21 Sagittarii
Binary star system in the constellation Sagittarius / From Wikipedia, the free encyclopedia
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]
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 | |
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]