Capella - Capella (English spelling)

The proper name of the alpha star in the constellation Auriga. Capella means "female goat" in Latin, and is found in the part of the goat held by the charioteer Erichthonius (the ancient king of Athens). It is also called Amalthea, named after the female goat that fed the young Zeus. It is the brightest star in the "Winter Pentagon" and shines beautifully in yellow in the winter night sky. It is the sixth brightest star in the entire sky. Its apparent magnitude is 0.08, and its color index ( B - V ) is +0.80 (one of the indices that express the color of a celestial body, calculated by subtracting apparent magnitude V from blue magnitude B ). Its position on the celestial sphere in 2000 is 5 hours 16 minutes 41 seconds in right ascension and +45 degrees 59.9 minutes in declination, and it reaches its culmination in the south at midnight around December 10 every year. Its parallax is 77.3 arc milliseconds, and its distance from Earth is 42.2 light years. Its proper motion is 434 arcseconds per year, with a position angle of 170 degrees, moving southward. Its radial velocity is 29.5 kilometers per second. Capella is the third brightest binary star after Sirius and Alpha Centauri, with a period of 104.0 days. It consists of two giant stars, a primary (Capella A or Aa) of spectral type G8Ⅲ and a companion (Capella B or Ab) of G1Ⅲ. Their luminosity is V (apparent magnitude) 0.71 (variable) and 0.96, respectively. The angular separation between the two stars has been measured as 0.05647 arcseconds (0.73 AU). Its orbital inclination (the tilt of the orbital axis as seen from Earth) is 43 degrees, so it does not undergo eclipses. The primary star has a mass 2.7 times that of the Sun, a radius 12.2 times larger, a luminosity 74 times larger, and a surface temperature of 4,800 K (Kelvin), while the companion star has a mass 2.6 times that of the Sun, a radius 9.2 times larger, a luminosity 59 times larger, and a surface temperature of 5,700 K. Both stars emit strong X-rays and ultraviolet light. The primary star is an active star of the RS Canes Venatici type, and emits X-rays from its corona. The companion star is sometimes observed to emit stronger X-rays than the primary star. It also emits ultraviolet light from its chromosphere and transition zone (the outer atmosphere of a star, between the chromosphere and the corona or the cold outer atmosphere). The spectrum of the primary star is deficient in lithium, with only 160 times less lithium than the companion star. Capella H and Capella L are located about 12 arcmin away on the celestial sphere. They are of spectral type M1 and M5, with apparent magnitudes of 10.2 and 13.7, respectively, and are observed in X-rays and ultraviolet light. These four stars (ADS3841 = Capella Aa, Ab, H, L) are said to form a multiple binary star system.

[Atsuo Yamazaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

ぎょしゃ座のα(アルファ)星の固有名。カペラとはラテン語で「雌の子ヤギ」の意で、馭者(ぎょしゃ)エリクトニウス（古代アテネ王）が抱えるヤギにあたる部分にある。幼いゼウスに乳を与えて育てた雌ヤギの名をとってアマルテアともよばれる。「冬の五角形」のうちもっとも明るい星で、冬の夜空に美しく黄色に輝く。全天で6番目に明るい恒星。実視等級は0.08等、色指数（B－V）はプラス0.80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5時16分41秒、赤緯プラス45度59.9分、毎年12月10日ごろ真夜中に南中する。視差は77.3ミリ秒角で、地球からの距離42.2光年。固有運動は434ミリ秒角／年、位置角170度で南に動いている。視線速度は29.5キロメートル／秒。カペラは、シリウス、ケンタウルス座α星に次いで3番目に明るい分光連星で、周期104.0日。スペクトル型G8Ⅲの主星（カペラAまたはAa）とG1Ⅲの伴星（カペラBまたはAb）の二つの巨星よりなる。明るさはそれぞれV（実視等級）0.71等（変光がある）、0.96等。2星の間隔は角距離0.05647秒（0.73天文単位）と測定された。軌道傾斜角（地球から見た公転軸の傾き）は43度で食現象はおこさない。主星は、質量が太陽の2.7倍、半径が12.2倍、光度が74倍、表面温度が4800K（ケルビン）、伴星は質量が太陽の2.6倍、半径が9.2倍、光度が59倍、表面温度5700K。両方の星からは、強いX線や紫外線が放射されている。主星はりょうけん座RS星型の活動を示しコロナからはX線が出ている。伴星からは、主星より強いX線が観測されることもある。また彩層や遷移層（恒星の外層大気で、彩層とコロナや低温外周大気圏の間の大気）から紫外線が出ている。主星のスペクトルにはリチウムの欠乏がみられ、伴星の160分の1しかない。両巨星から天球上を12分角ほど離れてカペラHおよびカペラLがある。スペクトル型はそれぞれM1とM5、実視等級は10.2等と13.7等で、X線や紫外線が観測されている。これら4星（ADS3841＝カペラAa、Ab、H、L）は多重連星系をなしているとされる。

［山崎篤磨］

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