Arcturus

Arcturus is the brightest star in the northern constellation of Boötes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere. It is designated α Boötis, which is Latinized to Alpha Boötis. Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli. When viewed from Earth, it appears to be positioned almost at the north galactic pole of the Milky Way.

Located relatively close at 36.7 light-years from the Sun, Arcturus is a single[7] 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 and is around 170 times as luminous. Its diameter is 35 million kilometres.