Chapter XIV: The Spectra of Double Stars

The spectra of double stars stand in the closest relations to their colours. This, indeed, is almost a truism, since spectra merely show in detail what is summarised in mixed tints. Yet the two forms of statement are not tautological. The result of prismatic analysis cannot be wholly anticipated from the visual impression. The eye makes no attempt to reduce its sensations to their elements. Totally different rays may be blended and balanced so as to produce an identical sum-total to the perception of the optic nerve. Nor would it be in all cases easy to pronounce upon the colour of a star from the simple inspection of its spectrum. One cannot tell beforehand, so to speak, how the eye will take things. Some scarcely measurable reinforcement of selective stoppage, a few rays of absorption added or removed, may make the difference between rosy and golden, or purple and pink. Thus neither direct nor prismatic observations are superfluous; but the latter, as affording scientifically accurate and—through photographic means—permanent records, are by far the more important.

The spectra of double stars unlike in colour are usually of different types; and here a remarkable rule applies. Contrasted pairs are, with few and doubtful exceptions, notably unequal in brightness, and the warmer tint invariably belongs to the larger component. Blue, green, or violet stars are always the satellites of red or yellow primaries; and, in accordance with these indications, they give first-type spectra, while their brighter and more ruddy neighbours show Antarian flutings or solar lines. We are thus led to the unexpected conclusion that, of two globes simultaneously contracting, the larger, which should naturally cool more gradually, and therefore run through its evolutionary stages at a more leisurely pace, attains solar standing while its companion still remains a “white star.” This relation is the very crux of cosmic growth-theories; something more will be said about it in the next chapter.

The separate spectral examination of coupled stars is far from easy, and has indeed rarely been attempted. Only by Sir William and Lady Huggins has the subject been prosecuted systematically and with success. Their application to it of photography, rendered possible by the completion, in 1897, of an ingeniously devised reflecting slit, constituted in itself an immense advance. Previously, only the superposed spectra of double stars had been chemically recorded, and these, for discriminatory purposes, were of no more than provisional use. One coloured pair, however, presents less difficulty in this respect than the rest. The components of β Cygni lie far enough apart to give distinct spectrographic images, formed by an objective prism, on the Draper Memorial plates.[515] Specially inviting as well to direct scrutiny, they were among the earliest objects subjected to Sir William Huggins’s light-analysis.

The pair consists of a third-magnitude “topaz” star and a fifth-magnitude “sapphire” at 34″. The unaltered value of this interval since Bradley’s measurement of it in 1755 almost assures us that they drift together through space under the stress of a physical bond. For their proper motion, though very small, would have sufficed, in the course of a century and a half, to produce unmistakable relative displacement. Blue stars, besides, are never solitary; and the companionship upon which their uncommon hue depends must evidently be real, not simply optical. Plate IX. Fig. iv. shows the spectra of these stars as photographed by Sir William and Lady Huggins.

Their complete diversity is apparent at a glance.[516] The hydrogen series is writ large and strong on that of the minor luminary; helium absorption is not apparent; the Sirian type is pronounced. Of solar quality, no less decidedly, is the golden light of the primary. Yet it cannot escape notice that the photographed spectra do not explain the vivid colouring of β Cygni; they might have been taken (speaking broadly) from any two stars of the types represented. This, indeed, was just what should have been expected, since the special absorption differentiating them from the common run of stars was known to lie outside the range of sensitiveness of ordinary plates. As regards the blue member of the pair, at any rate, there could be no doubt of the fact. A set of dark bands, cutting out a goodly proportion of its yellow and orange rays, were observed by Sir William Huggins in 1864,[517] and again by Dr. Vogel in 1872,[518] and they correspond with, and fully explain, its chromatic peculiarity. The topaz hue of the primary cannot be so directly associated with the subtraction of particular qualities of light; it is more probably due to an enhancement of that veiling of the higher spectral reaches to which sunshine owes its primrose tinge. Further investigation is, however, desirable; above all, the photographic registration on isochromatic plates of the unfamiliar absorption-bands from which the companion of β Cygni derives its distinction as an azure star.

