Arcturus

Arcturus is a red giant star in the northern constellation of Boötes, and the brightest star in the constellation. It has the Bayer designation α Boötis, which is Latinized to Alpha Boötis and abbreviated Alpha Boo or α Boo. With an apparent visual magnitude of −0.05, it is the fourth-brightest star in the night sky and the brightest in the northern celestial hemisphere. Arcturus forms one corner of the Spring Triangle asterism. Located relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K1.5III—an aging star around 7.1 billion years old that has used up its core hydrogen and evolved off the main sequence. It is about the same mass as the Sun, but has expanded to 25 times its size (around 35 million kilometers) and is around 170 times as luminous.