Achernar - Akernar (English spelling)

The proper name of the alpha star of Eridanus. Its name comes from the Arabic Akhir Al-Nahr (end of the river), and it is a pale blue star at the southwest end of the Eridanus River, which meanders from near Orion to the southwest on the celestial sphere to a declination of minus 58 degrees. Its apparent magnitude is 0.47 (variable), making it the 10th brightest star in the entire sky. Its color index ( B - V ) is minus 0.16 (one of the indices that express the color of a celestial object, calculated by subtracting apparent magnitude V from blue magnitude B ). In 2000, its position on the celestial sphere was 1 hour 37 minutes 43 seconds at right ascension and minus 57 degrees 14.2 minutes at declination, making it a southern star. It reaches its zenith in the sky at midnight around October 15 every year. Its parallax is 22.7 arc milliseconds, and its distance from Earth is 144 light years. Its proper motion is 97 milliarcseconds per year, and its radial velocity is +16 kilometers per second. It is a spectral type B3 IV-Vpe emission star (a star with emission lines in its spectrum), with a surface temperature of 18,000 K (Kelvin). An optical interferometer measured the star's diameter to be 2.5 milliarcseconds in the long axis and 1.6 milliarcseconds in the short axis. These are 12 and 8 times the diameter of the Sun, respectively, making it an oblate star, with its long axis 50% larger than its short axis. Its rotational speed v is v sin i = 225 kilometers per second ( i is the tilt of the star's rotational axis as seen from Earth), and the centrifugal force caused by its fast rotation causes the star's shape to be flattened, rather than spherical, with the equator bulging out.

[Atsuo Yamazaki]

"Masao Segawa's 'Constellation Museum: Winter' (1989, Gyosei)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

エリダヌス座のα(アルファ)星の固有名。名前はアラビア語で「川の終わり」（アキル・アル・ナールAkhir Al-Nahr）に由来し、オリオン座の近くから天球上を南西に大きく蛇行しながら赤緯マイナス58度まで走るエリダヌス川の南西の果てにある青白い恒星。実視等級は0.47等（変光がある）で、全天で10番目に明るい。色指数（B－V）はマイナス0.16等（天体の色を表す指標の一つで、青色B等級から実視V等級を引いたもの）。2000年の天球上の位置は赤経1時37分43秒、赤緯マイナス57度14.2分で南天の星。毎年10月15日ごろ真夜中に南中する。視差は22.7ミリ秒角で、地球からの距離は144光年。固有運動は97ミリ秒角／年、視線速度はプラス16キロメートル／秒。スペクトル型B3Ⅳ～Ⅴpeの輝線星（スペクトルに輝線がみられる星）で、表面温度は1万8000K（ケルビン）。光干渉計で星の直径が長径2.5ミリ秒角、短径1.6ミリ秒角と測定された。それぞれ太陽の直径の12倍と8倍で、長径が短径より50％も大きい扁平(へんぺい)な星。自転速度vはv sin i＝225キロメートル／秒（iは地球から見た星の自転軸傾斜角）もあり、速い自転による遠心力のため星の形が球ではなく赤道付近が膨らみ扁平になっている。

［山崎篤磨］

『瀬川昌男著『星座博物館　冬』（1989・ぎょうせい）』

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