Theta Eridani
 Acamar is the θ star in the lower right of the map |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Eridanus |
| Right ascension | 02h 58m 15.67525s |
| Declination | −40° 18′ 16.8524″ |
| Apparent magnitude (V) | 3.2 |
| Characteristics | |
| Spectral type | A3IV-V |
| U−B color index | 0.14 |
| B−V color index | 0.14 |
| Variable type | suspected |
| Astrometry | |
| Radial velocity (Rv) | 11.9 km/s |
| Proper motion (μ) |
RA: −52.89 mas/yr Dec.: +21.98 mas/yr |
| Parallax (π) | 20.23 ± 0.55mas |
| Distance | 161 ± 4 ly (49 ± 1 pc) |
| Absolute magnitude (MV) | 0.44 |
| Details | |
| θ1 Eri | |
| Mass | 2.6 M☉ |
| Radius | 16.0 R☉ |
| Luminosity | 96 L☉ |
| Temperature | 8,200 K |
| Rotation | 569 days |
| Rotational velocity (v sin i) | 70 km/s |
| θ2 Eri | |
| Mass | 2.4 M☉ |
| Luminosity | 36 L☉ |
| Temperature | 9,200 K |
| Rotational velocity (v sin i) | 90 km/s |
| Other designations | |
| θ1 Eri: HD 18622, HR 897, Acamar | |
| θ2 Eri: HD 18623, HR 898 | |
| Database references | |
| SIMBAD | data |
Theta Eridani (θ Eridani, abbreviated Theta Eri, θ Eri) is a binary system in the constellation of Eridanus. Its two components are designated θ¹ Eridani, also named Acamar, and θ² Eridani. The system's distance from the Sun as measured by the Hipparcos astrometry satellite is approximately 120 light-years.
Theta Eridani is the system's Bayer designation; θ¹ and θ² Eridani those of its two components.
The system bore the traditional name Acamar, derived from the Arabic آخِر النَّهْرĀkhir an-nahr which means "the end of the river", via a Roman-alphabet handwriting misread "rn" to "m". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN approved the name Acamar for θ¹ Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.
The term Ākhir an-nahr or Achr al Nahr appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.
Historically, Acamar represented the end of the constellation Eridanus. Now that distinction is held by the star Achernar, which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North), hence the choice of Acamar as the river's end during the time of Hipparchus and later Ptolemy.
In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of Theta Eridani, Chi Eridani, Phi Eridani, Kappa Eridani, HD 16754, HD 23319, HD 24072, HD 24160, Upsilon4 Eridani, 43 Eridani, Upsilon2 Eridani and Upsilon1 Eridani. Consequently, Theta Eridani itself is known as 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)
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