Chapter XIII: Colour Variability

Colour variation in stars is a somewhat elusive phenomenon. It cannot be measured; no “colorimeter” yet constructed has given satisfactory results. Then it is subject to adventitious modifications depending upon the state of the atmosphere, the fluctuating sensitiveness of the retina, the nature and aperture of the telescope employed. The same observer after a prolonged vigil will often receive quite different chromatic impressions from those derived with unfatigued sight; nay, his right and left eyes may sometimes pronounce incongruous judgments upon a colour-harmony or a colour-contrast. Such counterfeit changes, however, are slight and evanescent; with due care they can always be separated from intrinsic variations. The endless individualities of colour-vision have, indeed, also to be taken into account. There is no branch in which personal equation tells so heavily, yet so intangibly. Hence casual anomalies of description hardly raise a presumption of actual change. Evidence that it has occurred can only be admitted with extreme caution. The difficulty is to disengage what really deserves consideration from the multitude of floating statements tending only to bewilderment.

Three kinds of colour-variation may be discriminated. They severally affect periodical stars, red stars fairly constant in light, and tinted star couples.

(1) Stars with a light-cycle of less than a hundred days are usually of an unchanging yellowish hue; but “long-period” variables are characteristically red, and redness in stars appears to be rarely a fixed or stable property. It might be compared to an external covering capable of alteration in opacity, or even of entire removal, and connected in its effective action with complex, more or less unsettled conditions. Very commonly, the rises and descents in magnitude of such stars are associated with fadings and flushings of colour, a deeper tint generally accompanying a low light-phase. For this there is a double cause, in the diminution of brightness, and in the increase of absorption. The first acts physiologically. A faint ray strikes the eye as redder than one more brilliant, although both be of the same refrangibility. The second works objectively. Absorption in stellar atmospheres tells mainly on the blue end of the spectrum. Hence, as darkening closes in upon the shorter wave-lengths, the stars redden more and more. Mira, which is not properly a colour-variable, shows this effect markedly. Certain objects of the same class, however, change more radically, and less consistently. Their fluctuations in hue correspond very partially to their fluctuations in light. Colour-change seems to progress independently, and from a superficial point of view quite capriciously.

At Sir Cuthbert Peek’s observatory near Lyme Regis, a score of variable stars have been kept under watch since 1887. The data regarding them collected by Mr. Grover are remarkable and suggestive in several particulars, especially as regards the correlation of colour with magnitude. An important example is afforded by S Herculis, a star varying from the seventh to the thirteenth magnitude in about ten months. It is strongly red with a fine fluted spectrum, yet has, at sundry times, been seen completely blanched. We extract from the _Rousdon Observations_ some notes of its colour, with the corresponding dates and magnitudes.

_S Herculis_
┌──────────────┬──────────┬──────────────────────────────┐
│ Date. │Magnitude.│ Remarks. │
├──────────────┼──────────┼──────────────────────────────┤
│1886, Nov. 12 │ 9·4│White, sharp, and distinct. │
│ „ Nov. 29 │ 8·5│Decided red. │
│1887, May 16 │ 10·9│White; little, if any, colour.│
│ „ Dec. 20 │ 7·3│Deep ruddy. │
│1888, Oct. 15 │ 6·5│Fiery red. │
│1889, June 29 │ 9·0│Dull greyish. │
│ „ Sept. 30│ 8·3│Deep coppery red. │
│ „ Oct. 22 │ 9·1│Blood red, well defined. │
│1890, May 23 │ 9·1│Grey or ashen colour. │
│1891, May 12 │ 6·8│Brilliant scarlet. │
│1893, Aug. 17 │ 9·3│Dull white, well defined. │
│1894, Sept. 8 │ 7·5│Nearly white, sharp. │
└──────────────┴──────────┴──────────────────────────────┘

No trace of regularity is perceptible in these changes. The mean magnitude of the star when white was 9·0, when at its reddest, 8·5. They are then obviously unrelated to its light-phases.

