Theta Serpentis

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θ Serpentis
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Serpens
Right ascension 18h 56m 13.2s
Declination +04° 12' 13"
Apparent magnitude (V) combined: +4.03
ABC: 4.62 + 4.98 + 6.71
Distance 132 ± 9 ly
(40 ± 3 pc)
Spectral type A5V + A5Vn + G5
Other designations
Alya, 63 Serpentis, HR 7142 + HR 7141, HD 175639 + HD 175638 + HD 175726, SAO 124070 + SAO 124068, HIP 92951 + HIP 92946 + HIP 92984, ADS 11853, CCDM 18563+0413

Theta Serpentis (θ Ser, θ Serpentis) is a star system in the constellation Serpens. It is approximately 132 light years from Earth. Theta Serpentis also has the traditional name Alya, or Alga, from the Arabic الية ’alyah "fat tail (of a sheep)".

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.[1]

In Chinese, 天市左垣 (Tiān Shì Zuǒ Yuán), meaning Left Wall of Heavenly Market Enclosure, refers to an asterism which is represent eleven old states in China, consisting of θ Serpentis, δ Herculis, λ Herculis, μ Herculis, ο Herculis, 112 Herculis, ζ Aquilae, η Serpentis, ν Ophiuchi, ξ Serpentis and η Ophiuchi.[2] Consequently, θ Serpentis itself is known as 天市左垣七 (Tiān Shì Zuǒ Yuán qī, English: the Seventh Star of Left Wall of Heavenly Market Enclosure), represent the state Xu (徐).[3][4]

Both Theta-1 Serpentis and Theta-2 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 astronomical units 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 our Sun. Both star have a surface temperature of 8,000 kelvins.

The third star in the system, 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. ^ 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. 
  2. ^ (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.
  3. ^ (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.
  4. ^ (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.

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