Theta Serpentis

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θ Serpentis
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Serpens
Right ascension  18h 56m 13.18s[1]
Declination +04° 12′ 12.9″[1]
Apparent magnitude (V) +4.03 (4.62[2] + 4.98[2] + 6.71[3])
Spectral type A5V[4] + A5Vn[4] + G1V[5]
θ¹ Ser
Parallax (π)21.09 ± 2.86[6] mas
Distanceapprox. 150 ly
(approx. 47 pc)
θ² Ser
Parallax (π)19.25 ± 3.57[6] mas
Distanceapprox. 170 ly
(approx. 52 pc)
θ Ser C
Parallax (π)37.73 ± 0.51[6] mas
Distance86 ± 1 ly
(26.5 ± 0.4 pc)
Absolute magnitude (MV)+4.60[7]
θ Ser C
Mass1.097 M
Radius0.9870 R
Luminosity1.1817 L
Temperature6,067 K
Metallicity [Fe/H]−0.09 dex
Age200 Myr
Other designations
63 Serpentis, ADS 11853, CCDM 18563+0413, WDS 18562+0412
θ¹ Ser: Alya, θ Ser A, HR 7141, HD 175638, SAO 124068, HIP 92946, WDS 18562+0412A
θ² Ser: θ Ser B, HR 7142, HD 175639, SAO 124070, HIP 92951, WDS 18562+0412B
θ Ser C: HD 175726, HIP 92984, WDS 18562+0412C
Database references
θ¹ Ser
θ² Ser
θ Ser C

Theta Serpentis (θ Serpentis, abbreviated Theta Ser, θ Ser) is a triple star system in the constellation of Serpens.

It consists of a binary pair designated Theta Serpentis AB and whose two components are designated Theta¹ Serpentis or Theta Serpentis A (officially named Alya /ˈæliə/, the traditional name for the entire system)[8][9] and Theta² Serpentis or Theta Serpentis B, together with a third visual companion Theta Serpentis C.[10]

Based upon parallax measurements obtained during the Hipparcos mission, θ Serpentis AB is approximately 160 light-years, and θ Serpentis C approximately 86 light-years, from the Sun.


θ Serpentis (Latinised to Theta Serpentis) is the system's Bayer designation; θ¹ and θ² Serpentis those of the brightest two components. The designations of the two constituents as Theta Serpentis AB and C, and those of AB's components - Theta Serpentis A and B - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[11]

The system bore the traditional name Alya, or Alga, from the Arabic الية ’alyah "fat tail (of a sheep)".[12] In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[14] It approved the name Alya for the component Theta Serpentis A on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[9]

In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Dzaneb al Haiyet, which was translated into Latin as Cauda Serpentis, meaning 'the serpent's tail'.[15]

In Chinese, 天市左垣 (Tiān Shì Zuǒ Yuán), meaning Left Wall of Heavenly Market Enclosure, refers to an asterism which represents eleven old states in China, consisting of Theta Serpentis, Delta Herculis, Lambda Herculis, Mu Herculis, Omicron Herculis, 112 Herculis, Zeta Aquilae, Eta Serpentis, Nu Ophiuchi, Xi Serpentis and Eta Ophiuchi.[16] Consequently, the Chinese name for Theta Serpentis itself is 天市左垣七 (Tiān Shì Zuǒ Yuán qī, English: the Seventh Star of Left Wall of Heavenly Market Enclosure), representing the state Xu (徐).[17][18]


Both Theta¹ Serpentis and Theta² Serpentis are white A-type main sequence dwarfs. θ¹ has an apparent magnitude of +4.62 while the slightly dimmer θ² has a magnitude of +4.98. These two stars are 22 arcseconds apart on the sky, putting them at least 900 AU apart with an orbital period of at least 14,000 years. Both stars are similar to each other in all respects, having luminosities of 18 and 13 times solar respectively, radii of about twice solar and also masses of roughly 2 times that of the Sun. Both stars have a surface temperature of 8,000 kelvins.

Theta Serpentis C is a yellow G-type star with an apparent magnitude of +6.71. It is separated by 7 arcminutes from θ².


  1. ^ a b Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  2. ^ a b Hoffleit, Dorrit; Jaschek, Carlos (1991). "The Bright star catalogue". New Haven.
  3. ^ Høg, E.; et al. (2000), "The Tycho-2 catalogue of the 2.5 million brightest stars", Astronomy and Astrophysics, 355: L27, Bibcode:2000A&A...355L..27H, doi:10.1888/0333750888/2862, ISBN 0333750888
  4. ^ a b Abt, Helmut A.; Morrell, Nidia I. (1995). "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars". Astrophysical Journal Supplement. 99: 135. Bibcode:1995ApJS...99..135A. doi:10.1086/192182.
  5. ^ Houk, N.; Swift, C. (1999). "Michigan catalogue of two-dimensional spectral types for the HD Stars, Vol. 5". Michigan Spectral Survey. 05: 0. Bibcode:1999MSS...C05....0H.
  6. ^ a b c 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.
  7. ^ a b Boyajian, Tabetha S.; et al. (July 2013). "Stellar Diameters and Temperatures. III. Main-sequence A, F, G, and K Stars: Additional High-precision Measurements and Empirical Relations". The Astrophysical Journal. 771 (1): 31. arXiv:1306.2974. Bibcode:2013ApJ...771...40B. doi:10.1088/0004-637X/771/1/40. 40. See Table 3.
  8. ^ Davis, George A. (1944). "The pronunciations, derivations, and meanings of a selected list of star names". Popular Astronomy. 52: 8–30.
  9. ^ a b "Naming Stars". Retrieved 6 March 2018.
  10. ^ "Washington Double Star Catalog". United States Naval Observatory. Archived from the original on 14 February 2011. Retrieved 6 March 2018.
  11. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". 1012: arXiv:1012.0707. arXiv:1012.0707. Bibcode:2010arXiv1012.0707H. Cite journal requires |journal= (help)
  12. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  13. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  14. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  15. ^ 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 (8): 429. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  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 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. ^ (in Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name Archived 2010-08-10 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.

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