An analogous object is T Ursæ Majoris, which alternates between deep red and “creamy,” or pure white, though with a decided tendency to assume paler tints as brightness increases. A typical pair of observations were made on 5th and 14th February 1893. On the first evening T Ursæ was estimated as of 10·1 magnitude, and of a “deep, dull, ruddy hue”; on the second, it had risen to 9·3 magnitude, and become a “dull leaden colour,” showing “no trace of orange or red—a very curious change,” and one altogether unaccountable. Spectroscopic information, simultaneously procured, would have been likely to prove instructive, but none, unfortunately, is available.

A counter-example to T Ursæ is S Cephei, which, observed under the same conditions, and undergoing similar variations of brightness, was nevertheless recorded as at all times conspicuously red. On the other hand, χ Cygni, a flagrantly red variable of the Mira type, occasionally divests itself of colour as it brightens, although “scarlet” maxima are more common than “white.” Espin’s observations confirm the striking variability in hue of χ Cygni. On the whole, it cannot be doubted that temporary whiteness is a frequent feature of this class of ruddy stars, and the fact implies a great deal.

(2) In the second class of colour variables, light-change supervenes incidentally or not at all. It includes two historic examples—Sirius and Algol, both exceedingly unlikely, yet both attested on good authority to have been red within the scientific memory of man. The Sirian question has been exhaustively discussed by Dr. See[496] and by M. Schiaparelli;[497] their arguments are of most curious interest, but we can here only attempt to give what appears, on a fair view, to be their upshot. Two facts are incontestable; Seneca compared the colours of Mars and Sirius, and pronounced the star to be more intensely red than the planet, and Ptolemy applied to it his current epithet for “glowing ruddy” objects (ὑπόκιρρος), a piece of evidence vainly sought to be explained away as a transcriber’s error. Many other ancient authors imply, or are held to imply, what Seneca and Ptolemy definitely state; but even apart from these confirmatory hints, the simplest and perhaps the safest course appears to be to accept such definite statements. Their improbability does not in itself warrant their rejection. It has been suggested that the rapid scintillation of the Dog-star may have lent to it a fictitious redness, but it does not do so now. “Sirius is glancing blue-bright like a spirit,” Carlyle wrote from Templands in April 1842. And certainly the atmospheric disguise of colour cannot have been less effective in Dumfriesshire than at Rome or Alexandria. In the _Iliad_, a fiery nature and aspect are ascribed to Sirius; but Homeric indications are often loose or figurative. They, however, lend in this case countenance to the plausible surmise that the redness of the star was of antique standing. As to the date of its vanishing, nothing positive can be asserted; but the negative testimony of Al-Sûfi places it almost conclusively before the tenth century.

The same Persian astronomer supplies the only extant notice of Algol’s early redness. Perhaps a merely temporary phase, it seems nevertheless to have recurred after nine centuries. This was in 1841, when Schmidt at Athens perceived the star as yellowish red,[498] although its subsequent whiteness was patent to him as to all other observers. Was Schmidt deluded? It is very difficult to determine. Only the star itself can authenticate, by renewing, its evanescent glows of colour.

The pronounced redness of a seventh-magnitude star, No. 8 in Schjellerup’s “Red” Catalogue,[499] was recorded by Oeltzen during his revision of Argelander’s northern zones. Copeland, nevertheless, found it white, 1st January 1876; Espin, yellow, with a continuous spectrum, 14th November 1887; while Krüger registered on 6th October 1891 well-developed bands of the third type corresponding to an orange tint. Again, a ninth-magnitude star in Taurus[500] appeared to Hind “very red,” 3rd September 1848, but “bluish” 14th November 1850. Lost sight of for a quarter of a century, it was next observed by Copeland in January and February 1876 as pale yellow, and by Doberck, three years later, as reddish orange. Finally, on 10th January 1888, Espin saw it white, with a seemingly continuous spectrum; since when no attention—that the present writer is aware of—has been paid to it. A much brighter star in Aquila[501] (seventh magnitude) showed red to Schjellerup in 1863, but to Birmingham colourless in 1872 and 1874, and _blue_ 18th May and 20th July 1873. These changes were in a manner verified by subsequent spectroscopic observations; for the object, which had then recovered a ruddy tinge, was classed by Espin as of the fluted type, 20th September 1889, but by Krüger, 25th June 1892, as a solar star with a pale yellow cast. The colour-phases of an eighth-magnitude star, “63 Schjellerup,”[502] are attested by the best authorities; it is impossible to doubt their reality. Picked out for its redness at Copenhagen in 1863, the object, after numerous alternations, was described by Franks in 1885 as white. No later observations appear to be extant.

