Mu Sagittarii
{{short description|Multiple star system in the constellation Sagittarius}}
{{Starbox begin
| name=μ Sagittarii
}}
{{Starbox image
| image=
{{Location mark
|image=Sagittarius constellation map.svg|alt=|float=center|width=280
|label=|position=right
|mark=Red circle.svg|mark_width=12|mark_link=μ Sagittarius
|x=618|y=254
}}
|caption=Location of μ Sgr(circled)
}}
{{Starbox observe
| epoch=J2000.0
| constell=Sagittarius
}}
{{starbox character
| component=μ Sgr A
| class={{nowrap|B8Iap{{cite journal|bibcode=2010MNRAS.404.1306F|arxiv=1001.3337|title=Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants|journal=Monthly Notices of the Royal Astronomical Society|volume=404|issue=3|pages=1306|last1=Fraser|first1=M.|last2=Dufton|first2=P. L.|last3=Hunter|first3=I.|last4=Ryans|first4=R. S. I.|year=2010|doi=10.1111/j.1365-2966.2010.16392.x|doi-access=free |s2cid=118674151}} + B1.5V}}
|type=Blue supergiant (Aa)}}
{{starbox character|no_heading=y
| component=μ Sgr B
| variable=
}}
{{starbox character|no_heading=y
| component=μ Sgr C
| class=
| variable=
}}
{{starbox character|no_heading=y
| component=μ Sgr D
| variable=
}}
{{starbox character|no_heading=y
| component=μ Sgr E
| variable=
}}
{{starbox astrometry
| parallax=0.6437
| p_error=0.038
| absmag_v=A: −7.1
B: −1.2
D: −3.3
E: −2.1
}}
{{Starbox detail
| component1=μ Sgr Aa
| metal_fe=
| rotational_velocity=
| component2=μ Sgr Ab
| luminosity2=
| metal_fe2=
| rotational_velocity2=
}}
{{Starbox detail|no_heading=y
| component1=μ Sgr B
| mass=
| radius=
| gravity=
| metal_fe=
| rotational_velocity=
| component2=μ Sgr D
| mass2= {{Val|5.940|0.269|0.228}}
| temperature2= {{Val|16487|272|200|fmt=commas}}
| radius2= {{Val|4.95|0.30|0.45}}
| luminosity2= {{val|1764|398|270|fmt=commas}}
| gravity2= {{Val|3.82|0.07|0.05}}
| metal_fe2=
| rotational_velocity2=
}}
{{Starbox detail|no_heading=y
| component1=μ Sgr E
| temperature={{val|16700|200|300|fmt=commas}}
| radius={{val|6.36|0.36|0.31|fmt=commas}}
| luminosity={{val|2668|337|298|fmt=commas}}
| metal_fe=
| rotational_velocity=
}}
{{starbox catalogue
| names=Polis, μ Sagittarii, μ Sgr, Mu Sgr, 13 Sagittarii, CCDM J18210-2950, IDS 18078-2105, SAO 186497, WDS J18138-2104
| component1=μ Sgr A
| names1=HD 166937, HIP 89341, HR 6812, PPM 268080, BD−21°4908, FK5 682, GC 24856
| component2=μ Sgr D
| names2=HD 314059, BD−21°4907
| component3=μ Sgr E
| names3=HD 314057, BD−21°4909, 2MASS J18134906-2103528
}}
{{starbox reference
|Simbad=13+Sgr|sn=μ Sgr AB
|Simbad4=HD+314059|sn4=μ Sgr D
|Simbad5=HD+314057|sn5=μ Sgr E
}}
{{starbox end}}
Mu Sagittarii (μ Sagittarii, abbreviated Mu Sgr, μ Sgr) is a multiple star system in the constellation of Sagittarius. The brightest component, a blue supergiant designated Mu Sagittarii Aa, is formally named Polis {{IPAc-en|'|p|Q|l|I|s}}. The system is 5,000 light-years from the Sun and is part of the Sgr OB1 stellar association.
