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Admiral Smyth’s - Sidereal Chromatics

PART I. [*13]

ON THE COLOURS OF DOUBLE STARS


The Authors intent.

UNDER this title we are not about to inflict on the reader any of the extravagant notions of the ancients - aye, not even the best of them — concerning the essence of hues and tints, they being utterly unsupported either by reason or experiment. Nor is it intended to theorize under modern advantages upon light and rays, the seductions of harmony and contrast, nor to jump at any of the remaining uncertainties of the question. The present light sketch is merely handed forth to investigate that property of colours which affects the sight only : and, if the absolute tints of double stars are determined at special epochs, much will be contributed towards the mysterious laws which regulate such variations as may be observed. Upon this understanding, we will open our fire with the following extract from the Prolegomena to my [Cycle of Celestial Objects]

CYCLE OF CELESTIAL OBJECTS

Initiatory Remarks.

For the sake of the tyro, it may be proper to premise that the light of the two stars composing the double one is frequently found to be of very dissimilar intensity, and often of very dissimilar colours; and no one who has ever directed a telescope to the heavens can have failed to be struck with the brilliant hues they present, especially such lovely objects as γ Andromedæ, α Herculis and ε Boötis. The phenomenon of the tints displayed by the smaller stars is considered by M. Arago as owing to an excess of refrangible rays, acted upon by an absorbing force in the atmosphere of the larger star; but this cannot be the universal law. Sometimes the strongest of the two is a yellow, red, or orange tinge, still more frequently [*14] the secondary is blue, purple, or greenish, and those colours so palpable as to be visible in objects of the smallest magnitude. Now, as many of these pairs border on the extremes of the prismatic spectrum, the larger star being allied to the red, and the smaller to the violet, the exhibition may in such cases be the effect of contrasted complementary tints — corresponding to the male and female lights of Milton. [1] We all know that a white light appears greenish when near a strong red one, and becomes blueish when the neighbouring colour is yellow. In combinations of this nature some of the secondaries lose their colour on hiding the primary; but, as many of the examples defy this test, their colours are too decidedly indicated to be merely imaginary. For instance, as a α Leonis is of a brilliant white tint, the deep purple of its come, cannot be an illusion, and in δ Serpentis both the bodies are blue.

Struves Estimates.

It will thus be seen that this department of sidereal inquiry offers an interesting field for continuous investigation, and accordingly we find that the elder Struve has paid strict attention to it. By his observations it is confirmed that, besides the white, there exist stars of every shade of the prism; and that when the principal body is not white, it approaches the red side off the spectrum, while the satellite offers the bluish of the opposite extreme. Yet this apparent law is not without exceptions in the Catalogue before us; on the contrary, the mostly general case is that the two stars are of the same coloured as will be seen in the following summary, wherein he finds among 596 brilliant double stars:

375 pairs of the same colour and intensity.

101 pairs of the same colour, but different intensity.

120 pairs of totally different colours.

[*15] Among those of the same colour the white are the most numerous of 476 specimens of that species he found:

295 pairs, both white.

118 pairs both yellowish, or reddish.

68 pairs, both bluish.

Therefore the number of red or reddish stars is double that of the bluish tinge, and that of the white stars is twice and a half greater than the red ones. The combination of a blue companion with a coloured primary happens:

53 times, with a white principal star.

52 times, with a light yellow.

59 times, with a yellow or red.

16 times, with a green.

Obstacles to oncurrence.

Professor Struves chromatic designations are obscurissima, obscura, pallida, livida, alba, sub-flava, flava, sub-cærulea, caerulæ, rubicunda, and rubra. [2] He supposes the ninth magnitude to be the outside boundary in which he recognises colour, but I have been much struck with the beautiful blue tint of several of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity. The Professor found what I have also experienced, that Sir William Herschel saw most objects with a redder tinge than they have since proved to be of. This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both. We know there are many examples of very sharp eyes being unable to distinguish colours correctly, among whom may be instanced the late George Duke of Marlborough, who was an amateur astronomer, and possessed a good sight in other respects. There are others who have this singular physical defect with regard to particular colours only, as our estimable countryman Dalton, who, though so conversant with the laws of the spectrum, could not discriminate between scarlet and [*16] brown. So also those two celebrated men, Troughton and Dugald Stewart, were affected, but their peculiarity of vision consisted in confusing scarlet with green, and pink with blue. To the former the ripe cherry and its leaf were of one hue, only to be distinguished by their form, yet his eyesight was sharp enough for the examination of the minutest sub-divisions upon graduated instruments. Among other instances of this peculiarity of the sense of colours, I was greatly surprised on finding that an intimate friend of my own (Sir G. Musgrave) could not perceive the strong colours of γ Andromedæ and other remarkable stars with my telescope, as I was well aware of his exquisite taste and execution in missal-blazonry; but he assured me that he cannot easily discriminate between brown and green; albeit I have specimens of his art in which those colours are treated as well as they could be by any one in the Royal Academy. Sir John Herschel examined the eyes of an individual thus circumstanced, and satisfied himself that all the prismatic rays had the power of affecting them with the sensation of light, and producing distinct vision; so that he considers this defect as arising from no insensibility of the retina to rays of any peculiar refrangibility, but rather as residing in the sensorium, by which it is rendered incapable of appreciating exactly those differences between the rays on which these colours depend. [3]

Difficulty or deduction.

