Mu Sagittarii
 Location of μ Sgr(circled) |
|
| Observation data Epoch J2000.0 Equinox J2000.0 |
|
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 13m 45.8s |
| Declination | −21° 03′ 32″ |
| Apparent magnitude (V) | +3.85 |
| Characteristics | |
| μ Sgr A | |
| Spectral type | B8Iap + B1.5V |
| U−B color index | −0.52 |
| B−V color index | +0.22 |
| Variable type | EA + α Cyg |
| μ Sgr B | |
| Spectral type | B9III |
| U−B color index | −0.11 |
| B−V color index | −0.04 |
| μ Sgr C | |
| U−B color index | −0.30 |
| B−V color index | +0.23 |
| μ Sgr D | |
| Spectral type | B2IV |
| U−B color index | −0.57 |
| B−V color index | +0.11 |
| μ Sgr E | |
| Spectral type | B2.5V |
| U−B color index | −0.67 |
| B−V color index | +0.04 |
| Astrometry | |
| Parallax (π) | A: 0.09 ± 0.28mas |
| Distance | 920pc |
| Absolute magnitude (MV) |
A: −7.1 B: −1.2 D: −3.3 E: −2.1 |
| Details | |
| μ Sgr Aa | |
| Mass | 14.1 - 30 M☉ |
| Radius | 80.2 R☉ |
| Surface gravity (log g) | 2.0 cgs |
| Temperature | 12,000 K |
| μ Sgr Ab | |
| Mass | 9.8 - 15 M☉ |
| Radius | 14.5 R☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 23,000 K |
| Age | 10 Myr |
| μ Sgr B | |
| Luminosity | 603 L☉ |
| Temperature | 18,200 K |
| μ Sgr D | |
| Luminosity | 1,660 L☉ |
| Temperature | 20,400 K |
| μ Sgr E | |
| Luminosity | 2,450 L☉ |
| Temperature | 20,900 K |
| Other designations | |
| 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 | |
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, but others consider it to be part of a trapezium system of four gravitationally bound stars (plus an unseen companion).
μ Sagittarii varies in brightness and is classified as a variable star. The two spectroscopic components of μ Sgr A eclipse each other every 181 days, causing a 0.08 magnitude drop in brightness. In addition, it shows more irregular variations typical of an α Cyg variable, irregularly pulsating hot supergiants.
μ Sagittarii 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.
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