Rasalgethi — Double Star in Hercules
HIP 84345; Alpha1 Herculis; 64 Herculis
About Rasalgethi
Description
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.
Observing Tips
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.
History
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.
Fun Facts
Rasalgethi is close enough in the sky to Rasalhague (Alpha Ophiuchi) that their names — both Arabic — were intentionally paired in Ptolemy-era astronomy. The primary is one of the largest stars that can be split from its companion in amateur scopes: if placed at the center of our solar system, it would extend beyond the orbit of Mars. Rasalgethi is now losing mass heavily and will likely end its life as a supernova within a few hundred thousand years.
Observe
1Physical Properties
2Position & Identifiers
3How easy to split?
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | Easy | Easy | Easy |
| 150 mm Newton 150mm Newt. | Easy | Easy | Easy |
| Celestron C8 (203 mm SCT) C8 203mm | Easy | Easy | Easy |
Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs
4Visibility
Set a location in User Settings to see visibility data.
5Light Curve
6Comparison Stars for Rasalgethi (3.0–4.0)
Nearby stable stars for estimating brightness (AAVSO)
7Multiple Star System Quadruple C,D: optical
Separation over time
Apparent separation over time, computed from ORB6 orbital elements. Steep curves indicate fast-changing pairs — catch them while they're splittable.
Eyepiece View
A: 3.5 · B: 5.4 · Sep: 4.6″ · PA: 102° · N up, E right
Resolved · Rayleigh: 2.3″ · Dawes: 1.9″ · Eff: 2.3″
Explore
9
Size Comparison
10
Compare Stars
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Spectral Classification
12
Hertzsprung-Russell Diagram
13
Stellar Lifecycle
14
Blackbody Spectrum
15
Stellar Absorption Spectrum
Simulated absorption spectrum based on spectral type. Hover over lines to identify elements.
16
Stellar Fusion
Discover
17Stellar Notes
Nearby in the Sky
Other targets within a few degrees — pan your scope a little and keep exploring.
Visibility scores assume a 150 mm Newton at Bortle 4.
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