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.
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 (HD 124897). SIMBAD Astronomical Database
- For "horizontal branch", see Hertzsprung–Russell diagram.