Alpha Sagittarii is a blue, class Bdwarf 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.[8] 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.[11]
Nomenclature
α Sagittarii (Latinised to Alpha Sagittarii) is the star's Bayer designation. It is unclear why Bayer designated this star as the alpha in his Uranometria star atlas (placing it in the second magnitude class), rather than Kaus Australis or Nunki, which are at least five times brighter than α Sgr. 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 Arabicrukbat 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)[12] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[13] 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[10]).
A fictionalized version of the Rukbat system is the setting for Anne McCaffrey's Dragonriders of Pern series of novels. In the canon of this series, the system has five planets in standard orbits, two asteroid belts, an Oort cloud, and has also captured a rogue planet into a highly eccentric orbit which passes both through the Oort cloud and through the inner solar system on a 250 year orbit.
^ abcJohnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
^Buscombe, W. (1962), "Spectral classification of Southern fundamental stars", Mount Stromlo Observatory Mimeogram, 4: 1, Bibcode:1962MtSOM...4....1B
^Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick (eds.), "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and Their Applications, Proceedings from IAU Symposium No. 30, 30, University of Toronto: International Astronomical Union: 57, Bibcode:1967IAUS...30...57E
^ abcdeDavid, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID33401607.
^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