The following short list of the best-authenticated colour-variables may be useful to observers:—

┌───────────────────┬──────────┬──────────────────────────────────────┐ │ Designation. │Magnitude.│ Remarks. │ ├───────────────────┼──────────┼──────────────────────────────────────┤ │ 5 Schjellerup │ 7·0 │“Full garnet,” J. Herschel; red, │ │ = Krüger 75 │variable? │ Schjellerup, 1863; white, Dreyer, │ │ │ │ 1876. │ │ 8 Schjellerup │ 7·0 │Deep red about 1850; white, 1st │ │ = Krüger 102 │ │ January 1876. │ │ 63 Schjellerup │ 7·8 │Rubra, Schjellerup, 1863; blue, │ │ = Krüger 504 │ │ Birmingham frequently in 1873; │ │ │ │ decided red, Gould; colourless, │ │ │ │ Dreyer, 1880. │ │ 90 Schjellerup │ 7·7 │Rubra, Struve; bluish white, │ │ = Krüger 687 │ │ Birmingham, 1874; orange, Dreyer, │ │ │ │ 1879; white, Espin, 1888. │ │ 93 Schjellerup │ 9·0 │Blood red, Schjellerup, 1863; orange, │ │ = Krüger 698 │ │ Copeland, February 1876; colourless,│ │ │ │ Espin, 10th February 1888. │ │ γ Circini[503] │ 3·4–5·2 │Very red, Gould, about 1875; white, │ │ │ │ Stanley Williams, 1886. │ │64 _b_ Schjellerup │ 8·8 │Very red, Hind, 1848; bluish white, │ │ = Krüger 513 │ │ Hind, 1850; red, Dreyer, 1879; │ │ │ │ white, Espin, 1888. │ │ 148 Schjellerup │ 8·5–9·5 │Scarlet, Rosse, 1861; dark red, │ │ = Krüger 983 │ │ d’Arrest, 1866; colourless, │ │ │ │ Birmingham, 1874; red, intense │ │ │ │ bands, Dunér, 1878. │ │ 214 Schjellerup │ 7·0 │Red, Schjellerup, 1863; not red, │ │ = Krüger 1436 │ │ Birmingham, 1872, 1874; blue, │ │ │ │ Birmingham, 1873; orange, fluted │ │ │ │ spectrum, Espin, 1889; yellowish, │ │ │ │ solar spectrum, Krüger, 1892. │ │ _r_ Velorum │ 5·0 │Red, Gould, 1870·73; leaden white, │ │ │ │ 1888, A. M. Clerke; slight red │ │ │ │ tinge, Tebbutt, 1891. │ │222 _b_ Schjellerup│ 7·8 │Red, Lamont; yellow, Dreyer, 21st July│ │ = Krüger 1512 │ │ 1875; white, Dreyer, 18th August │ │ │ │ 1875; yellow, Espin, 1889; white, │ │ │ │ Krüger, 1891. │ └───────────────────┴──────────┴──────────────────────────────────────┘

Two stars[504] have been mentioned in an earlier chapter as anomalously white, considering that their spectra are of the fourth type. The possibility should not be overlooked that their paleness is only temporary. They are perhaps colour-variables, and will, at some future time, show the ruddy hue appropriate to the quality of their light.

(3) The colour changes of double stars are a peculiarly baffling subject of inquiry. Many have been recorded that can safely be dismissed as illusory; some that are unquestionably real. Yet in most cases there is a large element of doubt. Personal idiosyncrasies come strongly into play; meteorological influences, instrumental diversities, and all the chances and changes of existence swell the reckoning of uncertainty. To say nothing of the indeterminateness of language. Star tints are often so delicate as to defy verbal definition. Distinctions between rose-pink and amethyst, sea-green and apple-green, ashen, lilac, and grey, have only a nominal meaning. These tender shades, moreover, while escaping some eyes altogether, are enhanced by others into vivid contrasts; and hence observers, expecting to see star-couples glowing like fruits of the Hesperides, are apt to carry away the impression that the subtle coloration actually presented to them implies a marked change. To separate the kernel of fact from the husk of opinion or illusion is then no easy matter. Yet an inadequate attempt to banish confusion is almost always better than none, and may here be worth making.

