Arcturus

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. α Boötis (Latinised to Alpha Boötis) is the star's Bayer designation. The traditional name Arcturus derives from Ancient Greek Ἀρκτοῦρος (Arktouros) and means 'Guardian of the Bear', ultimately from ἄρκτος (arktos), 'bear' and οὖρος (ouros), 'watcher, guardian'. It has been known by this name since at least the time of Hesiod. One astronomical tradition associates Arcturus with the mythology around Arcas, who was about to shoot and kill his own mother Callisto who had been transformed into a bear. Zeus averted their imminent tragic fate by transforming the boy into the constellation Boötes, called Arctophylax 'bear guardian' by the Greeks, and his mother into Ursa Major (Greek: Arctos 'the bear'). The account is given in Hyginus's Astronomy. Aratus in his Phaenomena said that the star Arcturus lay below the belt of Arctophylax, although according to Ptolemy in the Almagest it lay between his thighs. An alternative lore associates the name with the legend around Icarius, who gave the gift of wine to other men, but was murdered by them, because they had had no experience with intoxication and mistook the wine for poison. It is stated this Icarius, became Arcturus, while his dog, Maira, became Canicula (Procyon), although 'Arcturus' here may be used in the sense of the constellation rather than the star. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Arcturus for this star. It is now so entered in the IAU Catalog of Star Names.