Arcturus

**Arcturus** 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. It is designated **α Boötis**, which is Latinized to **Alpha Boötis**. 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. When viewed from Earth, it appears to be positioned almost at the north galactic pole of the Milky Way.

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. It is about the same mass as the Sun, but has expanded to 25 times its size and is around 170 times as luminous. Its diameter is 35 million kilometres.

## Nomenclature

The name Arcturus is Latinised from the Greek star name Ἀρκτοῦρος (arktoyros) meaning "the Guard". The Greek name is found in ancient astronomical literature, e.g. Hesiod's *Work and Days*, Hipparchus's and Ptolemy's star catalogs. The folk-etymology connecting the star name with the bears (Greek: ἄρκτος, arktos) was probably invented much later. 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.

## Etymology

As one of the brightest stars in the sky, Arcturus has been significant to observers since antiquity.

In ancient Mesopotamia, it was linked to the god Enlil, and also known as Shudun, "yoke", or SHU-PA of unknown derivation in the *Three Stars Each* Babylonian star catalogues and later MUL.APIN around 1100 BC. The traditional name *Arcturus* is derived from the Ancient Greek Ἀρκτοῦρος (*Arktouros*) and means "Guardian of the Bear", ultimately from ἄρκτος (*arktos*), "bear" and οὖρος (*ouros*), "watcher, guardian". The name has been used since at least the time of Hesiod, circa 700 BC. It fell out of use in favour of Arabic names until it was revived in the Renaissance.

In Arabic, Arcturus is one of two stars called *al-simāk* "the uplifted ones" (the other is Spica). Arcturus is specified as السماك الرامح *as-simāk ar-rāmiħ* "the uplifted one of the lancer". The term *Al Simak Al Ramih* has appeared in Al Achsasi Al Mouakket catalogue (translated into Latin as *Al Simak Lanceator*). This has been variously romanized in the past, leading to obsolete variants such as *Aramec* and *Azimech*. For example, the name *Alramih* is used in Geoffrey Chaucer's *A Treatise on the Astrolabe* (1391). Another Arabic name is *Haris-el-sema*, from *حارس السماء* *ħāris al-samā’* "the keeper of heaven". or *حارس الشمال* *ħāris al-shamāl’* "the keeper of north".

In Indian astronomy, Arcturus is called Swati or Svati (Devanagari स्वाति, Transliteration IAST svāti, svātī́), possibly 'su' + 'ati' ("great goer", in reference to its remoteness) meaning very beneficent. It has been referred to as "the real pearl" in Bhartṛhari's kāvyas.

In Chinese astronomy, Arcturus is called *Da Jiao* (Chinese: 大角; *Dàjiǎo*; lit. 'great horn'), because it is the brightest star in the Chinese constellation called *Jiao Xiu* (Chinese: 角宿; *Jiǎo Xiǔ*; lit. 'horn star'). Later it became a part of another constellation *Kang Xiu* (Chinese: 亢宿; *Kàng Xiǔ*).