Alpha Sagittarii

Star in the constellation of Sagittarius

Alpha Sagittarii

Location of α Sagittarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	19h 23m 53.17483s[1]
Declination	−40° 36′ 57.3705″[1]
Apparent magnitude (V)	+3.97[2]
Characteristics
Spectral type	B8 V[3]
U−B color index	−0.33[2]
B−V color index	−0.10[2]
Distance		182 ± 2 ly (55.7 ± 0.7 pc)
Absolute magnitude (MV)	+0.23[5]
Details
Myr
PPM  353699
Database references
SIMBAD	data

Alpha Sagittarii (α Sagittarii, abbreviated Alpha Sgr, α Sgr), also named Rukbat /ˈrʌkbæt/,^[9][10] is a star in the constellation of Sagittarius.

Properties

Alpha Sagittarii is a blue,

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 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)^[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]).

In

This star, together with Beta¹ Sagittarii and Beta² Sagittarii, were Al Ṣuradain (ألسردين), the two Surad, desert birds.^[15]

In popular culture

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.

References