vendredi 19 septembre 2008

Ursa Major 10

Mizar (ζ UMa / ζ Ursae Majoris) is a star in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. The name comes from the Arabic ميزر mi'zar, meaning a waistband or girdle. Mizar's apparent magnitude is 2.23 and its spectral class is A1 V.
With normal eyesight one can make out a faint companion just to the east, named Alcor or 80 Ursae Majoris. Alcor is of magnitude 3.99 and spectral class A5 V. Mizar and Alcor together are sometimes called the "Horse and Rider," and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. Astronomer Sir Patrick Moore has suggested that this in fact refers to another star which lies visually between Mizar and Alcor. Mizar and Alcor lie three light-years apart, and though their proper motions show they move together (they are both members of the Ursa Major Moving Group), it is not believed they form a true binary star system, but simply an optical binary.
More components of the Mizar system were discovered with the advent of the telescope and spectroscopy; a fine, easily-split visual target, Mizar was the first telescopic binary discovered—most probably by Benedetto Castelli who in 1617 asked Galileo Galilei to observe it. Galileo then produced a detailed record of the double star. Later, around 1650, Riccioli wrote of Mizar appearing as a double. The secondary star, Mizar B, has magnitude 4.0 and spectral class A7, and comes within 380 AU of the primary; the two take thousands of years to revolve around each other. Mizar A was the first spectroscopic binary to be discovered, by Pickering in 1889. The two components are both about 35 times as bright as the sun, and revolve around each other in about 20 days. Mizar B was later found to be a spectroscopic binary as well. In 1996 the components of the Mizar A binary system were imaged in extremely high resolution using the Navy Prototype Optical Interferometer.
Beta Ursae Majoris (β UMa / β Ursae Majoris) is a star in the constellation of Ursa Major. It also has the traditional name Merak.
It is more familiar to northern hemisphere observers as one of the "pointer stars" in the Big Dipper, and a line connecting it with nearby Alpha Ursae Majoris (Dubhe) extends to Polaris, the north star. It is also one of the five stars in the Big Dipper asterism that form a part of a loose open cluster called the Ursa Major moving group, sharing the same area of space and not just the same patch of sky by our perspective.
Merak is fairly typical for a main sequence star of its type, although being slightly hotter and larger than our own Sun, it shines several times brighter than our home star. It is distinguished in the fact that evidence shows the star is surrounded by a cooling disk of dust, much like those discovered around Fomalhaut and most notably Vega. No planets have been discovered orbiting Merak, but the presence of the dust indicates they may exist or be in the process of forming.
The name is derived from the Arabic maraqq "loins" (of the bear).
The loins
Gamma Ursae Majoris (γ UMa / γ Ursae Majoris) is a star in the constellation Ursa Major. It also has the traditional names Phecda, Phekda or Phad and comes from the Arabic word فخذ fakhdh [faxð], "thigh" (of the Great Bear).
It is more familiar to most observers in the northern hemisphere as the lower-left star forming the bowl of the Big Dipper, together with Dubhe (upper-right), Merak (lower-right) and Megrez (upper-left). Along with four other stars in this well-known asterism, Phecda forms an actual loose, open cluster of stars known as the Ursa Major moving group. Like the other stars in the cluster, it is an average main sequence star not unlike our Sun, although somewhat hotter, brighter and larger.
Psi Ursae Majoris (ψ UMa / ψ Ursae Majoris) is a star in the constellation Ursa Major.
The name "Ta Tsun" of chinese origin supposedly translates as "the great wine-jar". Wasat, the delta star in the constellation Gemini can also be called by this name.
Mu Ursae Majoris (μ UMa / μ Ursae Majoris) is a star in the constellation Ursa Major. It also has the traditional names Tania Australis, and Alkafzah Australis.
Mu Ursae Majoris is a M-type red giant with a mean apparent magnitude of +3.06. It is approximately 249 light years from Earth. It is classified as a Semiregular variable star and its brightness varies from magnitude +2.99 to +3.33.
Spectroscopically it has been determined that this is a double star with a companion a mere 1.5 AU from the primary with a rotation period of 230 days.
Iota Ursae Majoris (ι UMa / ι Ursae Majoris) is a star system in the constellation Ursa Major. It is approximately 47.7 light years from Earth. It has the traditional names Talitha Borealis and Alphikra Borealis, and was also named Dnoces ("Second," backwards) after Edward H. White II, an Apollo 1 astronaut. The name was invented by his fellow astronaut Gus Grissom as a practical joke.[1]
The Iota Ursae Majoris system is composed of two binary stars. The brightest component, Iota Ursae Majoris A, is a white A-type subgiant with an apparent magnitude of +3.12. It is a spectroscopic binary whose components have an orbital period of 4028 days.
The companion binary is composed of the 9th magnitude Iota Ursae Majoris B and the 10th magnitude Iota Ursae Majoris C. These two stars orbit around each other with a period of 39.7 years, and are separated by roughly 0.7 arcseconds, or at least 10 Astronomical Units (AU). The two binary systems orbit around each other once every 818 years. The apparent separation between the two binaries is rapidly decreasing as they follow their orbits. In 1841 when the B component was first discovered, they had a separation of 10.7 arcseconds, or at least 156 AU. By 1971 their separation had decreased to 4.5 arcseconds, or at least 66 AU.

Talitha Australis Athalitha The southern one of the third leap القفزة الثّالثة
Talitha Borealis Ath-Thalithah The northern one of the third leap القفزة الثّالثة

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