The third brightest star
in the night sky, Arcturus
is overhead as darkness falls. Look to the north, find the handle of the dipper as illustrated above, and follow the arc as shown above. Continue down to Spica, and follow the ecliptic to the west to find Saturn, Mars and Venus, which will be close to the Moon.
Arcturus, or Alpha Boötis, is located about 36.7 light-years from the Sun. It is the brightest star of Constellation Boötes
, the Herdsman or Bear Driver, forming his left foot.Boötes
may be a hunter, on the tracks of the Great Bear, accompanied by his two dogs Asterion and Chara (the "Canes Venatici"). And yet the constellation was once known as Arctophylax which means the protector of the Bear. Perhaps it was the Romans who changed his role, for they called him Venator Ursae: the Bear Hunter.
Nowadays Boötes is generally considered to be a Herdsman (as in French: Le Bouvier), as he eternally shepherds the stars around the North Pole.
The constellation was known in antiquity, with the first recorded appearance being in Homer's Odyssey. In Book V Odysseus sails his ship by the stars, using the Pleiades, the Bear, and Boötes ("which set late") to reach his destination.
Arcturus is a type
K1.5 IIIpe orange giant
star — the letters "pe" stand for "peculiar emission," which indicates that the spectrum
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 visually 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 moving rapidly (122 km/s) relative to the solar system, and is now almost at its closest point to the Sun. Closest approach will happen in about 4000 years, when the star will be a few hundredths of a light year closer to Earth than it is today. 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 slightly larger than that of the Sun (1.1-0.4+0.6 solar mass). Arcturus is likely to be considerably older than the Sun, and much like what the Sun will be in its red giant phase.
53 probable members of a group of old disk stars appear to be moving with Arcturus.
Arcturus may be a Population II star and a member of the Milky Way galaxy's thick disk. The thick disk is generally formed of old stars that may lie several thousand light-years (ly) above or below the galactic plane, unlike thin disk stars such as the Sun which typically lie within a thousand ly. Such stars tend to have larger apparent motions, with rapid passages in highly inclined and elliptical orbits around the galactic core. Their movements tend to pierce the galactic disk that the Sun moves within, leading to a large relative motion. As they were born when the galaxy was less enriched or outside metal-rich thin disk areas, such stars also tend to be low in "metals," averaging around 25 percent of the Sun's. Thick disk stars make up only about four percent of all stars in the solar neighborhood
As the star is more massive than Sol, it has evolved faster into a helium-burning "clump" giant
, possibly within five to eight billion years since hydrogen ignition. As a star that has evolved out of the "main sequence
," Arcturus has fully shifted from the fusion of hydrogen to helium in at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. However, Arcturus is somewhat brighter than expected for a stable helium-burning star. Among the heavier elements and molecules detected in its stellar atmosphere have been iron (Fe-0.5) and CN. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass like the Sun).
Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion.
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
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
. For a return trip to Tahiti the navigators could use Sirius
, the zenith star of that island. Since 1976, the Polynesian Voyaging Society
has crossed the Pacific Ocean many times under navigators who have incorporated this wayfinding
technique in their non-instrument navigation.
The Koori people of southeastern Australia knew Arcturus as Marpean-kurrk, and its appearance in the north signified the arrival of larvae of the wood-ant (a food item) in spring. The beginning of summer was marked by the star's setting with the sun in the west and the disappearance of the larvae. The star was also known as the mother of Djuit (Antares
), and another star in Bootes, Weet-kurrk.