The rest are either absent or shrouded in overlying vapours.

In ρ Persei, a star capriciously variable between 3·4 and 4·2 magnitudes, the bands are perceptibly deeper than in α Orionis. On the other hand, the metallic rays seem rather less numerous and intense. But this diminution may be more apparent than real. The comparative faintness of the light they interrupt would partly account for it, and the added density of the associated flutings would help towards effacement.

Their still greater opacity in α Herculis occasions effects of contrast with the vividly tinted bright zones, described as “singular and magnificent” by Father Secchi, one of their earliest observers (see Plate XI. Fig. 1). Professor Keeler, whose study of fluted spectra was based on photographs of high dispersion showing manifold details, found the dark groovings, plentiful in α Orionis, to be present only as a comparatively scanty survival in α Herculis. Miss Maury reached a similar conclusion. Yet Dr. Vogel was struck with the richness in absorption-lines of this spectrum and the analogous one of β Pegasi.[391] Both these stars vary irregularly, α Herculis from 3·1 to 3·9, β Pegasi from 2·2 to 2·7 magnitude. Next to Antares, the brightest specimens of this class in the southern hemisphere are γ Crucis and β Gruis. Mr. McClean obtained spectrographs of all three at the Cape in 1897.[392]

Uncertainty has often been expressed as to the true nature of the luminosity in the open spaces of fluted spectra. Are they simply intervals of unshaded photospheric radiance, or is their brilliancy reinforced by the addition of bright lines? Just where they meet the black edges of the shafts of absorption, their splendour exceeds, in the opinion of some observers, what could be produced by contrast alone. It seems, nevertheless, unlikely that rays of emission should occur in these positions and nowhere else in the same spectra. Moreover, illusory impressions of the kind, both visual and photographic, are common and pertinacious. Nothing, however, is more inimical to truth than dogmatic denial; we must be ready to admit much that we should beforehand have deemed impossible, even the reality of far-fetched coincidences; for anticipation is often belied by fact. Professor Keeler, whose mind was singularly free from prepossessions, found it “impossible to avoid the conclusion” that in the spectrum of α Herculis “the edges of the zones bordering on the dark bands are bright—much brighter, that is, than the average continuous spectrum—and that they are due to a real predominance of emission at the regions of the spectrum in which they occur. In the case,” he wrote further, “of stars like α Orionis, of a less pure type, such a conclusion could not be safely drawn; yet the superior brightness of the spectrum at these places is obvious, and it can be traced even in second-type stars. May there not, after all, be bright regions in the solar spectrum, such as Draper supposed he had found in the places of the bright oxygen lines? And what is the relation between the dark bands in third-type stars and the bright zones which border on them?”[393]

Questions more easily asked than answered. They suggest doubts, not at once to be set at rest, as to the nature of so-called “continuous” spectra. May they not in certain cases include several maxima of radiation? The possibility is at least not excluded of individual differences in this respect between stellar photospheres. Yet of true gaseous emissions there seems to be no trace in Antarian stars. The admission of its presence could not be made for one without being extended to all; and many spectra of the class are clearly exempt from abrupt intensifications.

Chemical recognition has not been carried far in them. The familiar lines of only five substances—iron, calcium, magnesium, sodium, and hydrogen—are entirely unmistakable. The rest await more searching scrutiny. It will be of especial interest to determine whether titanium and its usual associate, vanadium, retain in these objects any share of their importance in sun-spots. Helium-absorption, too, which occasionally emerges to view in spot-spectra, might be looked for with some prospect of success, but is likely to be inconspicuous. The fundamental problem, however, in this connection relates to the origin of the flutings. It ought to prove capable of a definite and, so to speak, a simultaneous solution. For the members of the system show the coherence of a structural design. They form a marshalled array, an interdependent order. Their occurrence piecemeal need not then be expected. They will be recognised together or not at all.

Stars with fluted spectra have a fixity significant of immeasurable remoteness. Yet two—Antares and Betelgeux—are of the first order of apparent brightness. Their real magnitude must hence be prodigious. An approximate estimate of it can be arrived at in the case of Antares, which has an ostensible parallax of 0·021″, corresponding to a light journey of 155 years. This, at least, was the outcome of Mr. Finlay’s measurements at the Cape; but it is so small as to lie but slightly outside the margin allowed for their probable error. Its genuineness can then only be assumed for the purpose of fixing ideas; since the star may be indefinitely further off, while unlikely to be appreciably nearer, than this minute annual shift asserts it to be. Under this reserve we may compare Antares with our standard star, α_{2} Centauri, which, as we have seen, is equivalent to comparing it with the sun. The result is to show that four hundred suns, in its place, would barely supply the light we receive from the _alter ego_ of Mars.[394] And this is only what remains after a heavy absorption-toll has been levied—a toll of probably twice the amount paid by the sifted solar beams. The photospheric extent of Antares may then be set down as at least eight hundred times that of the sun, while the immense sphere it covers may be held, on good grounds, to have a comparatively high mean density. Even on the basis of equality in this respect, its mass would exceed that of the sun more than 22,000 times, while gravity at the surface of this unimaginable globe must possess at least twenty-eight times its solar power. Similar reasonings apply to Betelgeux, only with still further enlargement in measure of the conclusions they lead to. For Betelgeux is a more brilliant luminary than Antares, and its immobility in the sky is, if anything, more nearly absolute.

The processes of interior circulation in such bodies are extremely difficult to realise. The intensity of radiation must depend—other things being equal—upon the promptitude of delivery at the photospheric level of heated stuff from within. This must increase with the force of gravity, which is the driving power on ascending and descending currents; but it must, on the other hand, fall off as the sphere they traverse grows more compact. The manner in which the balance is struck in each individual star between these opposing influences transcends every rational conjecture. We can only see that it must vary widely, and that its variations necessarily affect the photospheric composition and the radiative characteristics of the globes in which they prevail.

Antarian stars obey no special law of distribution. They are scattered at large over the heavens, and usually in isolated positions. They show as a rule no tendency to gather into groups. A collection of nine specimens, located in the intervening space between the two grand clusters in Perseus, is perhaps unique. Discovered by Mr. Espin in 1891,[395] this nest of red stars appears like a garnet clasp linking together a pair of diamond aigrettes. They are of about the eighth magnitude, but may nevertheless, since they are assuredly vastly remote, be most majestic orbs. Not that we should ascribe to all stars of this spectral type the colossal dimensions of Betelgeux and Antares. The same degree of variety may be supposed to exist among them as among solar stars. Yet the analogy may not hold. It is conceivable that very great mass is a pre-requisite for the development of a fluted spectrum.