Achernar
Achernar is the brightest star in the constellation of Eridanus and the ninth-brightest in the night sky. It has the Bayer designation Alpha Eridani, which is Latinized from α Eridani and abbreviated Alpha Eri or α Eri. The name Achernar applies to the primary component of a binary system. The two components are designated Alpha Eridani A (the primary) and B (the secondary), with the latter known informally as Achernar B. As determined by the Hipparcos astrometry satellite, this system is located at a distance of approximately 139 light-years (43 parsecs) from the Sun.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Eridanus |
| Pronunciation | /ˈeɪkərnɑːr/ |
| Right ascension | 01h 37m 42.84548s |
| Declination | –57° 14′ 12.3101″ |
| Apparent magnitude (V) | 0.40–0.46 |
| Characteristics | |
| Spectral type | B3 Vpe + A1V-A3V |
| U−B color index | −0.66 |
| B−V color index | −0.16 |
| Variable type | Be |
| Astrometry | |
| Radial velocity (Rv) | +16 km/s |
| Proper motion (μ) | RA: 87.00±0.58 mas/yr Dec.: −38.24±0.50 mas/yr |
| Parallax (π) | 23.39 ± 0.57 mas |
| Distance | 139 ± 3 ly (43 ± 1 pc) |
| Absolute magnitude (MV) | −1.46 |
| Orbit | |
| Period (P) | 7.0389±0.0015 yr |
| Semi-major axis (a) | 0.17190±0.00025" (7.35±0.18 AU) |
| Eccentricity (e) | 0.7258±0.0015 |
| Inclination (i) | 30.32±0.35° |
| Longitude of the node (Ω) | 310.91±0.80° |
| Periastron epoch (T) | 2008.7582±0.0014 |
| Argument of periastron (ω) (secondary) | 172.05±0.87° |
| Details | |
| A | |
| Mass | 6.0 M☉ |
| Radius | 6.78 – 9.16 R☉ |
| Luminosity | 3,493 L☉ |
| Surface gravity (log g) | 2.772 – 3.561 cgs |
| Temperature | 12,673 – 17,124 K |
| Rotational velocity (v sin i) | 250 km/s |
| Age | 63 Myr |
| B | |
| Mass | 2.0 M☉ |
| Radius | 1.7 R☉ |
| Luminosity | 17.5 L☉ |
| Temperature | 9,064 K |
| Other designations | |
| Database references | |
| SIMBAD | data |
Of the ten brightest stars in the night-time sky by apparent magnitude, Alpha Eridani is the hottest and bluest in color because it is spectral type B. Achernar has an unusually rapid rotational velocity, causing it to become oblate in shape. The secondary is smaller, is spectral type A, and orbits Achernar at a distance of 7.35 AU.