Mu Sagittarii

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μ Sagittarii
Sagittarius constellation map.svg
Red circle.svg
Location of μ Sgr(circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Sagittarius
Right ascension 18h 13m 45.8s[1]
Declination −21° 03′ 32″[1]
Apparent magnitude (V) +3.85[2]
μ Sgr A
Spectral type B8Iap[3] + B1.5V[4]
U−B color index −0.52[5]
B−V color index +0.22[5]
Variable type EA + α Cyg[6]
μ Sgr B
Spectral type B9III[4]
U−B color index −0.11[5]
B−V color index −0.04[5]
μ Sgr C
U−B color index −0.30[5]
B−V color index +0.23[5]
μ Sgr D
Spectral type B2IV[7]
U−B color index −0.57[5]
B−V color index +0.11[5]
μ Sgr E
Spectral type B2.5V[7]
U−B color index −0.67[5]
B−V color index +0.04[5]
Parallax (π) A: 0.09 ± 0.28[1] mas
Distance 920[8] pc
Absolute magnitude (MV) A: −7.1[7]
B: −1.2[9]
D: −3.3[7]
E: −2.1[7]
μ Sgr Aa
Mass 14.1 - 30[10] M
Radius 67.9[10] R
Surface gravity (log g) 2.0[10] cgs
Temperature 12,000[10] K
μ Sgr Ab
Mass 9.8 - 15[10] M
Radius 12.2[10] R
Surface gravity (log g) 4.0[10] cgs
Temperature 23,000[10] K
Age 10[7] Myr
μ Sgr B
Luminosity 603[8] L
Temperature 18,200[8] K
μ Sgr D
Luminosity 1,660[8] L
Temperature 20,400[8] K
μ Sgr E
Luminosity 2,450[8] L
Temperature 20,900[8] K
Other designations
Polis, μ Sagittarii, μ Sgr, Mu Sgr, 13 Sagittarii, CCDM J18210-2950, IDS 18078-2105, SAO 186497, WDS J18138-2104
μ Sgr A: HD 166937, HIP 89341, HR 6812, PPM 268080, BD−21°4908, FK5 682, GC 24856
μ Sgr D: HD 314059, BD−21°4907
μ Sgr E: HD 314057, BD−21°4909, 2MASS J18134906-2103528
Database references
μ Sgr D
μ Sgr E

Mu Sagittarii (μ Sagittarii, abbreviated Mu Sgr, μ Sgr) is a multiple star system in the constellation of Sagittarius. The brightest component, designated Mu Sagittarii Aa, is named Polis,[11] the system is 3,000 light-years from the Sun and is part of the Sgr OB1 stellar association.


The components of the Mu Sagittarii system are designated 'A' through 'E', in order of their distance from the brightest, which is Mu Sagittarii A. 'A' is itself a spectroscopic binary with components designated Mu Sagittarii Aa and Ab. Of the five visible stars, component C is considered an optical double, not physically close to the other stars. Component D has also been listed as a purely optical double by some authors,[7] but others consider it to be part of a trapezium system of four gravitationally bound stars (plus an unseen companion).[9]

Component Apparent
from Mu Sagittarii A
Minimum distance
from Mu Sagittarii A
A +3.88 - -
B +8.04 16.9 arcseconds 42 200 AU or 0.67 ly
C +10.99 25.8 arcseconds 64 500 AU or 1.02 ly
D +9.63 48.5 arcseconds 121 200 AU or 1.92 ly
E +9.25 50.0 arcseconds 125 000 AU or 1.98 ly


μ Sagittarii (Latinised to Mu Sagittarii) is the system's Bayer designation. The designations of the five constituents as Mu Sagittarii A, B, C, D and E, and those of A's components - Mu Sagittarii Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[12]

The system bore the traditional name Polis, which is derived from a Coptic word meaning foal;[13] in 2016, the IAU organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN approved the name Polis for the component Mu Sagittarii Aa on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[11]

In Chinese, (Dǒu), meaning Dipper, refers to an asterism consisting of Mu Sagittarii, Phi Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Zeta Sagittarii. Consequently, Mu Sagittarii itself is known as 斗宿三 (Dǒu Sù sān, English: the Third Star of Dipper.)[15]



Mu Sagittarii A varies in brightness and is classified as a variable star,[6] the two spectroscopic components eclipse each other with an orbital period of 181 days, causing a 0.08 magnitude drop in brightness (from +3.84 to +3.96).[10] In addition, it shows more irregular variations typical of an Alpha Cygni variable, irregularly pulsating hot supergiant stars.


