jeudi 18 septembre 2008

Draco 6

The great serpent

Gamma Draconis (γ Dra / γ Draconis) is a star in the constellation Draco. It has the traditional name Eltanin (also Etamin).
Eltanin is an orange giant of spectral type K5, lying 148 light years away. Despite its Bayer designation of "gamma," it is actually the brightest star in Draco, outshining Rastaban (Beta Draconis) by nearly half a magnitude. In fact, the name "Rastaban" was formerly used for Eltanin, and the two terms share an Arabic root meaning "serpent" or "dragon." Its proximity to the point directly overhead of London has also earned it the name "Zenith Star."
In 1.5 million years, Eltanin will pass within 28 light years of Earth. At this point it will be the brightest star in the night sky, nearly as bright as Sirius (assuming its current absolute magnitude does not change).
In 1728, while unsuccessfully attempting to measure the parallax of this star, James Bradley discovered the aberration of light resulting from the movement of the Earth. Bradley's discovery proved Copernicus' theory that the Earth revolved around the Sun.

Eta Draconis (η Dra / η Draconis) is a star in the constellation Draco. It also has the traditional name Aldhibain (Arabic الذئبين al-Dhibain) meaning "The Two Wolves".
Eta Draconis has apparent magnitude +2.73 (third-magnitude) and is a yellow giant variable star belonging to spectral class G8III. The luminosity of Eta Draconis is neary 50 times that of the Sun. It is approximately 88 light years from Earth.

Ra's ath-Thu'ban
Head of the snake
رأس الثعبان

Beta Draconis (Beta Dra / β Draconis / β Dra) is the third brightest star in the constellation of Draco. It has the traditional name Rastaban, which confusingly has also been used for Gamma Draconis.
It is a G-type supergiant star, with spectral class G2 and an apparent visual magnitude of approximately 2.79. Its distance from the Sun is around 360 light years.[1] It is a binary star, with a binary star designation of ADS 10611, in which the supergiant is orbited by a dwarf companion once every four millennia or so.
The traditional name, from the Arabic phrase ra's ath-thu'ban ("head of the serpent") is less commonly written Rastaben, and has also been used for the star Gamma Draconis. Rastaban is also known as Asuia and Alwaid, the latter meaning "who is to be destroyed," though some trace it to Arabic al'awwad ("the lute player"). It is part of the asterism of the Mother Camels (Arabic al'awa'id), which may have influenced this alternate name.

The great serpent

Delta Draconis (δ Dra / δ Draconis) is a yellow star in the constellation Draco. It also has the traditional names Aldib, Altais and Nodus Secundus.
Delta Draconis has apparent magnitude +3.07 and belongs to spectral class G9III. It is approximately 100 light years from Earth.

Zeta Draconis (ζ Dra / ζ Draconis) is a star in the constellation Draco. It also has the traditional names Aldhibah or Nodus I (First Knot, the knot being a loop in the tail of Draco).
It is known as 紫微左垣四 (the Fourth Star of the Left Wall of the Purple Forbidden Enclosure), or simply 上弼 (the Star of the First Minister) in Chinese.
Aldhibah has an apparent magnitude of +3.17 and belongs to the spectral class B9III. Aldhibah is 300 light years from the Earth.

Iota Draconis (ι Dra / ι Draconis) is an orange giant star located 103 light-years (32 pc) away in the constellation Draco. A visually unremarkable star of apparent magnitude 3.31, it has recently been discovered to have a planet.
It also has the traditional name Edasich, with Eldsich recorded in the Century Cyclopedia. The Chinese knew it as Tso Choo 'Left Pivot'. It was called Al Ḍhiba' of Ulug Beg and the Dresden Globe, or Al dhīlī 'Male hyena' by Kazwini.

Chi Draconis (χ Dra / χ Draconis / Chi Draconis) is a star system in the constellation Draco. This star has been called by the traditional name of Batentaban Borealis, derived from the Arabic بطن الثعبان baţn al-thubān, meaning the Dragon's (or Serpent's) Belly.
The first companion is a yellow-white (class F) fourth-magnitude star with a mass approximately equal to that of the sun, but it is nearly twice as luminous. The second companion is an orange (class K) sixth-magnitude star, that is less massive and of lesser luminosity than the sun. In 1898 this system was reported to be a spectroscopic binary system, with an orbital period of 280.55 days. The two stars have an average separation of nearly an astronomical unit, which would disrupt the orbit of any Earth-like planet that was close enough to the primary to support liquid water. The two stars have about half the abundance of heavy elements as the Sun, but are approximately twice as old.

Thuban (α Dra / α Draconis / Alpha Draconis) is a star (or star system) in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star in ancient times.
Even though Thuban carries the Bayer designation Alpha Draconis, at apparent magnitude 3.65 it is over a magnitude fainter than the brightest star in the constellation, γ Dra (Etamin), whose apparent magnitude is 2.23. Thuban is not bright enough to be viewed from light-polluted areas.
Given good viewing conditions, Thuban is relatively easy to spot in the night sky, due to its location in relation to the Big Dipper asterism of Ursa Major. While it is well known that the two outer stars of the 'dipper' point to the modern-day pole star Polaris, it is less well known that the two inner stars, Phecda and Megrez, point to Thuban, just seven and a half degrees of arc from Megrez. The name comes from the Arabic ثعبان (θu‘bān, "the basilisk"), the Arabic name for the constellation Draco.

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