Theta Centauri or θ Centauri, officially named Menkent, is a single star in the southern constellation of Centaurus, the centaur. With an apparent visual magnitude of +2.06, it is the fourth-brightest member of the constellation. Based on parallax measurements obtained during the Hipparcos mission, it is about 58.8abbr=offNaNabbr=off distant from the Sun. It has a relatively high proper motion, traversing the celestial sphere at the rate of . This suggests that Menkent may have originated in the outer disk of the Milky Way and is merely passing through the solar neighborhood.
θ Centauri, Latinised to Theta Centauri, is the star's Bayer designation.
It bore the traditional name of Menkent derived from the Arabic word مَنْكِب (mankib) for "shoulder" (of the Centaur), apparently blended with a shortened form of "kentaurus" (centaur). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[1] to catalog and standardize proper names for stars. The WGSN approved the name Menkent for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.
In Chinese, Chinese: 庫樓 (Chinese: Kù Lóu), meaning arsenal, refers to an asterism consisting of Theta Centauri, Zeta Centauri, Eta Centauri, 2 Centauri, HD 117440, Xi¹ Centauri, Gamma Centauri, Tau Centauri, D Centauri and Sigma Centauri.[2] Consequently, the Chinese name for Theta Centauri itself is Chinese: 庫樓三 (Chinese: Kù Lóu sān, English: the Third Star of Arsenal).[3]
This is an evolved giant star with a stellar classification of K0 III and 1.27 times the mass of the Sun. It is believed to be fusing helium into carbon and heavier elements within its core, qualifying it as a horizontal branch or red clump star. It is a southern analog to Pollux, the brightest star in Gemini and the closest red giant to the Sun. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is, which, at its estimated distance, equates to a physical radius of about 10.6 times the radius of the Sun. The outer envelope has an effective temperature of 4,980 K, giving it the orange-hued glow of a cool, K-type star. Soft X-ray emission has been detected from this star, which has an estimated X-ray luminosity of .