Arcturus (α Boo / α Boötis / Alpha Boötis) is the brightest star in the constellation Boötes, and the third brightest star in the night sky, with a visual magnitude of −0.05, after Sirius and Canopus, although it is fainter than the combined light of the two main components of Alpha Centauri, which are too close together for the eye to resolve as separate sources of light, making Arcturus appear to be the fourth brightest. It is the second brightest star visible from northern latitudes and the brightest star in the northern celestial hemisphere. The star is in the Local Interstellar Cloud.
An easy way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find Spica (α Virginis) as well — hence the maxim, "Arc to Arcturus, then speed on to Spica."
Arcturus is a type K1.5 IIIpe red giant star — the letters "pe" stand for "peculiar emission," which indicates that the spectrum of light given off by the star is unusual and full of emission lines. This is not too uncommon in red giants, but Arcturus has a particularly strong case of the phenomenon. It is at least 110 times more luminous than the Sun, but this underestimates its strength as much of the "light" it gives off is in the infrared; total power output is about 180 times that of the Sun. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun.
Arcturus is notable for its high proper motion, larger than any first magnitude star in the stellar neighborhood other than α Centauri. It is now almost at its closest point to the Sun, and is moving rapidly relative to the solar system. Arcturus is thought to be an old disk star, and appears to be moving with a group of 52 other such stars. Its mass is hard to exactly determine, but may be about the same as the Sun, and is no more than 1.5 solar masses. Arcturus is likely to be considerably older than the Sun, and much like what the Sun will be in its red giant phase.
According to the Hipparcos satellite, Arcturus is 36.7 light years (11.3 parsecs) from Earth, relatively close in astronomical terms. Hipparcos also suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the very limits of our current ability to make it out. Recent results remain inconclusive, but do support the marginal Hipparcos detection of a binary companion.
As one of the brightest stars in the sky, Arcturus has been the subject of a number of studies in the emerging field of astroseismology.
Belmonte et al. (1990) carried out a radial velocity (Doppler shift of spectral lines) study of the star in April and May 1988, which showed variability with a frequency of the order of a few microhertz, the highest peak corresponding to 4.3 μHz (2.7 days) with an amplitude of 60 ms-1, with a frequency separation of ~5 μHz. They suggested that the most plausible explanation for the variability of Arcuturus is stellar oscillations.
High precision photometry from the Hipparcos satellite's observations showed Arcturus is now known to be slightly variable, by about 0.04 magnitudes over 8.3 days. It is believed that the surface of the star oscillates slightly, a common feature of red giant stars. In the case of Arcturus, this was an interesting discovery as it is known that the redder (more towards or within the M spectral class) a giant gets, the more variable it will be. Extreme cases like Mira undergo large swings over hundreds of days; Arcturus is not very red and is a borderline case between variability and stability with its short period and tiny range.
The name of the star derives from ancient Greek Αρκτοῦρος (Arktouros) and means "Bear Guard." This is a reference to it being the brightest star in the constellation Boötes (of which it forms the left foot), which is next to the Big and Little Bears, Ursa Major and Ursa Minor. There is also a Greek non-governmental environmental organization named Αρκτούρος that protects wild life.
In Arabic, it is called As-Simāk ar-Rāmiḥ (السماك الرامح) which has been translated as "leg of the lance-bearer" or "the lofty lance-bearer." This name has been variously romanized in the past, leading to obsolete variants such as Aramec and Azimech. Another Arabic name is Al-Hārith as-Samā' (الحارس السماء), "the keeper of heaven."
In Chinese astronomy, Arcturus is called Dah Jyaoo (大角, Great Horn, Pinyin: Dàjiǎo), because it is the brightest star in the Chinese constellation called Jyaoo Shiuh (角宿, Pinyin: Jiǎo Xiǔ). And later, it become a part of Kangh Shiuh (亢宿, Pinyin: Kàng Xiǔ), which is also a Chinese constellation.
