Mu Sagittarii

From Wikipedia, the free encyclopedia
Jump to: navigation, search
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 80.2[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 14.5[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 (Mu Sgr, μ Sagittarii, μ Sgr) is a multiple star system in the constellation Sagittarius. It also has the traditional name Polis. μ Sgr is 3,000 light years from Earth and is part of the Sgr OB1 stellar association.


The stars in the μ Sgr system are designated A through E, in order of their distance from the brightest component, which is μ Sgr A. μ Sgr A is also a spectroscopic binary. 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 Polis A
Minimum distance
from Polis 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 varies in brightness and is classified as a variable star.[6] The two spectroscopic components of μ Sgr A eclipse each other every 181 days, causing a 0.08 magnitude drop in brightness.[10] In addition, it shows more irregular variations typical of an α Cyg variable, irregularly pulsating hot supergiants.


is a type B giant star with a total luminosity of 180,000 times that of the Sun and a radius of 115 times solar. Its mass is 23 times the solar mass while it has a surface temperature of 11,100 kelvins.

Polis A is an eclipsing binary, bringing the total of stars in the system to six. The primary component is a spectral type B8 supergiant and the companion is a type B2 giant. The orbital period of the binary is 180.55 days. Due to occultation of the primary by the companion, the apparent magnitude of Polis A varies between +3.84 and +3.96.

The remaining components are very weakly bound to the Polis system.

Name and etymology[edit]


Although μ Sagittarii is a naked eye star, 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,[13] 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.[14][15]

Component D has an early B spectral type, near B3.[16] 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. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (rep. ed.). New York City: Dover Publications Inc. ISBN 0-486-21079-0. 
  12. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
  13. ^ 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. 
  14. ^ 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. 
  15. ^ 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. 
  16. ^ 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. 

External links[edit]