Arcturus

Arcturus in the constellation of Boötes

Arcturus (or α Boo, α Boötis, Alpha Boötis) of the constellation Boötes is the brightest star in the northern celestial hemisphere. With an apparent magnitude of −0.04, it is the fourth brightest star in the night sky, after −1.46 magnitude Sirius, −0.86 magnitude Canopus, and −0.27 magnitude Alpha Centauri.

It is a relatively close star at only 36.7 light-years from Earth, and, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood.

Arcturus is a type K1.5 IIIpe red giant star, with an absolute magnitude of −0.30. It has likely exhausted its hydrogen from the core and is now in its active red giant phase. It will continue to expand before entering the horizontal branch stage of its life cycle. The diameter of Arcturus is around 35 million kilometers.

Arcturus Media

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  Arcturus is the brightest star in the constellation of Boötes.

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  Optical image of Arcturus (DSS2 / MAST / STScI / NASA)

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  Size comparison between the Sun, Beta Ursae Majoris, Pollux, and Arcturus.

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  Arcturus next to the head of Comet Donati in 1858

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  The en:kabbalistic en:astrological sign "Alchameth". Corresponding to en:Arcturus, a star in the Bootes constellation. Originally described in en:Libri tres de occulta philosophia, 1531 by en:Heinrich Cornelius Agrippa.