Rasalgethi — ヘルクレスの二重星

Rasalgethi, Alpha Herculis, is a red supergiant semiregular variable of spectral type M5 Ib-II, paired with a close yellow-and-blue-green telescopic companion. It lies approximately 360 light-years away. The primary pulsates irregularly between magnitudes 2.7 and 4.0 over an average 128-day cycle superimposed on a longer 6-year trend. Its radius of roughly 280 solar radii and luminosity near 7,000 Suns rival those of Betelgeuse. The companion, Alpha Her B, is itself a spectroscopic binary with a G-type giant and an A-type dwarf.

Rasalgethi is one of the most beautiful color-contrast doubles within reach of small telescopes. The components lie 4.6 arcseconds apart at magnitudes 3.5 and 5.4 — easily split at 100x in a 3-inch refractor. The primary shows a deep warm orange, and the companion typically looks green or blue-green due to the color contrast. It marks the "head" of Hercules, conveniently close to the border with Ophiuchus. Best observed May through October.

The name comes from the Arabic "Ra's al-Jathi," meaning "the head of the kneeler" — Hercules in old Arabic astronomy was pictured kneeling. The double nature was discovered by William Herschel in 1759, and the variability was first established by Sir William Herschel's son John in 1821. The IAU formally adopted "Rasalgethi" for Alpha Her A in 2016.