Theta Capricorni

Theta Capricorni
Location of θ Capricorni (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Capricornus[1]
Right ascension 21h 05m 56.82783s[2]
Declination −17° 13′ 58.3021″[2]
Apparent magnitude (V) +4.07[3]
Characteristics
Evolutionary stage main sequence[4]
Spectral type A1 V[5]
U−B color index +0.01[3]
B−V color index −0.01[3]
Astrometry
Radial velocity (Rv)−10.9[6] km/s
Proper motion (μ) RA: +79.33[2] mas/yr
Dec.: −62.01[2] mas/yr
Parallax (π)20.11±0.28 mas[2]
Distance162 ± 2 ly
(49.7 ± 0.7 pc)
Absolute magnitude (MV)+0.60[1]
Orbit[7]
PrimaryA
NameB
Period (P)143 days
Semi-major axis (a)0.78 AU
Details[7]
A
Mass2.54 M☉
Radius2.35 R☉
Luminosity65[8] L☉
Surface gravity (log g)4.19±0.14[9] cgs
Temperature10,221 K
Metallicity [Fe/H]+0.26[1] dex
Rotational velocity (v sin i)104[9] km/s
Age280 Myr
B
Mass0.56 M☉
Radius0.52 R☉
Temperature3,900 K
Age280 Myr
Other designations
Udang, Dorsum, θ Cap, 23 Cap, BD−17°6174, FK5 1552, HD 200761, HIP 104139, HR 8075, SAO 164132[10]
Database references
SIMBADdata

Theta Capricorni, Latinized from θ Capricorni, (Theta Cap or θ Cap) formally named Udang,[11] is a binary star system in the southern constellation of Capricornus,[12] positioned 0.58° south of the ecliptic.[13] It can be seen with the naked eye, having an apparent visual magnitude of +4.07.[3] Based upon an annual parallax shift of 20.11 mas as seen from the Earth,[2] the star is about 162 light-years (50 pc) from the Sun. It is drifting closer with a radial velocity of −11 km/s.[6]

The star or star system is almost eclipsed by the Sun on about 3 February, when it will figure behind the Sun's corona if there is a full solar eclipse.[14] Thus the star can be viewed the whole night, crossing the sky, in early August (in the current epoch). It can be occulted by the Moon.[15]

Nomenclature

Theta Capricorni is the star's Bayer designation, which is Latinized from θ Capricorni and abbreviated Theta Cap or θ Cap. Sometimes (primarily in astrological sources), this star is called by the name Dorsum,[16] meaning the back (of the goat) in Latin.

Udang, the Shrimp, is a constellation from Bali (Indonesia), identified with Capricornus. It is anchored in the cultural calendar called Palelintangan which is attested for ~1300 years.[17] The IAU Working Group on Star Names approved the name Udang for Theta Capricorni A on 22 February 2026.[11]

In Chinese, 十二國 (Shíer Guó), meaning Twelve States, refers to an asterism which represents twelve ancient states in the Spring and Autumn period and the Warring States period, consisting of θ Capricorni, φ Capricorni, ι Capricorni, 38 Capricorni, 35 Capricorni, 36 Capricorni, χ Capricorni, 30 Capricorni, 33 Capricorni, ζ Capricorni, 19 Capricorni, 26 Capricorni, 27 Capricorni, 20 Capricorni, η Capricorni and 21 Capricorni.[18] Consequently, the Chinese name for θ Capricorni itself is 秦一 (Qin yī, English: the First Star of Qin), meaning that this star (together with 30 Capricorni[19]) and δ Serpentis in Right Wall of Heavenly Market Enclosure (asterism)[20] represents the state Qin (秦) (or Tsin).[20]

Characteristics

Radial velocity variations indicated it may be a binary star system,[21] but when the system was examined in the infrared, no companion was detected.[22] However, a companion was subsequently confirmed in 2023 by direct observations by an interferometer.[7]

