Theta Eridani

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Theta Eridani
Eridanus constellation map.png
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]
Spectral type A3IV-V[3]
U−B color index 0.14[4]
B−V color index 0.14[4]
Variable type suspected
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
Distance161 ± 4 ly
(49 ± 1 pc)
Absolute magnitude (MV)−0.59[5]
θ1 Eri
Mass2.6[6] M
Radius16.0[7] R
Luminosity145[5] L
Temperature8,200[6] K
Rotation569 days[7]
Rotational velocity (v sin i)70[3] km/s
θ2 Eri
Mass2.4[6] M
Luminosity36[6] L
Temperature9,200[6] K
Rotational velocity (v sin i)90[3] km/s
Other designations
HIP 13847, CCDM J02583-4018, CD-40° 771, SAO 216113, WDS 02583-4018
θ1 Eri: HD 18622, HR 897, Acamar
θ2 Eri: HD 18623, HR 898
Database references

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, formally named Acamar /ˈækəmɑːr/ (the traditional name of the system),[8][9] and θ² Eridani. The system's distance from the Sun as measured by the Hipparcos astrometry satellite is approximately 120 light-years.[1][2][10]


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 النَّهْر[citation needed] Ākhir 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)[11] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[12] It approved the name Acamar for θ¹ Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.[9]

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.[13]

Historically, Acamar represented the end of the constellation Eridanus.[6][14] 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),[15] 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, Upsilon3 Eridani, Upsilon2 Eridani and Upsilon1 Eridani.[16] Consequently, the Chinese name for Theta Eridani itself is 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)[17]

Stellar system[edit]

Theta Eridani is a binary system with some evidence suggesting it is part of a multiple star system.[18][19] The main star, θ¹ Eridani, is of spectral class A4 with a +3.2 apparent magnitude. Its companion star, θ² Eridani, is of spectral class A1 with an apparent magnitude of +4.3. The angular separation of the two stars is equal to 8.3 arcseconds.[6]

Ptolemy described Theta Eridani as a first-magnitude star, which some have taken to be in fact Achernar, which today appears just above the horizon in Alexandria where Ptolemy lived. Achernar, however, was not visible to Ptolemy (it is from Alexandria today due to precession); its declination in 100CE was -67, making it invisible even at Aswan. Ptolemy's Eridanus thus without doubt ended at Theta Eridani, which rose about 10 degrees above the horizon from Alexandria. This fact besides positively identifying Theta Eridani as the original "end of the river", also strengthens the case for it having been first magnitude in ancient times.[citation needed]


  1. ^ a b c d e f 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. ^ a b c d e f 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. ^ a b c d e "ACAMAR -- Star in double system". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
  4. ^ a b Hoffleit; et al. (1991). "Bright Star Catalogue". VizieR (5th Revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
  5. ^ a b Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  6. ^ a b c d e f g Kaler, James B. "ACAMAR (Theta Eridani)". Stars. University of Illinois. Retrieved 2010-02-24.
  7. ^ a b 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
  8. ^ Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. San Francisco, California. 48 (283).
  9. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  10. ^ Perryman, Michael (2010), The Making of History's Greatest Star Map, Astronomers' Universe, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  11. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  12. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  13. ^ 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. doi:10.1093/mnras/55.8.429.
  14. ^ Rogers, J. H. (1998). "Origins of the ancient constellations: II. The Mediterranean traditions". Journal of the British Astronomical Association. 108 (2): 79–89. Bibcode:1998JBAA..108...79R.
  15. ^ Larry Sessions. "Achernar: End of the River". EarthSky Tonight Post 06-29-2009. Retrieved 2010-02-26.
  16. ^ (in 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.
  17. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2010-09-03 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. ^ NSV 01002, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.
  19. ^ "Acamar". Alcyone Bright Star Catalogue. Retrieved 2010-02-26.

External links[edit]

Jim Kaler's Stars: Acamar

Coordinates: Sky map 02h 58m 15.70s, −40° 18′ 17.0″