The permanency or variation of star-tints requires still closer observation before any correct deductions can be drawn. The ancients recognised no blue stars; they only spoke of white or red ones, classing among the latter Arcturus, Aldebaran, Pollux, Antares, and Betelgeuze, which also appear red to us; but they added Sirius, the rubra canicula of the poets, which, though expressly declared to be red both by Ptolemy and Seneca, is now decidedly white, and brilliantly white too. This instance affords a strong presumption that these colours undergo [*17]changes; there are two remarkable examples of a recent date γ Leonis and γ Delphini which, at Sir William Herschels time of observing them, seem to have been perfectly white, though the first pair is now of a golden yellow and reddish green, and the second of a bright yellow and bluish green. Blue stars are of modern introduction, since they are first mentioned by Mariotte in 1656, who supposes that they owe this colour to their being fainter, and free from exhalations. Mr. Dunlop, in the Catalogue made at Paramatta, in Sir Thomas Brisbanes observatory, mentions a large group of stars, all the individuals of which are blue − also a bluish nebulosity; but we have no such object in the northern hemisphere. Nor is it less remarkable that amidst this infinite variety of tints, although single red stars are frequently met with, there is not an instance of a solitary green, purple, blue, or violet-coloured one being found; and, among other singularities, the irregularity and even absence of the principal prismatic shades during the phases of most of those stars that vary in their magnitude, is a striking phenomenon, whence may be yet deduced important conclusions respecting the velocity of different coloured rays.

Sir John Herschel on coloured suns.

Herschel II. treating of contrasted colours in his admirable Outlines of Astronomy, finely remarks:– It is by no means, however, intended to say, that in all such cases one of the colours is a mere effect of contrast, and it may be easier suggested in words than conceived in imagination, what variety of illumination two suns — a red and a green, or a yellow and a blue one — must afford a planet circulating about either; and what charming contrasts and grateful vicissitudes — a red and a green day, for instance, alternating with a white one and with darkness — might arise from the presence or absence of one or other, or both, above the horizon. Indeed the beautiful effects of such glorious prismatic effulgence among the probable wanderers, offers a tempting arena to the fanciful contemplation of imaginative powers.

Notions of old. [*18]

Zahn “ a zealot is opponent of the Copernican system ” in his Syntagma, remarks that the skies shine more like torches burning with eternal flame before the altar of the Most High, than the lamps of the ætherial vault, or the funeral lights of the setting sun ; and he descants on their colours, asserting that from the various hues of the fixed stars, their nature may be inferred, and the planets they imitate at once known. Hence some are designated Saturnine, some Jovial, and others Martial. The Saturnine are those of a leaden or livid colour, and dullish; the Jovial are bright and white; and the rusty-coloured ones are assigned to Mars. The Solar ones, partly yellow and partly red, shine very splendidly; those of Venus are of a box-coloured glow -Venerece sunt buxece, sen buxeo splendore clarescunt; and the Lunar are pale and dim. Such was the knowledge of stellar colours in 1694; a century and a half has done somewhat to alter all this.

Arago on Green and Blue Stars.

Our redoubtable colleague, Arago, treating of this interesting subject in his Astronomie Populaire, says: The phenomenon of the colour of multiple slurs has been observed for too short a time to expect a plausible explanation of it in the present day. It will be reserved for time and precise observations to inform us — whether green or blue stars are not suns already in course of decay. If the different shades of these stars do not indicate a process of combustion in different stages: if the tinge with an excess of the more refrangible rays which the smaller star frequently exhibits, does not arise from the absorbing power of an atmosphere which might be developed by the action of the usually more brilliant star which it accompanies, &c. Euge ! Now here is blue rain with such a vengeance that we can but marvel at the temerity of a worthy savant in ventilating so hasty and wild a supposition; but theory, especially when unsupported by crucial or instantial facts, is mighty prone to treading upon hot ashes.

Variable Stars.

However, in our present thirst for exact knowledge, it will [*19] be as well to admit the reasonableness of the De Coverley maxim, in that there is something to be said on all sides, for, since the foregoing remarks were first printed, much has been achieved among the Variable Stars, as well those which are proved to be periodical in their changes of light or brilliancy, as others which are yet only to be termed versatile. It is a promising field for exploration, even though the physical nature of the fluctuating bodies may continue to remain a latent mystery; at present they must be classed with those postulates regarding which Sir John Herschel pithily observes, it is easier to profound questions than to offer satisfactory replies to them.

Hinds ruddy tinted stars.

