Arcturus is the brightest star visible in the Northern Hemisphere, and the fourth brightest in the entire night sky, with a magnitude of zero. It and two other extremely bright stars, Vega and Capella, form a triangle dividing the northern sky into thirds. It is a red-orange supergiant star with a diameter 25 times wider than our sun and with four times its mass, and it shines with 180 times the sun's brightness. Its relatively small mass comes from the fact that Arcturus has passed through the hydrogen-fusion phase of its life cycle, and has begun to fuse helium into carbon.
Arcturus is 37 light years away from us, and moving quickly on an extremely large orbit around the centre of the galaxy. This orbit has carried it far out into the halo of the Milky Way, and it has been visible from Earth for only 500,000 years. In approximately 500,000 years more it will be invisible to us again, having charged off on a totally different path to ours. Its current closeness and brightness has made it important to human astronomers and mythologists alike, and the light from Arcturus was used to open the "Century of Progress" World Fair in Chicago in 1933, by tripping the switch for the fair's floodlights.
Arcturus (which means "the bear guardian" in Greek) is found in the constellation of Boötes, the herdsman, and is so called due to its proximity in the sky to Ursa Major and Ursa Minor, the Big and Little Bears (or Big and Little Dippers, if you're from the US). In 1635, it became the first star to be visible through a telescope during the day. Because of its great speed and motion across the sky compared to most other stars, Arcturus was instrumental in suggesting to the great astronomers of the early 18th century (such as Edmond Halley) that the stars were not fixed in relation to the planets, but were in fact free agents moving on their own paths through space. Halley was brought to this conclusion by noting the great difference between the position of Arcturus on ancient and modern maps of the heavens.
Arcturus figures in many world mythologies, including the ancient Babylonian, Vedic, Egyptian, Greek and Arabic cultures, and is mentioned in the Bible's Book of Job, Chapter 9 (King James edition):
8. Which alone spreadeth out the heavens, and treadeth upon the waves of the sea.
9. Which maketh Arcturus, Orion, and Pleiades, and the chambers of the south.
10. Which doeth great things past finding out; yea, and wonders without number.
Arcturus was formed along with the Milky Way itself, and at ten billion years old is probably the most ancient thing easily visible to the naked eye. We know how old it is because its spectrum reveals a lack of heavier elements such as aluminium, iron and silicon, which are shed by dying stars and absorbed into younger ones. It is thought that Arcturus will eventually shed its outer layers when its helium fuel runs out, and collapse into a white dwarf star - however, it is difficult to know for sure, as stars of its size frequently end as supernovas. The helium-burning cycle of any star is considerably shorter than its hydrogen phase, and Arcturus may have less than a billion years of brightness left.
References and Further Reading:
The Book of Job