Theta Eridani
Star system in the constellation Eridanus
Theta Eridani
Location of θ Eridani (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Eridanus
θ1 Eri
Right ascension 02h 58m 15.6764s[1]
Declination −40° 18′ 16.839″[1]
Apparent magnitude (V) 3.18[2]
θ2 Eri
Right ascension 02h 58m 16.4037s[3]
Declination −40° 18′ 16.906″[3]
Apparent magnitude (V) 4.11[2]
Characteristics
θ1 Eri
Evolutionary stage Main sequence[4]
Spectral type A3IV-V[5]
U−B color index +0.14[6]
B−V color index +0.128±0.012[7]
Variable type Rotating ellipsoidal variable[4]
θ2 Eri
Evolutionary stage Main sequence[4]
Spectral type A1V[5]
B−V color index +0.08[6]
Variable type suspected[8]
Astrometry
θ1 Eri
Radial velocity (Rv)+11.9±2.6[7] km/s
Proper motion (μ) RA: −52.273 mas/yr[1]
Dec.: +23.058 mas/yr[1]
Parallax (π)19.9047±0.2062 mas[1]
Distance164 ± 2 ly
(50.2 ± 0.5 pc)
θ2 Eri
Radial velocity (Rv)+7.57±0.33[3] km/s
Proper motion (μ) RA: −50.502 mas/yr[3]
Dec.: +16.613 mas/yr[3]
Parallax (π)19.5270±0.1372 mas[3]
Distance167 ± 1 ly
(51.2 ± 0.4 pc)
Absolute magnitude (MV)−0.59 (combined)[7]
Orbit[4]
Primaryθ1 Eri A
Companionθ1 Eri B
Period (P)4.1077 days
Semi-major axis (a)0.084 au (18.1 R☉)
Details
θ1 Eri A
Mass2.39[4] M☉
Radius3.9[4] R☉
Temperature7,900[4] K
Rotational velocity (v sin i)65.1[9] km/s
θ1 Eri B
Mass2.35[4] M☉
Radius3.6[4] R☉
Temperature8,100[4] K
θ2 Eri
Mass2.30[4] M☉
Radius3.2[4] R☉
Luminosity37[10] L☉
Surface gravity (log g)3.83[11] cgs
Temperature8,300[4] K
Metallicity [Fe/H]−0.197[11] dex
Rotational velocity (v sin i)102.5[12] km/s
Age630[4] Myr
Other designations
Theta Eri, θ Eri, CD−40°771, HIP 13847, SAO 216113, CCDM J02583-4018, WDS 02583-4018
θ1 Eri: Acamar, HD 18622, HR 897
θ2 Eri: HD 18623, HR 898
Database references
SIMBADThe system
θ1 Eri
θ2 Eri

Theta Eridani, Latinized from θ Eridani, is a triple star system in the constellation of Eridanus, with a combined apparent magnitude of 2.88.[7] The primary component has the proper name Acamar /ˈækəmɑːr/,[13] the traditional name of the system.[14] The system's distance based on parallax measurements is 164–167 light-years.

Stellar system

θ Eridani is a visual binary formed by the components θ1 Eridani and θ2 Eridani, alternatively called θ Eridani A and θ Eridani B, respectively. They have individual apparent magnitudes of +3.18 and +4.11,[2] spectral classes of A3IV-V and A1V,[5] and are separated by 8.3" in the sky, corresponding to a projected separation of 425 astronomical units (au).[4]

θ1 is itself a double-lined spectroscopic binary,[15] bringing the number of known stars to three. Its components take 4.1077 days to complete an orbit, with a semi-major axis of 0.084 au (12,600,000 km; 18 R☉). They are so close to each other that their shapes are distorted by tidal forces, and during the orbit the surfaces visible from Earth, and hence the luminosities, vary, making the system a rotating ellipsoidal variable. Both components are near the end of the main sequence. The primary, θ1 A, has 2.39 times the mass and 3.9 times the radius of the Sun, while the secondary, θ1 B, has 2.35 times the mass and 3.6 times the Sun's radius. Their effective temperatures are 7,900 and 8,100 K respectively,[4] giving them a white color typical of A-type stars.[16] As θ1 A evolves and becomes larger than its Roche lobe, mass exchange between the components is expected to occur within a few tens of millions of years.[4]

The spectroscopic binary nature of θ1 was initially uncovered by W. H. Wright in 1905.[17] In 2025, the system was resolved directly using interferometry by the GRAVITY instrument aboard the Very Large Telescope.[4]

θ2 Eridani is also near the end of its main sequence lifetime, having an estimated age of 630 million years, 2.3 times the mass and 3.2 times the Sun's radius. It has an effective temperature of 8,300 K,[4] giving a white color typical of A-type stars.[16] It appears to be a single star itself, and interferometric observations constrain the mass of any main sequence companion between 0.001″–0.2″ to be less than 0.55 solar masses.[4]

