21 Sagittarii is a binary star[3] system in the southern zodiac constellation of Sagittarius. It is visible to the naked eye as a faint point of light with a combined apparent visual magnitude of 4.81.[2] The system is located approximately 410 light years away from the Sun based on parallax.[1] It is moving closer to the Earth with a heliocentric radial velocity of −11.80 km/s.[6]

21 Sagittarii

Location of 21 Sagittarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18h 25m 21.04075s[1]
Declination	−20° 32′ 30.0385″[1]
Apparent magnitude (V)	4.81[2] (5.03 + 7.43)[3]
Characteristics
Spectral type	K2II + A5:[4][3]
U−B color index	+0.93[5]
B−V color index	+1.30[5]
Astrometry
Radial velocity (Rv)	−11.80[6] km/s
Proper motion (μ)	RA: +8.46[1] mas/yr Dec.: −28.07[1] mas/yr
Parallax (π)	7.95 ± 0.72 mas[1]
Distance	410 ± 40 ly (130 ± 10 pc)
Absolute magnitude (MV)	−0.67[2]
Details
21 Sgr A
Mass	7.9[7] M☉
Luminosity	320[2] L☉
Temperature	4,234[8] K
Rotational velocity (v sin i)	4.4[9] km/s
Other designations
21 Srg, BD−20°5134, GC 25132, HD 169420, HIP 90289, HR 6896, SAO 186794, CCDM J18254-2033AB, WDS J18254-2033AB, GSC 06274-01663[10]
Database references
SIMBAD	data

As of 2008, the pair had an angular separation of 1.7″ along a position angle of 280°. The brighter member of the pair, designated component A, is a K-type bright giant with a stellar classification of K2II[4] and visual magnitude 5.03.[3] It has eight[7] times the mass of the Sun and is radiating 320[2] times the Sun's luminosity from its photosphere at an effective temperature of 4,234 K[8] Its companion, component B, is magnitude 7.43 with type reported as A5:, where the ':' indicates some uncertainty about the classification.[11][3]

References