dimanche 5 octobre 2008

Lyra 10

. . . azure Lyra, like a woman's eye,
Burning with soft blue lustre.

Willis' The Scholar of Thebet ben Khorat.
α, 0.3, pale sapphire.

Wega, less correctly Vega, originated in the Alfonsine Tables from the Wāḳiʽ of the Arabs, Bayer having both titles; Scaliger, Waghi; Riccioli, Vuega vel Vagieh; and Assemani, Veka.
The Greeks called it Λύρα, which, in the 16th‑century Almagests and Tables, was turned into Allore, Alahore, and Alohore.
Among Latin writers it was Lyra, in classical days as in later, seen in the Almagest of 1551 as Fulgens quae in testa est & vocatur Lyra; and in Flamsteed's Testa fulgida dicta Lyra; but Cicero also used Fidis specially for the star, as did Columella and Pliny [XVIII, passim], Fides and Fidicula, its preëminent brightness fully accounting for the usurpation of so many of its constellation's titles, indeed undoubtedly originating them. In Holland's translation of Pliny it is the Harp-star.
The Romans made much of it, for the beginning of their autumn was indicated by its morning setting. It was this star that, when the hour of its rising was alluded to, called forth Cicero's remark, "Yes, if the edict allows it," — a contemptuous reference to Caesar's arbitrary, yet sensible, interference with the course of ancient time in the reformation of the calendar, an interference that occasioned as much dissatisfaction in his day as did Pope Gregory's reform2 in the 16th century.
Sayce identifies Wega, in Babylonian astronomy, with Dilgan, the Messenger of Light, a name also applied to other stars; and Brown writes of it:
At one time Vega was the Pole-star called in Akkadian Tir-anna ("Life of Heaven"), and in Assyrian Dayan-same ("Judge of Heaven"), as having the highest seat therein;
but fourteen millenniums have passed since Wega occupied that position!
The Chinese included it with ε and ζ in their Chih Neu, the Spinning Damsel, or the Weaving Sister, at one end of the Magpies' Bridge over the Milky Way, — Aquila, their Cow Herdsman, being at the other; but the story, although a popular one not only in China, but also in Korea and Japan, is told with many variations, parts of Cygnus sometimes being introduced.
These same three stars were the 20th nakshatra, Abhijit, Victorious, the most northern of these stellar divisions and far out of the moon's path, but apparently utilized to bring in this splendid object; or, as Mueller says, because it was of specially good omen, for under its influence the gods had vanquished the Asuras; these last being the Hindu divinities of evil, similar to the Titans of Greece. It was the doubtful one of that country's lunar stations, included in some, but omitted in others of their lists in all ages of their astronomy, and entirely different from the corresponding manzil and sieu, which lay in Capricorn. The Hindus figured it as a Triangle, or as the three-cornered nut of the aquatic plant Cringata, Wega marking its junction with the adjoining Çravana.
Hewitt says that in Egypt it was Maʽat, the Vulture-star, when it marked the pole, — this was 12000 to 11000 B.C. (1), — and Lockyer, that it was the orientation point of some of the temples at Denderah long antecedent to the time when γ Draconis and α Ursae Majoris were so used, — probably 7000 B.C., — one of the oldest dates claimed by him in connection with Egyptian temple worship.
Owing to precession, it will be the Polaris of about 11500 years hence, by far the brightest in the whole circle of successive pole-stars, and then 4 1/2 ° from the exact point, as it was about 14300 years ago. In 1880 it was 51°20ʹ distant. Professor Lewis Boss and Herr Stumpe place near it the Apex of the Sun's Way.
Picard failed in his efforts to obtain its parallax in the 17th century, but Struve thought that he had succeeded in this by his observations previous to 1840; still much discrepancy exists in the recent determinations. Elkin, in 1892, gave it as 0ʺ.092; or, to put it in popular language, if the distance from the earth to the sun be regarded as one foot, that from Wega would be 158 miles. The 10th‑magnitude companion, about 48ʺ away, used for some of these determinations, is entirely independent of it, although difficult to be seen owing to the great brilliancy of Wega. At least two other still fainter companions also have been found.
This was the first star submitted to the camera, by the daguerreotype process, at the Harvard Observatory on the 17th of July, 1850.
It lies on the western edge of the constellation figure, and, after Sirius, is the most prominent of the stars showing spectra of the Sirian type; yet, with all its splendor, affords but 1/9 of the latter's light. Still it is supposed to be enormously larger than our sun, proportionately very much hotter. It is moving toward our system at the rate of about 9 1/2 miles a second, and makes "the nearest approach in the northern hemisphere to an independently blue star"; while its flashing brilliancy justifies its being called the Arc-light of the sky. Miss Mitchell strangely called it pale yellow.
Wega rises at sunset far toward the north on the 1st of May, and, being visible at some hour of every clear night throughout the year, is an easy and favorite object of observation. It culminates on the 12th of August.
With ε and ζ it formed one of the Arabs' several Athāfiyy, this one being "of the people," while the others, fainter in Aries, Draco, Musca, and Orion, were "of the astronomers"; for sky objects are often very plain to them that are invisible to the ordinary observer.
β, Variable and binary, 3.4 to 4.5, very white.

