What kind of star is Rasalhague?
Rasalhague, catalogued as Alpha Ophiuchi A (α Oph A), appears in the official constellation Ophiuchus. Its apparent visual magnitude is about 2.08 combined, while its estimated distance is about 48.6 light-years. Astronomers classify the object as rapidly rotating A5 IV primary with K-type companion. 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.
Interferometric observations resolve Rasalhague A’s rotationally distorted photosphere: the equator bulges outward and is cooler than the poles because effective gravity differs across the surface. This gravity darkening means a single temperature or radius cannot fully describe the star. A lower-mass companion follows an eccentric orbit of roughly eight and a half years; combining astrometry with spectroscopy constrains both stellar masses. The system is relatively nearby, and its A-type primary is evolving away from the main sequence. Its measured appearance also depends on viewing angle, a central issue when interpreting other rapidly rotating stars.
The coordinate pair RA 17h 34m 56.07s, Dec +12° 33′ 36.1″ 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.
Rasalhague data at a glance
| Proper name | Rasalhague | Catalogue designation | Alpha Ophiuchi A (α Oph A) |
|---|---|---|---|
| Constellation | Ophiuchus | Apparent magnitude | about 2.08 combined |
| Spectral or system type | rapidly rotating A5 IV primary with K-type companion | Estimated distance | about 48.6 light-years |
| Right ascension (J2000) | 17h 34m 56.07s | Declination (J2000) | +12° 33′ 36.1″ |
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 Rasalhague
Rasalhague comes from Arabic raʾs al-ḥawwāʾ, “head of the serpent-bearer,” precisely describing its place at the top of Ophiuchus.
Ophiuchus represented a serpent-bearer in the Greco-Roman tradition, while the star’s surviving name is Arabic and simply identifies the figure’s head. Bayer labelled it Alpha Ophiuchi. It later became a key modern interferometric target because its proximity, brightness and fast rotation allow instruments to resolve departures from a spherical photosphere. The IAU standardized Rasalhague for component A, separating the traditional name from the system-level designation.
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 Rasalhague
Look southeast of Hercules and north of the dense Milky Way fields of Scorpius and Sagittarius. Rasalhague is Ophiuchus’s brightest star and forms a broad triangle with Vega and Altair. Its close companion is not a routine visual target.
Visible from most inhabited latitudes; best during northern summer and southern winter 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
- Find the top of Ophiuchus, north of Antares and east of Hercules.
- Use Vega, Altair and Rasalhague as a broad summer orientation triangle, not a formal asterism.
- Observe near culmination because the constellation spans a large north–south range.
- The star remains point-like visually; its oblate shape requires optical interferometry.
Interesting facts about Rasalhague
- Rasalhague is the brightest star in Ophiuchus.
- Rapid rotation makes its primary measurably oblate.
- Gravity darkening leaves the equator cooler than the poles.
- A lower-mass stellar companion orbits on an eccentric path.
- The proper name translates directly as “head of the serpent-bearer.”
Continue from Rasalhague into the night sky
Use the Ophiuchus 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.