What kind of star is Achernar?
Achernar, catalogued as Alpha Eridani (α Eri), appears in the official constellation Eridanus. Its apparent visual magnitude is about 0.46; variable, while its estimated distance is about 139 light-years. Astronomers classify the object as B6 Vep; binary system. 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.
Achernar A rotates near the speed at which equatorial material would become unbound, producing strong oblateness and gravity darkening. During Be phases, hydrogen emission reveals a decretion disc fed by gas expelled from the star; the disc can weaken or disappear, so measurements from different years need not describe the same state. Interferometry and spectroscopy also map Achernar B on a roughly seven-year orbit with eccentricity near 0.73, carrying it from about 2 to 13 astronomical units from the primary. Research finds the orbital and equatorial planes nearly aligned in projection, but the companion does not simply explain every episode of disc formation.
The coordinate pair RA 01h 37m 42.85s, Dec −57° 14′ 12.3″ 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.
Achernar data at a glance
| Proper name | Achernar | Catalogue designation | Alpha Eridani (α Eri) |
|---|---|---|---|
| Constellation | Eridanus | Apparent magnitude | about 0.46; variable |
| Spectral or system type | B6 Vep; binary system | Estimated distance | about 139 light-years |
| Right ascension (J2000) | 01h 37m 42.85s | Declination (J2000) | −57° 14′ 12.3″ |
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 Achernar
From Arabic ākhir al-nahr, “the end of the river,” describing its position at the southern end of Eridanus.
The Arabic name ākhir al-nahr means end of the river, identifying Eridanus’s bright southern terminus. In antiquity the constellation’s end was sometimes assigned farther north because Achernar was invisible from much of the classical Mediterranean. European southern-sky mapping fixed the modern extension. ESO’s Very Large Telescope Interferometer measured its extreme flattening in the early 2000s, making Achernar an iconic demonstration that rapidly rotating stars are not spheres. Continued interferometry later established the orbit and properties of its companion.
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 Achernar
From southern latitudes, trace winding Eridanus away from Orion toward its brilliant endpoint. Northern observers need both a low clear southern horizon and a location below the star’s geographic visibility limit.
Predominantly southern; invisible north of about 33° N and circumpolar south of about 33° S. 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
- From southern latitudes, trace winding Eridanus away from Orion to its brilliant endpoint.
- The star never rises north of roughly 33° N; observers near that limit need an unobstructed southern horizon.
- Compare Achernar with Canopus and nearby redder stars to appreciate its blue-white colour.
- Its flattened surface and compact gaseous disc cannot be seen in an eyepiece; explore them through interferometric observations.
Interesting facts about Achernar
- Achernar is among the ten brightest stars in the night sky.
- Extreme rotation produces a strongly oblate star with a much wider equator than polar diameter.
- It is a Be star: expelled material can form a gaseous equatorial disc and create emission lines.
- Interferometry made Achernar a landmark directly measured example of rotational flattening.
- A detected stellar companion means Achernar is not an isolated star.
Continue from Achernar into the night sky
Use the Eridanus 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.