Phi Sagittarii

Phi Sagittarii, Latinized from φ Sagittarii, is an interferometric binary star in the southern constellation of Sagittarius. With an apparent visual magnitude of 3.17, it is the ninth-brightest star in the constellation and is readily visible to the naked eye. Parallax measurements place it at a distance of roughly 239 ly from the Earth. It is receding with a radial velocity of +21.5 km/s.

The stellar classification of this star has been rated at B8.5 III, with the luminosity class of III indicating it is a giant star evolved away from the main sequence after it has exhausted the hydrogen at its core. This energy is being radiated from the star's outer envelope at an effective temperature of $12,487$, which produces the blue-white hue typical of B-type stars.

This star has been catalogued as a spectroscopic binary and a companion was apparently detected through lunar occultation. However, the latter was pointed out as spurious. Interferometric observations taken in 2017 finally revealed that Phi Sgr is indeed a binary by resolving a companion with a near-infrared K band flux ratio of 6% (corresponding to a mass of ) at a separation of $12,487 K$ ($17.7 mas$).

Name and etymology
In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Aoul al Sadirah, which was translated into Latin as Prima τού al Sadirah, meaning first returning ostrich.

In Chinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of φ Sagittarii, λ Sagittarii, μ Sagittarii, σ Sagittarii, τ Sagittarii and ζ Sagittarii. Consequently, the Chinese name for φ Sagittarii itself is 斗宿一 (Dǒu Xiù yī, the First Star of Dipper.)

This star, together with γ Sgr, δ Sgr, ε Sgr, ζ Sgr, λ Sgr, σ Sgr and τ Sgr comprising the Teapot asterism. φ Sgr, σ Sgr, ζ Sgr, χ Sgr and τ Sgr were Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches. According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Naʽām al Ṣādirah or Namalsadirah was originally the title for four stars: φ Sgr as Namalsadirah I, τ Sgr as Namalsadirah II, χ1 Sgr as Namalsadirah III and χ2 Sgr as Namalsadirah IV (except σ Sgr and ζ Sgr).