Epoch J2000 Equinox J2000
|Right ascension||19h 23m 53.17483s|
|Declination||−40° 36′ 57.3705″|
|Apparent magnitude (V)||+3.97|
|Spectral type||B8 V|
|U−B color index||−0.33|
|B−V color index||−0.10|
|Radial velocity (Rv)||−0.7 km/s|
|Proper motion (μ)||RA: +30.49 mas/yr
Dec.: −119.21 mas/yr
|Parallax (π)||17.94 ± 0.22 mas|
|Distance||182 ± 2 ly
(55.7 ± 0.7 pc)
|Absolute magnitude (MV)||+0.23|
|Radius||2.49 R☉|
|Surface gravity (log g)||4.11 cgs|
|Metallicity [Fe/H]||−0.02 dex|
|Rotational velocity (v sin i)||71 km/s|
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. However, this is due to its distance; in reality, the star is more than double the effective temperature of the Sun and nearly three times as massive, with a luminosity in visible wavelengths about 117 times greater than that of the Sun. Based on an excess emission of infrared radiation, it may have a debris disk, much like Vega.
This 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.
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, 天淵 (Tiān Yuān), meaning Celestial Spring, refers to an asterism consisting of Alpha Sagittarii, Beta¹ Sagittarii and Beta² Sagittarii. Consequently, Alpha Sagittarii itself is known as 天淵三 (Tiān Yuān sān, English: the Third Star of Celestial Spring.)
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