The theory of “composite stellar spectra” was proposed by Professor Pickering in 1891.[519] Spectrographic impressions showing a mixture of types should, he explained, result from the superposition of dissimilar spectra derived from close or telescopically indivisible stars. The forecast was verified by Miss Maury’s detection of eighteen self-imprinted images of the compound sort.[520] “In spectra of this class,” she writes, “the K-line appears either unduly narrow or overspread with a peculiar haziness. This appears to be due to the presence of an additional star, having a spectrum which belongs to some group earlier in the series. It is also significant that in such spectra the first-type characteristics predominate in the ultra-violet, the second or third-type features in the green and blue.” These duplex effects demonstrably own, in certain of the instances enumerated, a duplex cause; for they include γ Andromedæ, ε Boötis, and α Piscium, all three remarkable pairs. The presumption is accordingly strong that spectra appearing hybrid in small-scale delineations really emanate from a double source, although visual evidence of duplicity be wanting. Indeed, several of Miss Maury’s _crypto-doubles_, ο Leonis, ο Andromedæ, and α Equulei among the number, have been spectroscopically resolved by Professor Campbell into unlike pairs. And even should the motion-test fail, it need not be inferred that the star recalcitrant to it is single; for a negative result may signify merely that the method is inapplicable owing to the high inclination of the plane in which coupled stars revolve.

One of the show-objects of the heavens is γ Andromedæ, composed of a chrome-yellow star of 2·2 magnitude, and a sea-green fifth-magnitude attendant at 11″. The attendant itself can be divided with a good telescope into a blue and a green star, considerably advanced along an elliptic track since their first observation by Otto Struve in 1842; while the wide pair, discovered by Christian Mayer in 1777, remains relatively fixed, although their systematic union is attested by an identical secular progress of about 7″. Their spectra, photographed at Tulse Hill, closely resemble those of the components of β Cygni, the different patterns of absorption stamped on them forming almost as striking a contrast in the negatives as the colours of the original objects do in the sky. A similar combination is offered by ε Boötis, but on a reduced scale. The ultramarine satellite is here poised at a distance of only 3″ from its golden primary. Their spectra have, indeed, been no more than inferentially distinguished. Miss Maury’s scrutiny of the joint impression left by them upon the Harvard plates made it, however, fairly certain that, as usual, the blue star is of Sirian, the yellow star of solar quality; so that a relation of development is again indicated just the converse of that held, on _à priori_ grounds, to be probable.

The theoretical incongruity is, in some cases, heightened by the substitution for the sun-like primary of a red star giving a fluted spectrum. Such a pair is α Herculis. An “emerald” star of the sixth magnitude, at a distance of 5″ from its glowing leader-orb, yielded to Sir William Huggins’s early examination a spectrum of precisely the same character as that of the satellite to β Cygni. Antares, too, is quite similarly coupled with a green star, the spectrum of which, judging by the duplex impressions obtained at Harvard College, resembles that of Sirius, with, it may be, some approximation to that of Procyon.[521]

Mr. Burnham performed in 1881 the unprecedented feat of dividing a third-type star into a very close pair. He detected a satellite of the ninth magnitude situated within just one second of arc of η Geminorum, a fine red star, variable in a period of 229 days, although its maxima are unmarked by any signs of gaseous emission, doubtless because of the comparatively slight extent of the light-change. The spectrum of the small star cannot, of course, be directly observed, but its nature may be indicated by colour-observations. Should a glint of blue or green be caught under favourable circumstances, the inference that it proceeds from a source of the Sirian quality can be unhesitatingly drawn. A particular interest attaches to η Geminorum as the only Antarian star with a companion likely to prove sensibly mobile within a reasonable lapse of time.

The spectra of double stars that are unstable in colour have an importance both evidential and explanatory. They illustrate and tend to expound chromatic phenomena. The diversity in light-quality of 95 Herculis is then of extreme significance. These stars, as our readers will remember, are now perfectly matched. They are of equal brightness, and of the same yellow shade. But half a century ago they displayed brilliant complementary radiance in red and green. And their spectra correspond, not to their present uniformity, but to their historic contrast. Vogel in 1899 recorded for one component—presumably the star formerly green—a Sirian, for its twin a solar spectrum. Additional weight is thus lent to the old observations; and a hint, not to be neglected, is given as to the probability of future change.

It is less surprising to meet with spectral dissimilarity in the components of γ Delphini. For they differ in magnitude, and very markedly in colour, notwithstanding past intervals of agreement. And it was just during one of their periods of agreement, in 1883, that Vogel found the larger star to be of solar type, while its companion, now green, but then colourless, gave a Sirian spectrum. Here again, as in 95 Herculis, spectral distinctions seem to persist while chromatic distinctions are alternately effaced and restored.

