Achernar (Alpha Eridani)
Coordinates, Hipparcos distance (139 ly), magnitude 0.46, extreme 1.35:1 equatorial flattening, and observing guide for Achernar in Eridanus.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Apparent visual magnitude (V) | 0.46 mag | ± 0.03 mag | Johnson UBV photometry (varies between 0.40 and 0.49 during Be disk outbursts) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 23.39 mas | ± 0.57 mas | Hipparcos New Reduction astrometric solution (42.75 ± 1.04 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 139.4 ly | ± 3.4 ly | Derived from parallax 23.39 ± 0.57 mas | definitive | Hipparcos New Reduction Catalog |
| Equatorial radius (oblate photosphere) | 9.16 R☉ | ± 0.23 R☉ | VLTI/VINCI & PIONIER interferometry (polar radius = 6.78 ± 0.17 R☉) | definitive | SIMBAD Astronomical Database (CDS) |
| Primary stellar mass (Achernar A) | 6.1 M☉ | ± 0.6 M☉ | VLTI interferometric 7.04-year orbit of companion Achernar B (Kervella et al. 2022) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Achernar (Alpha Eridani Oblate Be Star) at RA 01h 37m 42.85s, Dec -57° 14′ 12.3″ within Eridanus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Achernar (Alpha Eridani, from the Arabic Ākhir al-Nahr meaning 'the End of the River') is the ninth-brightest star in the night sky at magnitude 0.46 and the brightest and hottest star among the ten brightest stars in Earth's sky after Rigel. Situated 42.75 ± 1.04 parsecs (139.4 light-years) away at the southern terminus of the winding constellation Eridanus, Achernar is a 6.1-solar-mass B6 Vep main-sequence star shining with roughly 3,150 solar luminosities.
In 2003 astronomers using the Very Large Telescope Interferometer (VLTI) in Chile made the astonishing discovery that Achernar is the least spherical star studied in the Milky Way: spinning at an equatorial velocity of ~298 km/s (over 96% of its critical centrifugal breakup speed), its equatorial radius bulges out to 9.16 solar radii while its polar radius is only 6.78 solar radii—an equatorial-to-polar ratio of 1.35 to 1! Centrifugal decompression cools its equator to 12,600 K while its high-gravity poles blaze at 17,100 K, sporadically slinging ionized gas into an equatorial Be decretion disk.
Adaptive-optics and VLTI infrared imaging also discovered a physical companion, Achernar B—an ~2.0-solar-mass A1–A3 main-sequence star orbiting Achernar A every 7.04 years along an eccentric path (mean separation ~6.7 AU, passing within 4.2 AU at periastron).
Practical Visual Observing Guide
- Lying at declination −57°14′, Achernar never rises north of +32°46′N latitude (making it invisible from most of the United States and Europe except southern Texas, southern Florida, and North Africa).
- For Southern Hemisphere observers, Achernar is circumpolar south of −33°S and shines as a solitary, piercing blue-white beacon midway between Fomalhaut and the Small Magellanic Cloud.
Catalog Cross-Identifications & Aliases
Common Aliases: α Eridani · HD 10144 · HIP 7588
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 7588 |
| HD | HD 10144 |
| SIMBAD | * alf Eri |
| WIKIDATA | Q12185 |
Related Objects, Comparisons & Guides
Vega (Alpha Lyrae Zero-Point Star)
Coordinates, distance (25.04 ly), magnitude 0.03 photometric zero point, rapid oblate rotation, and observing guide for Vega in Lyra.
Altair (Alpha Aquilae Rapid Rotator)
Coordinates, Hipparcos distance (16.73 ly), magnitude 0.76, 8.9-hour oblate spin imaged by CHARA, and observing guide for Altair in Aquila.
Fomalhaut (Alpha Piscis Austrini Debris Disk)
Coordinates, Hipparcos distance (25.13 ly), magnitude 1.16, JWST triple debris-belt system, and observing guide for Fomalhaut.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Scientific Sources & Dataset Provenance
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg