Arcturus

Arcturus is the brightest star in the northern celestial hemisphere and fourth brightest in the night sky at magnitude -0.04. It is an orange giant of spectral type K1.5III, located about 37 light-years from Earth in the constellation Boötes. Arcturus is about 25 times the diameter of the Sun and 170 times more luminous. It is a Population II star — older and metal-poor compared to the Sun — suggesting it originally formed in the thick disk or halo of the Milky Way and is now passing through the solar neighborhood.

Easy to find by following the arc of the Big Dipper's handle — 'arc to Arcturus' is one of the most well-known star-hopping mnemonics. Its distinctive deep orange color is immediately apparent to the naked eye and intensifies through binoculars. Arcturus is visible from virtually all inhabited latitudes. Best observed from March through August when Boötes is high in the evening sky.

The name Arcturus comes from the Greek 'Arktos Ouros' meaning 'Guardian of the Bear,' referring to its position near Ursa Major. It was one of the first stars whose proper motion was detected, by Edmond Halley in 1718. In 1933, light from Arcturus was focused onto a photoelectric cell to activate a switch that turned on the floodlights at the Chicago World's Fair — the light had traveled 40 years, having left Arcturus around the time of the previous Chicago fair in 1893.