With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27). Its diameter is 35 million kilometres.Īrcturus is the brightest star in the constellation of Boötes. It is about the same mass as the Sun, but has expanded to 25 times its size and is around 170 times as luminous. Located relatively close at 36.7 light-years from the Sun, Arcturus is a single 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. When viewed from Earth, it appears to be positioned almost at the north galactic pole of the Milky Way. 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. It is designated α Boötis, which is Latinized to Alpha Boötis. Alramech, Alramech, Abramech, α Boötis, Alpha Boo, α Boo, 16 Boötis, BD+19☂777, GJ 541, HD 124897, HIP 69673, HR 5340, SAO 100944, LHS 48, GCTP 3242.00Īrcturus 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.
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