North Star - Polaris

It is the alpha star of the constellation Ursa Minor. It has this Chinese name because it is a bright star located very close to the north celestial pole (the direction of the Earth's rotation axis extended northward). Another Chinese name, "Hokushin" (Tatsu means star), has been passed down in Japan since ancient times, and other Japanese names include "Ne no Hoshi" (Ne is the zodiac sign for north) and "Hitotsu Boshi" (One Star). The English word Polaris comes from the Latin name for "Stella Polaris". It hardly moves during its diurnal motion and is always visible almost due north, so it has been used to determine direction since ancient times. Its apparent magnitude is almost 2.0 and its distance is about 430 light years. Its position on the celestial sphere is the 2000th equinox, with right ascension 2 hours 32 minutes and declination +89 degrees 16 minutes. When viewed through a telescope, a faint 8.6 magnitude star can be seen very close by, forming a visual binary star. The companion star (star B) is a main sequence star of spectral type F3 (surface temperature about 6800 K, mass about 1.5 times that of the Sun, radius about 1.4 times). Furthermore, the main body of Polaris itself is a spectroscopic binary star, with the main star (star A) being a supergiant star of spectral type F8 (surface temperature about 5800 K, mass about 6 times that of the Sun, radius about 80 times), and the companion star (star P) being a main sequence star of spectral type F0 (surface temperature about 7300 K, mass about 1.5 times that of the Sun, radius about 1.5 times). Stars A and P orbit each other in an elliptical orbit with an eccentricity of 0.63 and an orbital period of 30.46 years. The main star, a supergiant star of type F8, is a pulsating variable star that expands and contracts repeatedly with a period of 3.9697 days. About 100 years ago, the magnitude of the variation was about 0.15, but recently it has decreased to about 0.05, and the pulsation period has also lengthened by more than 5 minutes over the past 100 years. The North Star is approaching the solar system at a speed of 17 kilometers per second. Due to the Earth's precession, the direction of the Earth's axis of rotation (the North Celestial Pole) is gradually moving relative to the celestial sphere. As of the year 2000, the position of the North Celestial Pole is about three-quarters of a degree away from the North Star, but by around 2100 it will be about half a degree closer, and then it will gradually move away from the North Star. Therefore, the North Star will only have meaning as the North Star for another 1,000 years or so.

[Akira Okazaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

こぐま座のα(アルファ)星。天の北極（地球自転軸を北側に延長した方向）のすぐ近くに位置する明るい星なので、この漢名がある。別の漢名「北辰(ほくしん)（辰は星の意）」も古くから日本に伝わり、このほかに和名としては「子の星(ねのほし)（子は十二支で北のこと）」「一つ星(ひとつぼし)」などがある。英語のポラリスは「極の星 stella polaris」のラテン名に由来。日周運動でほとんど動かず、いつもほぼ真北に見えているので、昔から方位を定めるのに利用されてきた。実視等級はほぼ2.0等で、距離は約430光年。天球上の位置は2000年分点で、赤経2時32分、赤緯プラス89度16分である。望遠鏡で見ると、ごくそばに暗い8.6等星が見え、実視連星を形成する。伴星（B星）はスペクトル型F3の主系列星（表面温度約6800K、質量は太陽の約1.5倍、半径は約1.4倍）である。さらに、北極星の本体自身が分光連星であり、主星（A星）は、スペクトル型F8の超巨星（表面温度約5800K、質量は太陽の6倍程度、半径は約80倍）、伴星（P星）は、スペクトル型F0の主系列星（表面温度約7300K、質量は太陽の約1.5倍程度、半径も約1.5倍）である。A星とP星は公転周期30.46年で離心率0.63の楕円(だえん)軌道を描いて互いに回っている。主星のF8型超巨星は脈動変光星で、3.9697日の周期で膨張・収縮を繰り返している。約100年前は変光幅が0.15等ほどあったが、最近では0.05等程度に減少し、また脈動周期もこの100年間で5分余り長くなった。北極星は全体として毎秒17キロメートルで太陽系に近づきつつある。地球の歳差運動のため地球の自転軸の方向（天の北極）は天球に対して徐々に移動している。天の北極の位置は2000年の時点で北極星から角度で約4分の3度離れているが、2100年ころには約2分の1度まで近づき、その後はしだいに北極星から離れてゆく。そのため北極星が北極の星としての意味をもつのは、あと1000年ぐらいであろう。

［岡崎　彰］

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