Mu Sagittarii A appears as a type-B giant star with a total luminosity of 180,000 times that of the Sun and a radius of 115 times the Sun's. Its mass is 23 times the solar mass while it has a surface temperature of 11,100 K. Mu Sagitarii Aa is a type B8 supergiant and the companion (Mu Sagitarii Ab) is a type B2 giant.

The remaining components are very weakly bound to the Polis system, and although Mu Sagittarii is visible to the naked eye, the properties of the secondary components are highly uncertain.

The apparent magnitude for component B has been measured at between +8.04[5] and 10.481,[16] leading to uncertainties about its physical properties, distance, and membership of the system. The Washington Double Star Catalog gives a magnitude of 10.48 and the Catalog of Components of Double and Multiple Stars a magnitude of 11.5.[17][18]

Component D has an early B spectral type, near B3,[19] the full MK spectral type has been measured as B2 IV, and the assumption of a subgiant luminosity suggests that it is more distant than the other stars of the system.[7] The spectral type has also been estimated photometrically as B2 V, and a main sequence luminosity matches the distance of the other stars.[9]


  1. ^ a b c Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D. 
  3. ^ Fraser, M.; Dufton, P. L.; Hunter, I.; Ryans, R. S. I. (2010). "Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants". Monthly Notices of the Royal Astronomical Society. 404: 1306. arXiv:1001.3337Freely accessible. Bibcode:2010MNRAS.404.1306F. doi:10.1111/j.1365-2966.2010.16392.x. 
  4. ^ a b Zasche, P.; Wolf, M.; Hartkopf, W. I.; Svoboda, P.; Uhlař, R.; Liakos, A.; Gazeas, K. (2009). "A Catalog of Visual Double and Multiple Stars with Eclipsing Components". The Astronomical Journal. 138 (2): 664. arXiv:0907.5172Freely accessible. Bibcode:2009AJ....138..664Z. doi:10.1088/0004-6256/138/2/664. 
  5. ^ a b c d e f g h i j k l Echevarria, J.; Roth, M.; Warman, J. (1979). "Photometric Study of Trapezium-Type Systems". Revista Mexicana de Astronomia y Astrofisica. 4: 287. Bibcode:1979RMxAA...4..287E. 
  6. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S. 
  7. ^ a b c d e f g h Abt, H. A.; Cardona, O. (1983). "Confirmation among visual multiples of an increase of AP stars with age". Astrophysical Journal. 272: 182. Bibcode:1983ApJ...272..182A. doi:10.1086/161276. 
  8. ^ a b c d e f g Lindroos, K. P. (1986). "A study of visual double stars with early-type primaries. V - Post-T Tauri secondaries". Astronomy and Astrophysics. 156: 223. Bibcode:1986A&A...156..223L. 
  9. ^ a b c Lindroos, K. P. (1985). "A study of visual double stars with early type primaries. IV Astrophysical data". Astronomy and Astrophysics Supplement Series. 60: 183. Bibcode:1985A&AS...60..183L. 
  10. ^ a b c d e f g h i Polidan, R. S.; Plavec, M. J. (1984). "A hot companion to MU Sagittarii - an opportunity to sound the atmosphere of a B8 IA supergiant". Astronomical Journal. 89: 1721. Bibcode:1984AJ.....89.1721P. doi:10.1086/113678. 
  11. ^ a b "Naming Stars". Retrieved 16 December 2017. 
  12. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707Freely accessible [astro-ph.SR]. 
  13. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (rep. ed.). New York City: Dover Publications Inc. ISBN 0-486-21079-0. 
  14. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  15. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
  16. ^ Lindroos, K. P. (1983). "A study of visual double stars with early type primaries. II - Photometric results". Astronomy and Astrophysics Supplement Series. 51: 161. Bibcode:1983A&AS...51..161L. 
  17. ^ 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. 
  18. ^ Dommanget, J.; Nys, O. (1994). "Catalogue of the Components of Double and Multiple stars (CCDM). First edition". Obs. R. Belg. 115. Bibcode:1994CoORB.115.....D. 
  19. ^ Nesterov, V. V.; Kuzmin, A. V.; Ashimbaeva, N. T.; Volchkov, A. A.; Röser, S.; Bastian, U. (1995). "The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars". Astronomy and Astrophysics. 110. Bibcode:1995A&AS..110..367N. 

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