Achird — カシオペヤの二重星

Achird, Eta Cassiopeiae, is a nearby G-type main-sequence binary only 19.4 light-years away, making it one of the closest solar analogs visible to the naked eye. The primary is a G0V dwarf very similar to the Sun in mass, temperature, and luminosity. Its K7V companion orbits every 480 years at a present separation of 12 arcseconds. The pair shines at combined magnitude 3.44.

Achird is one of the finest yellow-and-red double stars for small telescopes. Any 3-inch scope at 50x cleanly splits the 12-arcsecond pair. The primary appears sun-yellow at magnitude 3.5, and the companion shows a distinct orange-red tint at magnitude 7.5. Achird lies between Schedar (Alpha Cas) and Cih (Gamma Cas), near the center of Cassiopeia's "W" pattern. Circumpolar from mid-northern latitudes — visible every clear night.

The name Achird first appears in a 17th-century Latin manuscript but its etymology is uncertain — possibly a Hebrew or Persian loan word. The binary nature was discovered by William Herschel in 1779. Achird is sometimes called the "Sun's neighbor" in popular astronomy writing because the primary is so close in properties to the Sun.