Ancient Japanese astronomy adopted the Chinese name Dah Jyaoo (大角, Tai Roku), but its western name, Arcturus (アルクトゥルス), is more common now.
In Inuit astronomy, Arcturus is called the Old Man.
In Greek mythology, Arcturus is a star created by Zeus to protect the nearby constellations, Arcas and Callisto (Ursa major and Ursa minor). According to the myth, Callisto was the daughter of Lycaon, the king of Arcadia. As a young girl, she vowed to the goddess Artemis to be forever faithful and devoted to her. She was to remain a virgin forever in order to serve and accompany Artemis while hunting animals in the forest. However, one day, Zeus, the king of the gods, fell in love with Callisto and forced her to have sexual intercourse with him. Callisto gave birth to a son whom she named Arcas. Zeus knew that if Hera, his wife, learned of his disloyalty she would be angry with Callisto, so in order to protect her he transformed her into a brown bear. Callisto, as a bear, roamed around the forest looking for her son, Arcadian. After years of searching she found Arcas, who was now a grown man. She finally came upon him and, overjoyed, stood on her hind legs and tried to embrace him. Arcas, however, did not recognize his mother and thought he was being attacked, so he drew his sword to defend himself. Zeus, watching everything, as usual, felt sorry for them and in order to prevent this tragedy he transformed Callisto and Arcas into constellations (now known as Ursa major and Ursa minor) and placed them near to him in the sky. Hera, who had learned the truth and was furious, asked Ocean, the river that surrounds the earth, not to permit them to wash themselves in his waters; therefore these two constellations are always seen high in the night sky, and never drop into the ocean. Moreover, in order to protect them from Hera's jealousy, Zeus placed another star near to them: Arcturus (which means the guardian of Arctos, the bear). It protects and accompanies them for eternity.
Prehistoric Polynesian navigators knew Arcturus as Hōkūleʻa, the "Star of Joy". Arcturus is the zenith star of the Hawaiian Islands. Using Hōkūleʻa and other stars, the Polynesians launched their double-hulled canoes from Tahiti and the Marquesas Islands. Traveling east and north they eventually crossed the equator and reached the latitude at which Arcturus would appear directly overhead in the summer night sky. Knowing they had arrived at the exact latitude of the island chain, they sailed due west on the trade winds to landfall. If Hōkūleʻa could be kept directly overhead, they landed on the southeastern shores of the Big Island of Hawaiʻi. For a return trip to Tahiti the navigators could use Sirius, the zenith star of that island. Since 1976, the Polynesian Voyaging Society's Hōkūle‘a has crossed the Pacific Ocean many times under navigators who have incorporated this wayfinding technique in their non-instrument navigation.
The star achieved fame when its light was used to open the 1933 Chicago World's Fair. The star was chosen as it was thought that light from Arcturus had started its journey at about the time of the previous Chicago World's Fair in 1893. Technically the star is 36.7 light years away so the light would have started its journey in 1896.
In a reading in which the 'sleeping prophet' describes philosophical concepts as they relate to religious tenets, Edgar Cayce mentions Arcturus.
(Q) The sixth problem concerns interplanetary and inter-system dwelling, between earthly lives. It was given through this source that the entity Edgar Cayce, after the experience as Uhjltd, went to the system of Arcturus, and then returned to earth. Does this indicate a usual or an unusual step in soul evolution?
(A) As indicated, or as has been indicated in other sources besides this as respecting this very problem, - Arcturus is that which may be called the center of this universe, through which individuals pass and at which period there comes the choice of the individual as to whether it is to return to complete there - that is, in this planetary system, our sun, the earth sun and its planetary system - or to pass on to others. This was an unusual step, and yet a usual one. (5749-14)
In the astrology of the Middle Ages, Arcturus was one of the 15 Behenian fixed stars, associated with jasper and the plantain herb. Cornelius Agrippa lists its symbol under the alternate name Alchameth.