Arcturus; Print; Arcturus; designation a Bo鰐is (Latinized to Alpha Bo鰐is; abbreviated Alpha Boo; a Boo); is the brightest star in the constellation of Bo鰐es; 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.