Procyon (English spelling)

The proper name of the alpha star in the constellation Canis Minor. This star, along with Betelgeuse and Sirius, form the "Winter Triangle." The name means "before the dog" in Greek, and comes from the fact that it rises in the eastern sky before Sirius (Dog Star) in the constellation Canis Major. Its Arabic name is Al-She'ara Al-Shamia, meaning "Sirius of the North."

Its celestial position in 2000 was right ascension 7 hours 39 minutes 18 seconds and declination +5 degrees 13.5 minutes. It reaches its zenith at midnight around January 15 every year. Its apparent magnitude is 0.36, making it the eighth brightest star in the sky. Its color index ( B - V ) is +0.42 (a measure of the color of a celestial object, calculated by subtracting apparent magnitude V from blue magnitude B ). It is a yellowish, subgiant or main sequence star of spectral type F5 IV-V. Its parallax is 285.9 arc milliseconds, and it is a nearby star 11.4 light years from Earth. Its mass is 1.50 times that of the Sun, and its surface temperature is 6500 Kelvin. Interferometer measurements have determined that the star's angular diameter is 5.2 arc milliseconds. Its radius is 2.1 times that of the Sun. Its luminosity is 7.2 times that of the Sun. The rotational speed v is v sin i = 5.7 km/s (where i is the tilt of the star's axis as seen from Earth). Its proper motion is large, moving at a rate of 1258.5 arc milliseconds per year in a direction along a position angle of 214.7 degrees.

Due to the wobble of its proper motion, the existence of a faint companion star was predicted in 1840, and a companion star (Procyon B) with a magnitude of 10.7 was discovered in 1896. It is a famous white dwarf star along with the companion star Sirius, with a mass 0.60 times that of the Sun and a radius 0.0123 times that of the Sun, just a little larger than the Earth. It has a high average density of 0.45 tons per cubic centimeter. The surface temperature is 7740 K. The primary and companion stars revolve around their common center of mass with a period of 40.38 years, an elliptical orbit with an eccentricity of 0.365, and an inclination (the angle of the orbital axis as seen from the Earth) of 31.9 degrees. The semi-major axis of the primary star's orbit is 1180 arcseconds. The radial velocity of the center of mass of the binary system is -4.12 km/s, and the radial velocity of the primary star periodically changes with an amplitude of 1.70 km/s around the radial velocity of the center of mass as it orbits. Precise spectrophotometric measurements have been performed on the primary star, and various vibrational modes and chemical composition analyses have been performed. X-rays have been observed from the less active corona.

[Atsuo Yamazaki]

"The Universe of Stars: A Modern Introduction" by Kunitomo Sakurai (1987, Kyoritsu Publishing)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

こいぬ座のα(アルファ)星の固有名。この星とベテルギウス、シリウスで「冬の大三角」をつくる。名前はギリシア語で「イヌの前に」の意で、おおいぬ座のシリウス（Dog Star）に先駆けて東天に昇ってくることに由来する。アラビア名はアル・シァラ・アル・シャミイアAl-She'ara Al-Shamiaで「北のシリウス」の意。

　2000年の天球上の位置は、赤経7時39分18秒、赤緯プラス5度13.5分。毎年1月15日ごろ真夜中に南中する。実視等級0.36等で全天で8番目に明るい。色指数（B－V）はプラス0.42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）。黄色に輝くスペクトル型F5Ⅳ～Ⅴの準巨星または主系列星。視差は285.9ミリ秒角で地球からの距離11.4光年の近距離星。質量は太陽の1.50倍で表面温度は6500K（ケルビン）。干渉計で星の角直径が5.2ミリ秒角と測定された。半径は太陽の2.1倍である。光度は太陽の7.2倍。自転速度vはv sin i＝5.7キロメートル／秒（iは地球から見た星の自転軸傾斜角）。固有運動は大きく、位置角214.7度の方向に1258.5ミリ秒角／年で動く。

　固有運動のふらつきから、1840年に暗い伴星の存在が予想され、1896年に10.7等の伴星（プロキオンB）が発見された。シリウスの伴星とともに有名な白色矮星(わいせい)で、質量は太陽の0.60倍、半径は太陽の0.0123倍で地球よりも少し大きいだけである。平均密度は大きく、1立方センチメートルあたり0.45トンにもなる。表面温度は7740K。主星と伴星は、周期40.38年、楕円(だえん)軌道の離心率0.365、軌道傾斜角（地球から見た公転軸の傾き）31.9度で共通重心の周りを公転している。主星の公転軌道長半径1180ミリ秒角。連星系の重心の視線速度はマイナス4.12キロメートル／秒で、主星の視線速度は公転に伴い重心の視線速度を中心に振幅1.70キロメートル／秒で周期的に変化している。主星については精密な分光測光がなされており、さまざまな振動モード解析や化学組成解析が行われている。あまり活発でないコロナからX線が観測されている。

［山崎篤磨］

『桜井邦朋著『星々の宇宙――その現代的入門』（1987・共立出版）』

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