The more closely the chromatics of double stars are studied, the more clearly emerges an irreducible minimum of change. A satisfactory example is afforded by one of the most carefully watched binaries in the heavens. The primary in 70 Ophiuchi is of 4·5, the satellite of 6·5 magnitude, and it is unquestionably the satellite which conspicuously varies in hue. Sir William Herschel in 1779 perceived in it a very slight reddish suffusion, and J. Herschel and South described the pair in 1824 as “white and livid.” Yet the elder Struve, an incomparable authority, considered their “yellow and purple” tints remarkable enough to warrant their inclusion in a restricted list of objects showing _colores insignes_,[505] and they were still “topaz and violet” when observed successively by Smyth and Webb.[506] “Gold and purple” again they appeared in July 1883 to Perrotin at Nice, although less than a month previously he had noted them “greenish yellow and reddish yellow,” while a year later he recorded them as “golden and orange.”[507] This vesture they continue to wear. They are ordinary yellowish stars with an ordinary solar spectrum. Sooner or later, however, the companion may be expected to put off its crocus-veil and shine Tyrian-hued.

The stars of γ Delphini are now finely contrasted in orange and green. They appeared, nevertheless, white to the elder Herschel in 1779; white and yellowish to Herschel and South in 1824; “reddish yellow and greyish lilac” to Gore in 1874;[508] pale rose and light green to Dembowski in 1876–77; orange and green to Flammarion in 1877. Moreover, the companion showed “light emerald” during the years 1831–39, but “flushed grey” in 1850. Doberck found it bluish in 1882, and the primary yellow;[509] Vogel in 1883 recorded both stars as creamy white; while in 1895—according to Mr. Franks—the colours were “very pronounced, the chief star being a strong yellow and the companion greenish.”[510] They are of fourth and fifth magnitudes respectively, and a sky-gap of 11″ divides them. Their mutual revolutions have made little sensible progress during a century and a quarter, but their common drift through space certifies their systematic connection.

The case of 95 Herculis is somewhat perplexing.[511] This is an equal pair of fifth-magnitude stars, rigidly fixed during the last twelve decades at an apparent distance of 6″. Their “magnificent tints of orange and green” excited Father Secchi’s admiration in 1855; and Piazzi Smyth was accustomed to see them “apple green and cherry red” until 29th July 1856, when he perceived with stupefaction, from his point of vantage on the Peak of Teneriffe, that both were of the undistinguished white attributed to them by Herschel in 1780. Fitful and partial displays of their original chromatic brilliancy appear to be vouched for by Dunér’s and Flammarion’s[512] observations of the stars as “bright green and yellow,” and “gold and azure”; but their pale primrose is now unrelieved by a shade of difference. There is no good reason to doubt that, in the earlier part of the century, they were marked by vivid complementary colours. Obvious to Webb, they were remarked by Admiral Smyth as an unusual instance of diversity in tint “between components so nearly equal in brightness.”

Instances are not infrequent of the small star in pairs of disparate brightness varying in colour; but the relation is never inverted; no primary is exclusively subject to change of tint. The satellite of δ Herculis, a greenish star of the fourth magnitude, appeared to Struve alternately grape red and ashen white; to Dembowski, blue; to Knott, bluish green in 1850, ruddy purple in 1871; to Fletcher, in 1851, red; to Flammarion, violet. The conjunction of these stars is thought to be merely fortuitous. They are moving along divergent straight lines, and hence seem destined to definitive separation. Yet colour-changes of the kind affecting the satellite do not occur in isolated objects, and would rather imply a physical connection with a dominating orb. It will then be of particular interest to determine quite certainly whether δ Herculis is a truly gravitational, or simply an optical couple.

