vendredi 19 septembre 2008

Ursa Major 9

Epsilon Ursae Majoris (ε UMa / ε Ursae Majoris) is the brightest star in the constellation Ursa Major (despite its Bayer designation being merely "epsilon"), and at magnitude 1.76 is the thirty-first brightest star in the sky. It also has the traditional name Alioth (from the Arabic word alyat—fat tail of a sheep).
It is known as 北斗五 (the Fifth Star of the Northern Dipper) or 玉衡 (the Star of Jade Sighting-tube) in Chinese.
It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.[1]
According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A0p; the "p" stands for peculiar, as the spectrum of its light is quite odd, of a kind characteristic of an Alpha2 Canum Venaticorum variable. Alioth, as a representative of this type, is believed to look the way it does because of two interacting processes: first, the star's strong magnetic field separating different elements salting the star's hydrogen fuel, then a rotation axis at an angle to the magnetic axis spinning different bands of magnetically sorted elements into the line of sight between Alioth and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Alioth to have very strange spectral lines that fluctuate over a period of 5.1 days. In the case of Alioth, the rotational and magnetic axes are at almost 90 degrees to one another; in the map of Alioth linked below, note how the darker (denser) regions of chromium form a band at right angles to the equator. A recent study suggests the 5.1 days variation may be due to a substellar object with around 14.7 Jupiter masses in eccentric orbit (e=0.5) and average separation of 0.055 Astronomical units.
For its type, Alioth has a relatively weak magnetic field (15 times weaker than α CVn's), but it is still 100 times stronger than that of the Earth.
The fatty tail of a sheep

Alpha Ursae Majoris (α UMa / α Ursae Majoris) is the second-brightest star in the constellation of Ursa Major (despite its Bayer designation of "alpha"). It also has the traditional name Dubhe.
It forms part of the Big Dipper (also known in English as the Plough or the Great Bear), and is the northern of the pointers, the two stars of Ursa Major which point towards Polaris. However, it is not part of the Ursa Major moving group. Instead, it is an evolved helium-burning star, about 124 light years away. It is a multiple star, orbited by a main sequence companion, Dubhe B, at a distance of about 23 astronomical units (AU), as well as a close pair, Dubhe C, at a distance of about 8000 AU.
The traditional name comes from the Arabic for "bear", dubb, from the phrase ظهر الدب الاكبر‎ żahr ad-dubb al-akbar "the back of the Greater Bear".
It is known as 北斗一 (the First Star of the Northern Dipper) or 天樞 (the Celestial Pivot) in Chinese.
Due to the precession of the equinoxes, Alpha Ursae Majoris was the northern pole star around 5100 BC, and will be again around the year 20,500 AD.
Dubhe is the official state star of the State of Utah.

Eta Ursae Majoris (η UMa / η Ursae Majoris) is a star in the constellation Ursa Major. It also has the traditional names Alkaid (or Elkeid) and Benetnash (Benetnasch).
It is known as 北斗七 (the Seventh Star of the Northern Dipper) or 搖光 (the Star of Twinkling Brilliance) in Chinese.
Alkaid is the most eastern (leftmost) star in the Big Dipper (Plough) asterism. However, unlike most stars of the Big Dipper it is not a member of the Ursa Major moving group.
Alkaid has apparent magnitude +1.9, making it the 35th brightest star in the sky. It is a young bluish-white main sequence star of spectral class B3 V. At 20,000 kelvins it is one of the hotter stars visible with the naked eye.
The name derives from "The leader of the daughters of the bier". The daughters of the bier; i.e., the mourning maidens, are the three stars of the handle of the Big Dipper, Alkaid, Mizar (star), and Alioth.
"Leader" of the mourning maidens

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