Mu Sagittarii

Mu Sagittarii

Location of μ Sagittarii (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagittarius
Right ascension	18h 13m 45.8s
Declination	−21° 03′ 32″
Apparent magnitude (V)	+3.84
Distance	> 3912 ly (> 920 pc)
Spectral type	B2III
Other designations
Polis, μ Sagittarii, μ Sgr, Mu Sgr, 13 Sagittarii, CCDM J18210-2950A, BD-21 4908, FK5 682, GC 24856, HD 166937, HIP 89341, HR 6812, IDS 18078-2105 A, PPM 268080, SAO 186497, WDS J18138-2104A.

Mu Sagittarii (Mu Sgr, μ Sagittarii, μ Sgr) is a star system in the constellation Sagittarius. It also has the traditional name Polis.

The stars in the Polis system are designated Polis A through Polis E. Polis is more than 3000 light years from Earth.

The primary component in this system, Polis A, 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.

Component	Apparent Magnitude	Separation from Polis A	Minimum distance from Polis A
Polis A	+3.84	-	-
Polis B	+11.5	16.9 arcseconds	42 200 AU or 0.67 ly
Polis C	+13.5	25.8 arcseconds	64 500 AU or 1.02 ly
Polis D	+9.9	48.5 arcseconds	121 200 AU or 1.92 ly
Polis E	+9.4	50.0 arcseconds	125 000 AU or 1.98 ly

Name and etymology

• The traditional name Polis is derived from a Coptic word meaning foal.
• In Chinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of μ Sagittarii, φ Sagittarii, λ Sagittarii, σ Sagittarii, τ Sagittarii and ζ Sagittarii. Consequently, μ Sagittarii itself is known as 斗宿三 (Dǒu Sù sān, English: the Third Star of Dipper.)

External links

• Polis