Alpha Sagittarii
Alpha Sagittarii (α Sagittarii, abbreviated Alpha Sgr, α Sgr), also named Rukbat , is a star in the constellation of Sagittarius. Properties. Alpha Sagittarii is a blue, class B dwarf star. It does not appear particularly bright in the sky to the naked eye, with a visual apparent magnitude of +3.97. The star has an effective temperature about twice that of the Sun and is nearly three times as massive, with a luminosity in visible wavelengths about 117 times that of the Sun. Based on an excess emission of infrared radiation, it may have a debris disk, much like  Vega. It is a single-lined spectroscopic binary system. The ROSAT All Sky Survey discovered that Alpha Sagittarii is emitting an excess flux of X-rays, which is not expected to originate from a star of this spectral class. The most likely explanation is that the companion is an active pre-main sequence star or else a star that has just reached the main sequence. Nomenclature. "α Sagittarii" (Latinised to "Alpha Sagittarii") is the star's Bayer designation. It is unclear why Bayer designated this star as the alpha, rather than Epsilon Sagittarii or Sigma Sagittarii. This led some old  star charts to occasionally depict Alpha and Beta Sagittarii as much brighter than they are in reality, as they are invisible from northern Europe, being too far south to see there. The star bore the traditional names "Rukbat" and "Alrami", derived from the Arabic "rukbat al-rāmī" 'the knee of the archer'. The star Delta Cassiopeiae also bore the traditional names "Ruchbah" or "Rukbat", from the Arabic word ركبة "rukbah" meaning "knee". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of  July 2016 included a table of the first two batches of names approved by the WGSN; which included "Rukbat" for this star (Delta Cassiopeiae was later given the name "Ruchbah"). In Chinese, (), meaning "Celestial Spring", refers to an asterism consisting of Alpha Sagittarii, Beta¹ Sagittarii and Beta² Sagittarii. Consequently, the Chinese name for Alpha Sagittarii itself is (, .) This star, together with Beta¹ Sagittarii and Beta² Sagittarii, were "Al Ṣuradain" (ألسردين), the two Surad, desert birds.