Sheliak, Shelyak, and Shiliak are from Al Shilyāk, one of the Arabian names for Lyra. The star lies about 8° southwest from Wega and 2 1/2 ° west from γ.
With δ and ι it was Tsan Tae in China.
The changes in its brilliancy, detected by Goodricke in 1784, were fully investigated by Argelander from 1840 to 1859, and showed a regularly increasing period of variability which now is 12 days, 21 3/4 hours, with several fluctuations of a somewhat complex nature.
Like γ Cassiopeiae and other variables of the Sirian type, it shows in its spectrum, — perhaps the best specimen of Pickering's 4th class, — not only the usual dark lines, but also the bright lines of glowing gases, hydrogen and helium being especially conspicuous. Pickering concluded, from the singular character and behavior in the shifting of these lines, that the chief star must consist of at least two luminous bodies rotating around a common centre of gravity at a very great rate of speed, perhaps three hundred miles a second, the period of revolution equaling the period of variability. Scheiner says of it, "There is great probability that more than two bodies are concerned in the case of β Lyrae"; and yet it may not be impossible, in view of the recent discoveries at the Johns Hopkins Laboratory, that variations of pressure may be concerned in this remarkable shifting of lines. A full and interesting discussion of this appears in Popular Astronomy for July, 1898.

γ, 3.3, bright yellow,

2 1/2 east of β is Sulafat, from another of the titles of the whole constellation.
Jugum, formerly seen for it, may have come from a misunderstanding of Bayer's text, where it probably is used merely to designate the star's position on the frame of the Lyre, his words being ad dextrum cornu, Ζυγόν, Iugum, — a fair example of the indefiniteness of much of his stellar nomenclature.
At a distance point 1/3 of the distance from β to γ is the wonderful Ring Nebula, NGC 6720, 57 M., discovered in 1772 by Darquier from Toulouse, although its apparent annular form was not revealed till later by Sir William Herschel's observations. In our day high-powers show its oval form somewhat undefined at the edges, with a dark opening in the centre containing a few very faint stars, among which, visible only in the largest telescopes, but prominent in photographs, is a central condensation of light like a star. p288The spectrum of nebula and central "star" is purely gaseous. Although appearing oval to us, it is supposed to be nearly circular, but seen obliquely. It is the only annular nebular visible through small telescopes, although there are six others now known.
ε1, or Fl. 4, Binary, 4.6 and 6.3, yellow and ruddy;
ε2, or Fl. 5, Binary, 4.9 and 5.2, both white.

These are the celebrated Double Double, each pair probably separately revolving in a period of over two hundred years, and both pairs perhaps revolving around their common centre of gravity; but if so, the period is to be reckoned only by millenniums, for the measures of the last fifty years show no sensible orbital motion. This is by far the finest object of the kind in all the heavens.
They are 207ʺ apart, and, to the ordinary eye, form an elongated star; but exceptionally sharp sight will resolve them without aid. The pairs are 3ʺ.2 and 2ʺ.45 apart respectively, and a good 2 1/4‑inch glass with a power of 140 will separate each pair. The position angle of the components of ε1 is 12°; and of those of ε2, 132°; while that of ε1 and ε2 is 173°. Their "double-double" character was first published by the Jesuit father Christian Mayer in 1779, although its discovery has generally been attributed to Sir William Herschel.
The distance between ε1 and ε2, small as it is, is nearly twice that noticed by astronomers, in 1846, — 128ʺ — between the actual and the computed positions of the planet Uranus, a discrepancy which convinced them of the existence of a still more remote planet and led to the discovery of Neptune. Such is the marvelous nicety of modern astronomical measurements!
Between these stars lie three very much fainter, two of which, of the 13th magnitude, are the Debilissima, Excessively Minute, of Sir John Herschel, discovered by him in 1823.
ε and ζ form an equilateral triangle with Wega, the sides about 2° long; ε being at the northern angle. These three stars were one of the Athāfiyy of the early Arabs.
η, a 4.4‑magnitude, is Aladfar in the Century Atlas, by some confusion with the star μ; and with θ, of the same brilliancy, was, in China, Leën Taou, Paths within the Palace Grounds.
μ, of the 5th magnitude, was Kazwini's Al Aṭhfār, the Talons (of the Falling Eagle), which he described as a fainter star in front of the bright one, i.e. west of Wega.

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