Albireo — はくちょうの恒星

Albireo, Beta Cygni, is perhaps the most celebrated double star in the northern sky — a dazzling contrast pair of an amber K-type giant and a blue-white B-type dwarf, separated by 34 arcseconds. The primary (Beta Cyg A) is itself a close binary, while the companion (Beta Cyg B) is a single rapidly-rotating star. Gaia parallax measurements published in 2018 suggested the two stars may be an optical, not physical, pair — but more recent analyses argue they share motion and likely do form a gravitationally bound wide system. Together they lie about 430 light-years away.

Albireo is the unchallenged showpiece of summer skies. Any small telescope at 40-80x splits the pair cleanly; the golden primary glows at magnitude 3.1, and the sapphire-blue secondary shines at 5.1. The color contrast is so striking that first-time observers routinely gasp at the eyepiece. Find Albireo at the head of the Northern Cross, at the foot of Cygnus the Swan. Best observed June through November; nearly overhead at temperate latitudes in August.

The name Albireo likely arose from a medieval Latin corruption of the Arabic "al-minhar al-dajajah" (the beak of the hen), filtered through Greek misreadings into "ab ireo" or "ireus." Father Johann Hevelius used the name in his 1687 Prodromus. The color contrast was first celebrated in 19th-century amateur literature, notably by William Henry Smyth in his 1844 "Cycle of Celestial Objects."