Last modified on January 10, 2018, at 12:59


000Bootes constellation map.jpg
Observational Data
Designation Alpha Boötis
Right ascension 14h 15m 39.7s
Declination +19o 10' 56"
Constellation Boötes
Type of object Star
Magnitude Absolute Mag: -0.29
Apparent Mag: -0.04
Distance from Earth 36.7 ±0.3 ly
Radial velocity +5 km/s
Proper motion RA: −1093.45 mas/yr
Dec.: −1999.40 mas/yr
Parallax 88.98 ±0.68 mas

Arcturus, also known as Alpha Boötis, is a star located some 36.7 light years from our Sun. It is the brightest star in the constellation Boötes. Arcturus is the fourth brightest star overall in the night sky after Sirius, Canopus, and Alpha Centauri AB (the two stars are too close to resolve separately with the unaided eye), and the brightest one located in the northern celestial hemisphere, with an apparent magnitude of -0.05.[1] Arcturus is a single star, although in the 1990s it was thought to possibly have a stellar companion.[2]

Arcturus has the second largest proper motion of first magnitude stars (only Alpha Centauri's is greater), as observed from Earth. This motion was first detected by Edmond Halley in 1718. Right now, Arcturus is at its closest proximity to our solar system, and will fade from view in another half million years as it moves away.

Arcturus in HistoryEdit

Arcturus is well known worldwide since antiquity, thanks to the fact that it is one of the brightest stars in the night sky.

The star was known by ancient Polynesian navigators as Hōkūleʻa, the "Star of Joy". The Polynesians knew the star was at its zenith over the Hawaiian islands and used this knowledge to sail from Tahiti and the Marquesas Islands to Hawaii. For their return trip, the Polynesians would use Sirius, the zenith star of Tahiti. This trip has been duplicated several times using the same wayfinding technique of navigation through the use of these same stars since 1976 by the Polynesian Voyaging Society.

The star was known as Marpean-kurrk to the Koori of Australia, its appearance signifying the birth of the larvae of the wood-ant in spring. The start of summer would be signaled by the star setting with the Sun. It was also seen as the mother of Antares (known to them as Djuit).[3]

The name Arcturus is a derivative of the Ancient Greek Αρκτοῦρος, meaning "Guardian of the Bear". This was in reference to the constellation Boötes, which is next to Ursa Major and Ursa Minor, the Greater and Lesser Bears. According to Greek mythology, the star was created by Zeus for the purpose of guarding these constellations, referred to as Callisto and her son Arcas.

The star is also mentioned in the Old Testament of the Bible in the King James Version:

Which maketh Arcturus, Orion, and Pleiades, and the chambers of the south. Job 9:9 (KJV)

Canst thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons? Job 38:32 (KJV)

In the actual Hebrew, it is translated as עיש, or Ayish, and it is typically associated with Ursa Major, although the exact meaning is disputed and controversial owing to the general obscurity of ancient terminology.[4]

In Arabic the star's full name is referred to as السماك الرامح as-simāk ar-rāmiħ, or "the uplifted one of the lancer". It is one of two stars called al-simāk, or "the uplifted one", the other being Spica.[5] This shortened name has been romanized in Medieval Europe to such names as Aramec, Azimech, and Alramih. Another Arabic term for the star is حارس السماء ħāris al-samā’ or "the keeper of heaven".[6]

For the Chinese, the star is located in the constellation known as 角宿, Jiao Xiu, of which it is the brighest star. Because of this, the star is called 大角, Da Jiao, or "Great Horn". It is also part of the constellation of 亢宿, Kang Xiu.

During the 1933 World Fair in Chicago, the light from Arcturus was collected and used to start a series of switches to illuminate and officially open the fair. This was based on the conclusion at the time that Arcturus was 40 light years distant, therefore light originating from the star during the 1896 World Fair in Chicago, 40 years prior, would have reached here for the 1933 fair.[7]

The StarEdit

Comparison in size between Arcturus and the Sun. Copyright © Windows to the Universe [1]. Used with permission.

Arcturus is an orange-red giant of the spectral type K1.5 IIIpe. The star has some 24.5 times the diameter of our Sun. Like all giants, it has a low mass compared to its size, having only 1.5 times the mass of our Sun.[8] Arcturus is 115 times as bright as our sun visually, although its absolute magnitude is 215 times as great counting the infrared.[9] The star has rather low metallicity, being only 17 to 32 percent as abundant as our Sun in elements heavier then hydrogen, based on its abundance of iron.[10]

As a helium-burning giant star that has consumed all its hydrogen core fuel and left the main sequence, it has fully shifted to fusing helium in its core into carbon and oxygen. Eventually the star will begin to lose its mass and the gases will puff out as a planetary nebula containing mostly hydrogen and helium, with smaller amounts of other elements. What will be left is a white dwarf star that will gradually cool and fade over time.

There are no known substellar companions orbiting Arcturus at this time. Any world that would have been Earth-like while Arcturus was on the main sequence would have long ago been destroyed. For such a world to exist now with liquid water on its surface, it would need to be centered around 11 AU away, or between the orbital distances of Saturn and Uranus in our solar system. Such a world would be extremely difficult to detect using current methods.