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 formally named Polis /ˈpɒlɪs/.^[11] The system is 3,000 light-years from the Sun and is part of the Sgr OB1 stellar association.

Quick Facts Constellation, Right ascension ...

μ Sagittarii

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]
Characteristics
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]
Astrometry
Parallax (π)	A: 0.09 ± 0.28 mas[1]
Distance	920[8] pc
Absolute magnitude (MV)	A: −7.1[7] B: −1.2[9] D: −3.3[7] E: −2.1[7]
Details
μ Sgr Aa
Mass	14.1 - 30[10] M☉
Radius	67.9[10] R☉
Luminosity	200 000 L☉
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
SIMBAD	μ Sgr AB
	μ Sgr D
	μ Sgr E

Close

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]

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 occurs in the lunar station that was given the name πολις polis in a Coptic manuscript list of lunar stations, all of which Crum concluded were of Greek origin, in this case from polis "city".^[13]

In 2016, the IAU organized a Working Group on Star Names (WGSN)^[14] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[15] It 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, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of Mu Sagittarii, Phi Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Zeta Sagittarii. Consequently, the Chinese name for Mu Sagittarii itself is 斗宿三 (Dǒu Sù sān, English: the Third Star of Dipper.)^[16]