What kind of star is Rasalgethi?
Rasalgethi, catalogued as Alpha Herculis (α Her; name applies to component Aa), appears in the official constellation Hercules. Its apparent visual magnitude is variable; combined light roughly 2.7–4.0, while its estimated distance is about 360 light-years; uncertain. Astronomers classify the object as M5 Ib–II + spectroscopic/visual companions. These labels describe temperature, luminosity class or system components; they should not be read as a claim that every naked-eye point is a single isolated star.
Alpha Herculis is a hierarchical triple. The dominant cool M-type bright giant or low-luminosity supergiant varies semiregularly as its enormous atmosphere pulsates and loses material. A visually resolvable companion contributes a contrasting yellow-green appearance in many eyepieces; spectroscopy shows that this companion is itself a binary. SIMBAD's parallax gives a rough distance near 360 light-years, but bright, variable, multiple giants challenge precision astrometry. The red primary's temperature is far below the Sun's, yet its greatly expanded surface makes it substantially more luminous. Molecular material and circumstellar gas reveal ongoing mass loss, offering a nearby laboratory for late stellar evolution before the system's most massive member reaches a compact-remnant stage.
The coordinate pair RA 17h 14m 38.86s, Dec +14° 23′ 25.2″ fixes the object's direction on the celestial sphere for the J2000 reference epoch. Right ascension works like celestial longitude measured in hours, while declination works like latitude north or south of the celestial equator. Proper motion and improved measurements can refine catalogue positions, but these coordinates are sufficiently precise for a finder chart or planetarium search.
Rasalgethi data at a glance
| Proper name | Rasalgethi | Catalogue designation | Alpha Herculis (α Her; name applies to component Aa) |
|---|---|---|---|
| Constellation | Hercules | Apparent magnitude | variable; combined light roughly 2.7–4.0 |
| Spectral or system type | M5 Ib–II + spectroscopic/visual companions | Estimated distance | about 360 light-years; uncertain |
| Right ascension (J2000) | 17h 14m 38.86s | Declination (J2000) | +14° 23′ 25.2″ |
Values are rounded for an observer-friendly overview. Consult the linked SIMBAD record for component-level identifiers, bibliography, measurement provenance and newer catalogue values.
The name Rasalgethi
From Arabic raʾs al-jāthī, “head of the kneeler,” describing the older figure represented by Hercules. The IAU applies Rasalgethi to component Aa.
The Arabic name describes Hercules as the Kneeling One, an orientation inherited from much older sky traditions. Bayer assigned Alpha to Rasalgethi even though Beta Herculis is normally brighter. William Herschel catalogued the visual pair, and generations of observers have valued it for colour contrast as much as for orbital study. Its changing red primary also connected classical double-star observing with the later systematic study of semiregular variable stars.
Modern professional catalogues retain identifiers such as the Bayer designation alongside a proper name. The International Astronomical Union standardizes which name belongs to which component—an important distinction for systems whose combined light looks like one star from Earth.
How to find Rasalgethi
Find Hercules between Vega and Arcturus, then look south of the Keystone for the figure's head. A small telescope at 80–150× can separate the warm primary and closer-looking companion when seeing is steady.
Visible from most of Earth and best on northern summer evenings. Visibility on a particular night still depends on latitude, season, time, horizon obstructions, weather and local light pollution. Use the interactive atlas below to check the surrounding field before observing.
Observation checklist
- Best from June through September evenings.
- Use moderate aperture and 100× or more for the double.
- Slight defocus can make the orange colour more obvious.
- Brightness estimates are complicated by combined light from the system.
Interesting facts about Rasalgethi
- Rasalgethi is a hierarchical triple system.
- Its cool primary is semiregularly variable.
- The named component has an extended, mass-losing atmosphere.
- Beta Herculis is usually brighter despite Rasalgethi's Alpha designation.
- It is a classic colour-contrast telescopic double.
Continue from Rasalgethi into the night sky
Use the Hercules constellation guide to place this star within its larger pattern. When you are ready to observe, compare dark-sky places, find a nearby observatory, or follow our practical guide to finding a star in the night sky. Symbolically registered names are kept separately in the searchable Star-Register catalogue.
Sources and further reading
Measurements are rounded for readability. Multiple systems, variable stars and distant supergiants can have component-specific or model-dependent values. Survey panels show photographic sky fields, not the physical surface of the star.