21 Sagittarii

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[1] mas
Distance410 ± 40 ly
(130 ± 10 pc)
Absolute magnitude (MV)−0.67[2]
Details
21 Sgr A
Mass7.9[7] M☉
Luminosity320[2] L☉
Temperature4,234[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
SIMBADdata

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
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. Vizier catalog entry
  2. Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. Vizier catalog entry
  3. Eggleton, P. P.; Tokovinin, A. A. (2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. Vizier catalog entry
  4. Hoffleit, D.; Warren, W. H. (1995). "VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR On-line Data Catalog: V/50. Originally Published in: 1964BS....C......0H. 5050. Bibcode:1995yCat.5050....0H.
  5. Mallama, A. (2014). "Sloan Magnitudes for the Brightest Stars". The Journal of the American Association of Variable Star Observers. 42: 443. Bibcode:2014JAVSO..42..443M.Vizier catalog entry
  6. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065.
  7. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410: 190. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. Vizier catalog entry
  8. McDonald, I.; Zijlstra, A. A.; Boyer, M. L. (2012). "Fundamental parameters and infrared excesses of Hipparcos stars". Monthly Notices of the Royal Astronomical Society. 427: 343. arXiv:1208.2037. Bibcode:2012MNRAS.427..343M. doi:10.1111/j.1365-2966.2012.21873.x. Vizier catalog entry
  9. Glebocki, R.; Gnacinski, P. (2005). "VizieR Online Data Catalog: Catalog of Stellar Rotational Velocities (Glebocki+ 2005)". VizieR On-line Data Catalog: III/244. Originally Published in: 2005csss...13..571G; 2005yCat.3244....0G. 3244. Bibcode:2005yCat.3244....0G. Vizier catalog entry
  10. "21 Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-06-08.
  11. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920. Vizier catalog entry
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