A good many yellow stars have purplish attendants of dubious spectroscopic standing. Their quality remains untried, and is difficult to conjecture. One of the best examples is η Cassiopeiæ, a revolving pair consisting of a 3·5 and a 7·5 star 5″ apart. The primary emits golden light marked with the solar stamp of absorption; its satellite has been variously described as violet, rosy, and garnet. These notes of colour, indeed, supply no hint as to the nature of the corresponding spectrum; but some indication that it is more “advanced” than that of the large star may be gathered from the mass-relations of the pair. Their gravitational disparity, as determined by Otto Struve, is 3·7 to 1, while their light disparity is 40 to 1. In other words, the satellite is nearly seventeen times less luminous than it would be if of the same mean density with its primary, and of equal areal lustre. In reality it is probably both more compressed and less brilliant. But these properties belong to a comparatively late stage of growth, and should be associated with a strongly absorptive atmosphere. The precise type of absorption characterising dependent stars of a violet hue it would be rash to predict, but it is very desirable to ascertain.

A pair closely resembling η Cassiopeiæ is ξ Boötis. Again in this case a yellow primary of solar type has a rose-purple attendant actively circulating round it. With it may be classed a couple in Pisces (Σ 3036), coloured “very little yellow and dusky red,”[522] and probably β Cephei, composed of a sulphur-tinted helium star of 3·4 magnitude,[523] and an eighth-magnitude violet attendant at 14″.

The great majority of double stars present, however, the same or similar tints; they are white and creamy, or sulphur-coloured and golden, and the spectra derived from them accord entirely with these indications. They are almost always variants of one type. But the rule observed in contrasted pairs that the smaller is the earlier star is here inverted. The subordinate members of systems undistinguished for colour often show signs of having progressed further on the road towards extinction than the larger orbs. This principle is finely illustrated by the grand southern binary, α Centauri. Now these stars are almost exactly equal in mass, yet one gives only a quarter of the other’s light. It is also more deeply tinted with yellow; we may, indeed, safely infer that it is dimmer mainly because of the additional absorption to which its colour testifies. The spectra of the pair, splendidly delineated in Sir David Gill’s photographs, are both of the solar class, but with a difference.

That of the brilliant component is an exact copy of the Fraunhofer spectrum (see Plate X. Fig. 2), while that of the inferior star might be called post-Arcturian, manifesting traces of affinity with the fluted type of Betelgeux.[524] The spectral relations of α Centauri doubtless prevail in many other systems, but they do not arise inevitably, even under quite similar conditions. Thus the unequal stars of γ Leonis give virtually identical spectra of the Arcturian or post-Arcturian species;[525] and the equal stars of γ Virginis, though of Sirian type, are unmarked by the smallest difference in the mode of absorption. It would then appear that, while two globes cast, as it were, in the same mould, like those united in α Centauri and γ Virginis, frequently proceed _pari passu_ along the life-course of suns, one may outrun the other under the influence of unknown circumstances. Couples unassorted in size comport themselves differently; but here, too, allowance has to be made for original diversities of constitution, or supervening incidents of development.

To resume. The colours of double stars afford preliminary indications as to the nature of their spectra, but cannot, in all cases, be interpreted with much confidence. Blue and green stars are, as a nearly invariable rule, the satellites of red or yellow primaries. They belong to the Sirian type, modified, probably, by special absorption serving to lend predominance to the more refrangible rays, and so produce their unusual tints. “Purple” attendant stars have also been observed; the quality of light, however, corresponding to this designation remains unknown. It may prove to be stamped with strong absorption, such as would be symptomatic of advanced condensation; and if so, purplish or violet satellites are a radically distinct class of bodies from azure stars; for they might be inferred to be proportionately more massive and less luminous than their primaries, while the inverse relation doubtless holds good in gold and green as well as in topaz and turquoise combinations. Stellar pairs of equal magnitudes are, with the rarest exceptions, alike in colour and spectrum. They are primrose-tinted—scarcely ever pure white—and of solar or Sirian type.

The spectra of couples no more than two seconds apart can be separately photographed with the Tulse Hill apparatus; and indications of duplicity are often obtained from the composite nature of the spectral impressions given by apparently single stars. Only dissimilar components, however, are capable of being thus distinguished; superposed spectra disclose themselves as such by their differences, among which opposite motion-displacements are occasionally met with. The discrimination of mixed qualities of light in single spectrographic records is a branch of research promising further developments.