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Iota Ursae Majoris (Iota UMa, ι Ursae Majoris, ι UMa) is a star system in the northern circumpolar constellation of Ursa Major. It has an apparent visual magnitude of 3.14,〔 making it visible to the naked eye and placing it among the brighter members of this constellation. Based upon parallax measurements, it is located at a distance of from Earth. It has the traditional names Talitha, Talitha Borealis and Alphikra Borealis, and was also dubbed 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.〔 The Iota Ursae Majoris system is composed of two binary stars. The brightest component is a white A-type subgiant. It is a spectroscopic binary whose components have an orbital period of 4028 days. The companion binary is composed of the 9th magnitude and 10th magnitude stars. 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 AU. This pair may be the source of the X-ray emission detected from this system.〔 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. This system appears to be dynamically unstable with a high likelihood and may become disrupted on a time scale on the order of 105 years.〔 ==Name and etymology== *The traditional name ''Talitha (share with κ UMa) comes from the Arabic phrase "the third spring, or leap, of the ghazal".〔(Richard Hinckley Allen :Star Names — Their Lore and Meaning - Ursa Major, the Greater Bear )〕 The term ''Borealis'' meaning "the north side" in Latin. *In Chinese, (), meaning ''Three Steps'', refers to an asterism consisting of ι Ursae Majoris, κ Ursae Majoris, λ Ursae Majoris, μ Ursae Majoris, ν Ursae Majoris and ξ Ursae Majoris. Consequently, ι Ursae Majoris itself is known as (, (英語:the First Star of Three Steps)) and (, (英語:Star of First Upper Step)).〔 ((Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 21 日 )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Iota Ursae Majoris」の詳細全文を読む スポンサード リンク
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