Mu Sagittarii (μ Sagittarii, abbreviated Mu Sgr, μ Sgr) is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Sagittarius. The brightest component, a

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blu ...

designated Mu Sagittarii Aa, is formally named Polis . The system is 5,000

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

and is part of the Sgr OB1

stellar association A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more visible stars. An association is primarily identified by commonalities in i ...

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 A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

with components designated Mu Sagittarii Aa and Ab. Of the five visible stars, component C is considered an

optical double In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a ...

, 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).

Nomenclature

''μ Sagittarii'' ( Latinised to ''Mu Sagittarii'') is the system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. 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

s, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU). 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 Polis (: poleis) means 'city' in Ancient Greek. The ancient word ''polis'' had socio-political connotations not possessed by modern usage. For example, Modern Greek πόλη (polē) is located within a (''khôra''), "country", which is a πατ ...

'' "city". In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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, (), meaning ''

Dipper Dippers are members of the genus ''Cinclus'' in the bird family Cinclidae, so-called because of their bobbing or dipping movements. They are unique among passerines for their ability to dive and swim underwater. Taxonomy The genus ''Cinclus'' ...

'', refers to an asterism consisting of Mu Sagittarii, Phi Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Zeta Sagittarii. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Mu Sagittarii itself is (, .)

Properties

Variability

Mu Sagittarii A varies in brightness and is classified as a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

. The two spectroscopic components

eclipse An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...

each other with an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

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 Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time other parts expand. They are supergiant Supergiants are among the most massive and m ...

, irregularly pulsating hot

supergiant star Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute magnitude, absolute visual magnitudes between about −3 and −8. The temperatures of supe ...

Physical

Mu Sagittarii A appears as a type-B

giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...

with a total luminosity of 180,000 times that of

the Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot Plasma (physics), plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as ...

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 astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

(the average distance from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

to the

). Its mass is 23 times the

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

while it has a surface temperature of 11,100 K. Mu Sagittarii Aa is a type B8

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 A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original obj ...

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, leading to uncertainties about its physical properties, distance, and membership of the system. The

Washington Double Star Catalog The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for (as of January 2024) 1 ...

gives a magnitude of 10.48 and the Catalog of Components of Double and Multiple Stars a magnitude of 11.5. Component D has an early B spectral type, near B3. 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

External links

Polis
{{DEFAULTSORT:Mu Sagittarii Algol variables Sagittarii, Mu B-type giants B-type supergiants Sagittarii, 13 166937 089341 6 Sagittarius (constellation) Eclipsing binaries B-type main-sequence stars J18134906-2103528 BD-21 4908 Alpha Cygni variables 6812

Polis Polis (: poleis) means 'city' in Ancient Greek. The ancient word ''polis'' had socio-political connotations not possessed by modern usage. For example, Modern Greek πόλη (polē) is located within a (''khôra''), "country", which is a πατ ...