My indefatigable friend Mr.Hind, who has worked to admiration among the Variables, has noted that these stars, especially the fainter ones, have generally a ruddy tint; upon which Mons. Arago thus dilates:-Might there not be some connexion between this remark and the observation made by the same astronomer, that variable stars at the instant of their minimum brightness appear surrounded with a kind of fog? Supposing the existence of this fog well established, we should be on the road to tile explanation of these singular phenomena. Perhaps we may arrive at the conclusion that a stars variations of brightness are due, not to a perfectly opaque planet revolving round it, but to cosmical clouds, which, by a similar movement of revolution, would be successively interposed between those bodies and the earth.
Now whatever may hereafter prove to be the upshot of our opinions quaint, it must be received as a crucial fact in the discussion, that nine of Hind's Variables are recorded as being red, and one absolutely crimson, like to a blood-drop on the black ground of the sky. The fog or haze at the minimum was palpably obvious, and in several some very decided changes of colour took place at different stages of their light-curve, for instance, R Geminorum, announced by its discoverer as passing through blue, yellow, and red, during the 371 days of [*20] the gradations in its lustre, which is fully confirmed by the accurate Mr. Norman Pogson. Besides the detection of the above-mentioned crimson star, Mr. Hind thus wrote me from Regents Park under date of the 14th January, 1850 :

I also avail myself of this opportunity to mention that, in October 1845, I remarked a most fiery or scarlet star on the confines of Lepus and Orion, AR, (1850) 4 h. 52 m. 45 s. and N P D 105° 2′. This is by far the most deeply-coloured of any that I have yet seen, and in striking contrast with a beautifully white star preceding it one minute. It is not inserted in Sir John Herschel’s Catalogue of Red Stars, there is no allusion to it in your Cycle, nor can I find any previous notice of it ; yet I am doubtful whether I can have first discovered this remarkable star.

Hind’s Mira Ophiuchi.

The same diligent observer on the night of the 28th of April, 1848, suddenly perceived a red star of between the fourth and fifth magnitude, very conspicuous to the naked eye, in the region of the Serpent-bearer, in AR 16 h. 51 m., and in Dec. −12° 39′, where, from a thorough acquaintance with the spot, he felt assured, and could state positively, that no star down to the tenth magnitude had previously existed. It was to unassisted vision as bright as ν Serpentis. I happened to be absent from home when Mr. Hind sent me notice of this Mira Ophiuchi, nor was it till the end of June that I was able to get a telescopic glimpse of it when it had dwindled to about the eighth degree of lustre only. From the time of its discovery the stranger continued to diminish, but without altering its position; and, before the advance of the season rendered observations impracticable, it had nearly disappeared. But, although it has been watched from 4.5 to below 13.5 magnitude, it cannot as yet be enrolled among the periodical Variables, and for the present must remain trader the designation of Hinds Changing Star.
In the present stage of inquiry – an inquiry of no small moment in universal physics — the Variables can hardly be placed in the same category with the Double Stars to which I am drawing attention; for, though some of them may be cosmically changeable in colour, the conditions must be essentially [*21] different between a single star like the above-cited R Geminorum, ranging to our eyes from the seventh to the eleventh degrees of brightness, and such a steady bold double object as 95 Herculis — for which see the Appendix. Both classes, however, are proved to be entities and it remains to watch the Variables through their curves of light with unremitting diligence, in order to arrive at more satisfactory results. Some of the temporary phenomena, noted in low stars, may be caused by the mutable conditions of the earths atmosphere and the interference of light; but it is certain that most of the variations above alluded to, are entirely independent of any effect which the said atmosphere could produce.

Variable Star Atlas.

In the Speculum Hartwellianum, page 128, it is shown time on engaging Mr. N. R. Pogson as Dr. Lees astronomical assistant, an express condition was, that a special scrutiny be made into the desiderata of those wondrous bodies relative to which we have just spoken; and such was his diligence that, within the short space of two years, twenty maps of known or suspected Variables, together with the chief recorded Temporary stars, were in so advanced a state of preparation as to be nearly ready for engraving. But, being suddenly called away, in order to assume the Directorship of the important Observatory at Madras, he carried the results of his former labours with him, to superintend their due completion; when, with the express permission of Government, the work will be published under the title of HARTWELL AND MADRAS ATLAS OF VARIABLE STARS.

ENDNOTES

[1] On this head Sir John Herschel has remarked that it is probably in virtue of that general law of optics which provides that when the retina is under the influence of excitement by any bright-coloured light, feebler lights, which when seen alone would produce no sensation but of whiteness, shall for the time appear coloured with the tint complementary to that of the brighter.

[2] That masterly observer, perplexed by the tint of the close companion of ξ Orionis, somewhat sesquipedalianly designates it oliuoceaserebicenda. I entered it as light purple, and Sestini called it azure.

[3] An intelligent seaman, Captain Joseph Huddart, wrote Of persons who could not distinguish colours. — See the Philosophical Transactions for 1777.


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