Which star is the brightest in the northern celestial hemisphere?

Arcturus, designation α Boötis (Latinized to Alpha Boötis, abbreviated Alpha Boo, α Boo), is the brightest star in the constellation of Boötes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere. 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.

Relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K0III—an ageing star around 7.1 billion years old that has used up its core hydrogen and moved off the main sequence. It is 1.08±0.06 times as massive as the Sun, but has expanded to 25.4±0.2 times its diameter and is around 170 times as luminous.