The companion of δ Cygni shows analogous variations. “Ashen grey” to Struve’s perception during the years 1826–33, it surprised him with a strong red glow in 1836; three years later, Dawes found it blue; Secchi, by turns red, blue, and violet in 1856–57; Dembowski, grey in 1862–63; Engelmann, red in 1865. Of late its blue aspect has predominated; yet Perrotin recorded it as yellow or orange with the great Nice refractor both in 1883 and in 1886. These stars make a very much closer pair than δ Herculis, and are in slow orbital movement.

Two at least of the four stars grouped in σ Orionis may be admitted to fluctuate in hue.[513] One of 7·5 magnitude appeared ashen grey in 1837, ruddy in 1851 and 1869, bluish in 1883. A more distant, somewhat brighter component, usually dust-coloured, was marked “grape red” by Smyth in 1832. Even the chief star is not of the perennial whiteness that should match its helium spectrum. Webb found it yellow in 1851, and Gould entered it as “red” in the Argentine Uranometry. It was divided by Burnham in 1888 into an excessively close pair (fourth and sixth magnitudes at 0·26″), which, already in 1891, gave indications of circulatory movement.[514]

The following is an enumeration of some double stars reputed, on good grounds, to be colour-variables:—

┌────────────┬───────────┬─────────┬──────────────────────────────────┐ │Designation.│Magnitudes.│Distance.│ Remarks. │ ├────────────┼───────────┼─────────┼──────────────────────────────────┤ │70 Ophiuchi │ 4·5, 6·5 │ 1·6″ │Primary white or yellow, satellite│ │ = Σ 2272 │ │ │ alternately purple, rosy, and │ │ │ │ │ yellow. Spectrum, solar. │ │ γ Delphini │ 4, 5 │ 11″ │Primary cowslip to orange; │ │ = Σ 2727 │ │ │ companion emerald to blue, │ │ │ │ │ lilac, and topaz. Spectra, solar│ │ │ │ │ and Sirian. │ │95 Herculis │ 5·3, 5·3 │ 6″ │Contrasted green and red to │ │ = Σ 2264 │ │ │ uniform yellow. Spectra, solar │ │ │ │ │ and Sirian. │ │ δ Herculis │ 4·0, 8·5 │ 26″ │Companion by turns ashen, red, │ │ = Σ 3127 │ │ │ violet. Chief star gives a │ │ │ │ │ helium spectrum. │ │δ Cygni = Σ │ 3, 8 │ 1·5″ │Satellite grey to red, blue, or │ │ 2579 │ │ │ green. Slow binary. Large star │ │ │ │ │ gives a Sirian spectrum. │ │ σ Orionis │ 4·1, 7·5, │13″, 41″ │Chief star white to reddish; │ │ = Σ 762 │ 7·0 │ │ helium spectrum. Companions grey│ │ │ │ │ to ruddy. Fixed. │ │38 Geminorum│ 5·5, 8·0 │ 6·8″ │Companion varies in magnitude, 7·5│ │ = Σ 982 │ │ │ to 10; in colour, from bluish │ │ │ │ │ (1829) to red (1856, 1863), and │ │ │ │ │ azure (1872). │ │ γ Leporis │ 4·0, 6·5 │ 93″ │Companion pale green, 1832; │ │ │ │ │ garnet, 1851 and 1874 (Webb). │ │ │ │ │ Chief star gives a solar │ │ │ │ │ spectrum. │ │γ Serpentis │ 4·5, 9·0 │ 51″ │Small star lilac, 1832; “native │ │ │ │ │ copper,” 1851 (Webb). │ └────────────┴───────────┴─────────┴──────────────────────────────────┘

Colour-variability has hitherto been only observed, as it were, in passing. And the casual study of a subject is seldom effectual. Here much more is required if any progress is to be made towards discovering the laws and cause of the phenomenon. What is essential to ascertain is the nature of the spectroscopic response to colour-change. On this side the problem can be attacked with some hope of getting nearer to a solution. If visual alterations of hue can satisfactorily be brought to the test of prismatic analysis, the way will be thrown open for an important gain of knowledge; while it is hard to see by what other means ignorance on the curious topic we have been discussing can be dissipated. It is not, indeed, always easy to combine work in different branches; yet the correlation of results is a vital need of astronomy, and scarcely ever fails to prove especially and widely illuminative.