Arcturus
red giant star in the northern constellation of Boötes
Arcturus is a red giant star in the northern constellation of Boötes, and the brightest star in the constellation. It has the Bayer designation α Boötis, which is Latinized to Alpha Boötis and abbreviated Alpha Boo or α Boo. With an apparent visual magnitude of −0.05, it is the fourth-brightest star in the night sky and the brightest in the northern celestial hemisphere. Arcturus forms one corner of the Spring Triangle asterism.
Located relatively close at 36.7 light-years from the Sun, Arcturus is a 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 (around 35 million kilometers) and is around 170 times as luminous.
Nomenclature
The traditional name Arcturus is Latinised from the ancient Greek Ἀρκτοῦρος (Arktouros) and means "Guardian of the Bear", ultimately from ἄρκτος (arktos), 'bear' and οὖρος (ouros), 'watcher, guardian'. As ἄρκτος also came to mean "north", the name can also translate to "Guardian of the North".
The designation of Arcturus as α Boötis (Latinised to Alpha Boötis) was made by Johann Bayer in 1603. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars.
The public source identifies “Arcturus” as red giant star in the northern constellation of Boötes. This brief keeps that definition visible, then builds a research path around Arcturus, giant and star.
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Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Sep 4, 2026. The linked authority identifier is Q12985. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1603 and 2016.
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This entry incorporates text from “Arcturus” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.