Epoch J2000.0 Equinox J2000.0
|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|
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 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. 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).
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.
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