The main component of this system is an A-type main sequence star with a stellar classification of A1 V.[5] Theta Capricorni has an estimated 2.54 times the mass of the Sun and around 2.35 times the Sun's radius. It is 280 million years old[7] and is spinning fairly rapidly with a projected rotational velocity of 104 km/s.[9] The star is radiating 65[8] times the solar luminosity from its photosphere at an effective temperature of around 10,000 K.[7]

The secondary component does not have a known stellar classification, but is much smaller and cooler than its primary, with about 50% the mass and radius of the Sun, and a temperature of 3,900 K. It takes 140 days (4.6 months) to complete an orbit around the barycenter, and is separated from its primary by 0.78 astronomical units.[7]

References

  1. ^ a b c 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.
  2. ^ a b c d e f van Leeuwen, F. (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, S2CID 18759600.
  3. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J.
  4. ^ 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.
  5. ^ a b Cowley, A.; et al. (April 1969), "A study of the bright A stars. I. A catalogue of spectral classifications", Astronomical Journal, 74: 375–406, Bibcode:1969AJ.....74..375C, doi:10.1086/110819.
  6. ^ a b Wilson, R. E. (1953), "General Catalogue of Stellar Radial Velocities", Carnegie Institute Washington D.C. Publication, Carnegie Institution for Science, Bibcode:1953GCRV..C......0W, ISBN 9780598216885, LCCN 54001336.{{citation}}: CS1 maint: ignored ISBN errors (link) CS1 maint: work parameter with ISBN (link)
  7. ^ a b c d e f Waisberg, Idel; et al. (June 2023), "Binarity and beyond in A stars – I. Survey description and first results of VLTI/GRAVITY observations of VAST targets with high Gaia–Hipparcos accelerations", Monthly Notices of the Royal Astronomical Society, 521 (4): 5232–5254, arXiv:2206.05251, Bibcode:2023MNRAS.521.5232W, doi:10.1093/mnras/stad872, ISSN 0035-8711.
  8. ^ a b McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x, S2CID 118665352.
  9. ^ a b c David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID 33401607.
  10. ^ "tet Cap", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2025-08-04.
  11. ^ a b "IAU Catalog of Star Names". Retrieved 22 February 2026.
  12. ^ Kaler, James B. (September 12, 2008), "Theta Capricorni", Stars, University of Illinois, retrieved 2017-05-09.
  13. ^ Pratt, John P. (February 10, 2016), "Zodiac Stars", John Pratt's Web Site, retrieved 2021-06-21.
  14. ^ In the Sky Earth astronomy reference utility showing the ecliptic and relevant date as at J2000 - present
  15. ^ Eitter, J. J.; Beavers, W. I. (November 1974), "Lunar Occultation Summary. I", Astrophysical Journal Supplement, 28: 405, Bibcode:1974ApJS...28..405E, doi:10.1086/190326
  16. ^ "Dorsum", www.constellationsofwords.com, retrieved 2017-05-16.
  17. ^ "Udang". All Skies Encyclopaedia. IAU Working Group on Star Names. Retrieved 22 February 2026.
  18. ^ (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.
  19. ^ Ridpath, Ian, "Capricornus the Sea Goat", Ian Ridpath's Startales, retrieved 2025-08-04.
  20. ^ a b Allen, Richard Hinckley (1963), "Capricornus", Star Names, Their Lore and Meaning, Dover, retrieved 2017-05-09.
  21. ^ Lagrange, A.-M.; et al. (February 2009), "Extrasolar planets and brown dwarfs around A-F type stars. VI. High precision RV survey of early type dwarfs with HARPS", Astronomy and Astrophysics, 495 (1): 335–352, arXiv:0809.4636, Bibcode:2009A&A...495..335L, doi:10.1051/0004-6361:200810105, S2CID 62894956.
  22. ^ Ehrenreich, D.; et al. (November 2010), "Deep infrared imaging of close companions to austral A- and F-type stars", Astronomy and Astrophysics, 523: A73, arXiv:1007.0002, Bibcode:2010A&A...523A..73E, doi:10.1051/0004-6361/201014763, S2CID 54913363.
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