Nomenclature

Theta Eridani, Latinized from θ 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 [18] 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)[19] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[20] 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.[13]

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.[21]

Historically, Acamar represented the end of the constellation Eridanus.[22][23] 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),[24] 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.[25] Consequently, the Chinese name for Theta Eridani itself is 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard).[26]

References

  1. ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c Fabricius, C.; Høg, E.; Makarov, V. V.; Mason, B. D.; Wycoff, G. L.; Urban, S. E. (1 March 2002). "The Tycho double star catalogue". Astronomy and Astrophysics. 384: 180–189. Bibcode:2002A&A...384..180F. doi:10.1051/0004-6361:20011822. ISSN 0004-6361.
  3. ^ a b c d e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  4. ^ a b c d e f g h i j k l m n o p q r s t Waisberg, Idel; Klein, Ygal; Katz, Boaz (December 2025). "Hidden Companions to Intermediate-mass Stars. XXXIII. Unveiling a 2.35M⊙, 0.06 au Companion to Acamar = Theta Eridani A". Research Notes of the AAS. 9 (12): 347. doi:10.3847/2515-5172/ae2f5d.
  5. ^ a b c Gray, R. O.; et al. (2006). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample". The Astronomical Journal. 132 (1): 161–170. arXiv:astro-ph/0603770. Bibcode:2006AJ....132..161G. doi:10.1086/504637. S2CID 119476992.
  6. ^ a b Hoffleit; et al. (1991). "Bright Star Catalogue". VizieR (5th Revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
  7. ^ a b c d 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. XHIP record for this object at VizieR.
  8. ^ NSV 01002, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.
  9. ^ Ammler-von Eiff, M.; Reiners, A. (June 2012). "New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars?". Astronomy & Astrophysics. 542: A116. arXiv:1204.2459. Bibcode:2012A&A...542A.116A. doi:10.1051/0004-6361/201118724. ISSN 0004-6361.
  10. ^ Stassun, Keivan G.; et al. (2019-10-01). "The Revised TESS Input Catalog and Candidate Target List". The Astronomical Journal. 158 (4): 138. arXiv:1905.10694. Bibcode:2019AJ....158..138S. doi:10.3847/1538-3881/ab3467. hdl:1721.1/124721. ISSN 0004-6256. S2CID 166227927.
  11. ^ a b Anders, F.; Khalatyan, A.; Chiappini, C.; Queiroz, A. B.; Santiago, B. X.; Jordi, C.; Girardi, L.; Brown, A. G. A.; Matijevič, G.; Monari, G.; Cantat-Gaudin, T.; Weiler, M.; Khan, S.; Miglio, A.; Carrillo, I.; Romero-Gómez, M.; Minchev, I.; de Jong, R. S.; Antoja, T.; Ramos, P.; Steinmetz, M.; Enke, H. (August 2019). "Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18". Astronomy & Astrophysics. 628: A94. arXiv:1904.11302. Bibcode:2019A&A...628A..94A. doi:10.1051/0004-6361/201935765. ISSN 0004-6361.
  12. ^ Díaz, C. G.; et al. (July 2011). "Accurate stellar rotational velocities using the Fourier transform of the cross correlation maximum". Astronomy & Astrophysics. 531: A143. arXiv:1012.4858. Bibcode:2011A&A...531A.143D. doi:10.1051/0004-6361/201016386. S2CID 119286673.
  13. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  14. ^ Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. 48 (283). San Francisco, California: 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681. S2CID 120743052.
  15. ^ Corbally, C. J. (August 1984). "Close visual binaries.I.MK classifications". The Astrophysical Journal Supplement Series. 55: 657–677. Bibcode:1984ApJS...55..657C. doi:10.1086/190973. ISSN 0067-0049.
  16. ^ a b "The Colour of Stars". Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Archived from the original on 2013-12-03. Retrieved 2012-01-16.
  17. ^ Wright, W. H. (May 1905). "A list of twelve stars whose radial velocities vary". The Astrophysical Journal. 21: 371–375. Bibcode:1905ApJ....21..371W. doi:10.1086/141223. ISSN 0004-637X.
  18. ^ "Acamar". All Skies Encyclopaedia. IAU Working Group on Star Names. Retrieved 23 December 2025.
  19. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  20. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  21. ^ 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.
  22. ^ Kaler, James B. "ACAMAR (Theta Eridani)". Stars. University of Illinois. Retrieved 2010-02-24.
  23. ^ 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.
  24. ^ Larry Sessions. "Achernar: End of the River". EarthSky Tonight Post 06-29-2009. Retrieved 2010-02-26.
  25. ^ (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.
  26. ^ (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.
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