System
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, but others consider it to be part of a trapezium system of four gravitationally bound stars (plus an unseen companion).
| class = wikitable |
| Component
! Separation ! Minimum distance |
|---|
| A
| +3.88 | - | - |
| B
| +8.04 | 16.9 arcseconds |
| 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 |
Nomenclature
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).{{cite arXiv |title=On the naming convention used for multiple star systems and extrasolar planets |date=2010 |eprint=1012.0707 |class=astro-ph.SR |last1= Hessman |first1=F. V. |last2= Dhillon |first2=V. S. |last3= Winget |first3=D. E. |last4= Schreiber |first4=M. R. |last5= Horne |first5=K. |last6= Marsh |first6=T. R. |last7= Guenther |first7=E. |last8= Schwope |first8=A. |last9= Heber |first9=U.}}
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".See station 22 in footnote 12 in Stefan Weinstock, "Lunar Mansions and Early Calendars", Journal of Hellenic Studies, Volume 69, November 1949, pp. 48-69.
In 2016, the IAU organized a Working Group on Star Names (WGSN){{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}} to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.{{cite web | url=https://www.iau.org/static/science/scientific_bodies/working_groups/280/wg-starnames-triennial-report-2015-2018.pdf | page=5 | title=WG Triennial Report (2015-2018) - Star Names |access-date=2018-07-14}} 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.
In Chinese, {{lang|zh|斗}} ({{lang|zh-Latn|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 {{lang|zh|斗宿三}} ({{lang|zh-Latn|Dǒu Sù sān}}, {{langx|en|the Third Star of Dipper}}.)
Properties
= Variability =
Mu Sagittarii A varies in brightness and is classified as a variable star. 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). In addition, it shows more irregular variations typical of an Alpha Cygni variable, irregularly pulsating hot supergiant stars.
= Physical =
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. This means that Mu Sagittarii Aa's diameter is greater than that of the orbit of Mercury, or a little more than one astronomical unit (the average distance from Earth to the Sun). Its mass is 23 times the solar mass while it has a surface temperature of 11,100 K. Mu Sagittarii Aa is a type B8 blue supergiant and the companion (Mu Sagitarii Ab) is a type B2 giant. Its high mass means it has sufficient mass to end its life in a core-collapse supernova explosion.
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 and 10.481,{{cite journal|bibcode=1983A&AS...51..161L|title=A study of visual double stars with early type primaries. II - Photometric results|journal=Astronomy and Astrophysics Supplement Series|volume=51|pages=161|last1=Lindroos|first1=K. P.|year=1983}} 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.{{cite journal|bibcode=2001AJ....122.3466M|title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog|journal=The Astronomical Journal|volume=122|issue=6|pages=3466|last1=Mason|first1=Brian D.|last2=Wycoff|first2=Gary L.|last3=Hartkopf|first3=William I.|last4=Douglass|first4=Geoffrey G.|last5=Worley|first5=Charles E.|year=2001|doi=10.1086/323920|doi-access=free}}{{cite journal|bibcode=1994CoORB.115.....D|title=Catalogue of the Components of Double and Multiple stars (CCDM). First edition|journal=Obs. R. Belg|volume=115|last1=Dommanget|first1=J.|last2=Nys|first2=O.|year=1994}}
Component D has an early B spectral type, near B3.{{cite journal|bibcode=1995A&AS..110..367N |title=The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars |journal=Astronomy and Astrophysics |volume=110 |pages=367 |last1=Nesterov |first1=V. V. |last2=Kuzmin |first2=A. V. |last3=Ashimbaeva |first3=N. T. |last4=Volchkov |first4=A. A. |last5=Röser |first5=S. |last6=Bastian |first6=U. |year=1995}} 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. The spectral type has also been estimated photometrically as B2 V, and a main sequence luminosity matches the distance of the other stars. Components D and E are located about 5,100 light-years from Earth based on their Gaia Data Release 3 parallaxes.
References
{{reflist|refs=
{{cite Gaia DR3}} [https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=I/355&Source=4093934243590140800 Gaia DR3 record for this source] at VizieR.
{{cite Gaia DR3}} [https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=I/355&Source=4093933448976732160 Gaia DR3 record for this source] at VizieR.
}}
External links
- [http://stars.astro.illinois.edu/sow/polis.html Polis]
{{Stars of Sagittarius}}
{{DEFAULTSORT:Mu Sagittarii}}
Category:Sagittarius (constellation)
Category:B-type main-sequence stars