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[1][2] |
| Declination | −40° 18′ 16.8524″[1][2] |
| Apparent magnitude (V) | 3.2[3] |
| Characteristics | |
| Spectral type | A3IV-V[3] |
| U−B color index | 0.14[4] |
| B−V color index | 0.14[4] |
| Variable type | suspected |
| Astrometry | |
| Radial velocity (Rv) | 11.9 [3] km/s |
| Proper motion (μ) | RA: −52.89[1][2] mas/yr Dec.: +21.98[1][2] mas/yr |
| Parallax (π) | 20.23 ± 0.55[1][2] mas |
| Distance | 161 ± 4 ly (49 ± 1 pc) |
| Absolute magnitude (MV) | 0.44 |
| Details | |
| θ1 Eri | |
| Mass | 2.6[5] M☉ |
| Radius | 16.0[6] R☉ |
| Luminosity | 96[5] L☉ |
| Temperature | 8,200[5] K |
| Rotation | 569 days[6] |
| Rotational velocity (v sin i) | 70[3] km/s |
| θ2 Eri | |
| Mass | 2.4[5] M☉ |
| Luminosity | 36[5] L☉ |
| Temperature | 9,200[5] K |
| Rotational velocity (v sin i) | 90[3] km/s |
| Other designations | |
| Database references | |
| SIMBAD | data |
Theta Eridani (θ Eri, θ Eridani) is a star in the constellation Eridanus. It has the traditional name Acamar from the Arabic آخِر النَّهْر[citation needed] Ākhir an-nahr which means "the end of the river". Historically, Acamar represented the end of the constellation Eridanus.[5][7] Now that distinction is held by the star Achernar, a star which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North),[8] hence the choice of Acamar as the river's end during the time of Hipparchus and later Ptolemy.
The term Ākhir an-nahr or Achr al Nahr was appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.[9]
In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of θ Eridani, χ Eridani, φ Eridani, κ Eridani, HD 16754, HD 23319, HD 24072, HD 24160, υ4 Eridani, 43 Eridani, υ2 Eridani and υ1 Eridani.[10] Consequently, θ Eridani itself is known as 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)[11]
Acamar is a double star with some evidence suggesting it is part of a multiple star system.[12][13] The main star, θ1 Eri, is of the spectral class A4 and has a +3.2 apparent magnitude. Its companion star, θ2 Eri, is of the spectral class A1 and has an apparent magnitude of +4.3. The angular separation of the two stars is equal to 8.3 arcseconds.[5] Acamar's distance from Earth as measured by the Hipparcos astrometry satellite is approximately 120 light-years.[1][2][14]
Ptolemy described Acamar as a first-magnitude star,[15] a fact which is all the more enigmatic since the brighter star Achernar can be found just above the horizon in Alexandria where Ptolemy lived and would have been easily visible to merchants trading farther up the Nile.[7][16] Al Sufi in the 10th century listed it as one of the thirteen brightest stars in the sky, and Ulugh Beg from Persia in the 15th century also listed it as a first-magnitude star.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode:1997A&A...323L..49P
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- ↑ 3.0 3.1 3.2 3.3 3.4 "ACAMAR -- Star in double system". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
- ↑ 4.0 4.1 "Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Kaler, James B. "ACAMAR (Theta Eridani)". Stars. University of Illinois. Retrieved 2010-02-24.
- ↑ 6.0 6.1 Setiawan, J. et al. (July 2004), "Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch", Astronomy and Astrophysics 421: 241–254, Bibcode:2004A&A...421..241S, doi:10.1051/0004-6361:20041042-1
- ↑ 7.0 7.1 Rogers, J. H. (1998). "Origins of the ancient constellations: II. The Mediterranean traditions". Journal of the British Astronomical Association, 108 (2,): p.79–89. Bibcode:1998JBAA..108...79R.
- ↑ Larry Sessions. "Achernar: End of the River". EarthSky Tonight Post 06-29-2009. Retrieved 2010-02-26.
- ↑ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society 55: 429. Bibcode:1895MNRAS..55..429K.
- ↑ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- ↑ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
- ↑ NSV 01002, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.
- ↑ "Acamar". Alcyone Bright Star Catalogue. Retrieved 2010-02-26.
- ↑ Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5
- ↑ Richard Hinckley Allen (1963). "Star Names: Their Lore and Meaning". Dover Publications. pp. 215–220. Retrieved 2010-06-11. "It is the solitary star visible from the latitude of New York City in early winter evenings, low down in the south, on the meridian with Menkar of the Whale; but Baily said that its brilliancy has probably lessened since Ptolemy's time, for the latter designated it by α — i.e. of the 1st magnitude."
- ↑ Arnold, H. J. P.; Doherty, P.; Moore, P.; (1997). The Photographic Atlas of the Stars. Waukesha, WI 53187-1612, USA: Kalmbach Publishing Company. p. 48. ISBN 978-0-7503-0378-1.
External links
Jim Kaler's Stars: Acamar
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Coordinates: 
02h 58m 15.70s, +40° 18′ 17.0″