Theta Capricorni
Binary star in the constellation Capricornus
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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]