mercredi 1 octobre 2008

Centaurus 10

α, Binary, 0.2 and 1.5, white and yellowish.

Baily's edition of Ulug Beg's catalogue gives this as Rigil Kentaurus, from Al Rijl al Kentaurus, the Centaur's Foot; describing it as on the toe of the right front hoof, and Bayer so illustrated it. Chrysococca had the synonymous ποῦς κοντούρος; and our Century Dictionary retains Rigel, although this is better known for the bright star in Orion. Burritt located on the left fore hoof a 4th‑magnitude star that he wrongly lettered α; and above the pastern our 1st‑magnitude, also lettered α, with the title Bungula, which I find only with him and the Standard Dictionary. He gives no explanation of this, nor can I trace it further; it may be a word specially coined by Burritt from β and ungula, the hoof, although even in this the letter is wrong.
Ideler said that α and β also have been the Arabic Ḥaḍar, Ground, and Wazn, Weight, as is explained at the star β; but he seemed at a loss as to the proper assignment of these words, although inclining to Ḥaḍar for β.
These two stars were among the much discussed Al Muhlifaïn described at γ Argūs and δ Canis Majoris.
Alpha's splendor naturally made it an object of worship on the Nile, and its first visible emergence from the sun's rays, in the morning at the autumnal equinox, has been connected by Lockyer with the orientation of at least nine temples in northern Egypt dating from 3800 to 2575 B.C., and of several in southern Egypt from 3700 B.C. onward. As such object of worship it seems to have been known as Serk‑t.
It bore an important part, too, in southern china as the determinant of the stellar division Nan Mun, the South Gate.
α lies in the Milky Way, 60° south of the celestial equator, culminating with Arcturus, but is invisible from north of the 29th parallel. It is of the greatest interest to astronomers, being, so far as is now known, the nearest to our system of all the stars, although more than 275,000 times the distance of the earth from the sun, — 92,892,000 miles, — and 100 millions of times the distance from the earth to the moon, — 238,840 miles. Its parallax, first taken at the Cape of Good Hope by Henderson in 1839, and later by Gill and Elkin, and now fixed at 0ʺ.75, shows a distance equal to that traveled by light in 4 1/3 years.
We can better realize the immensity of this distance from Professor Young's statement that if the line from the earth to the sun's centre be represented as 215 feet long, one to this star would be 8000 miles; and from Sir John Herschel's illustration:
to drop a pea at the end of every mile of a voyage on a limitless ocean to the nearest fixed star, would require a fleet of 10,000 ships of 600 tons burthen, each starting with a full cargo of peas.
The nicety of parallactic observation, too, is shown by the fact that "an angle of 2ʺ is that in which a circle of 6/10 of an inch in diameter would be seen at the distance of a mile."
Were our sun removed to the distance of α Centauri, its diameter of 866,400 miles would subtend an angle of only 1/143 of a second of arc, of course utterly inappreciable with the largest telescope; and if seen from that star, would appear as a 2d‑magnitude near the chair of Cassiopeia.
α was first discovered to be double by Richaud at Pondicherry, India, in 1689; but there seems discrepancy in the magnitudes respectively attributed to the components. Early astronomers thought the lesser star, α1, a 4th‑magnitude; even recently Gould has estimated it as 3 1/2; yet Miss Clerke writes, "the lesser, though emitting only 1/3 as much light as its neighbour, is still fully entitled to rank as of the 1st magnitude"; all of which may indicate an increase of brilliancy since its observation began. Together they give nearly four times as much light as the sun, while their mass is double that of the latter.
The period of orbital revolution is about eighty-one years; the position angle in 1897, 208°; and they now are 21ʺ.5 apart, — about 2700 millions of miles, — and yet connected! This distance is increasing.
Their proper motion, 3ʺ.7 annually, or about 446 millions of miles across the line of vision, will carry them to the Southern Cross in 12,000 years.
The spectrum of α2, the larger star, is midway between the Sirian and Solar.

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