Polis (Mu Sagittarii) - HD166937 - HIP89341

Polis (Mu Sagittarii) is a blue eclipsing binary system giant star that can be located in the constellation of Sagittarius. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Mu Sagittarii is the Bayer Classification for the star. HIP89341 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD166937. The Id of the star in the Gould Star Catalogue is 41. Stars in the southern hemisphere are more likely to have a Gould Id than the northern hemisphere. For example, there are no Gould classified stars in Ursa Major.

Polis has alternative name(s), 13 Sagittarii , mu._Sgr, 13 Sgr.

Location of Polis

The location of the star in the galaxy is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For Polis, the location is 18h 13m 45.81 and -21d03`31.8 .

Proper Motion of Polis

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving -0.48 ± 0.16 miliarcseconds/year towards the north and 0.30 ± 0.28 miliarcseconds/year east if we saw them in the horizon.

Physical Properties (Colour, Temperature, Radius) of Polis

Polis has a spectral type of B2III:. This means the star is a blue giant star. The star has a B-V Colour Index of 0.19 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,827 Kelvin.

Polis Radius has been calculated as being 786.72 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 547,399,838.02.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 963.44. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Polis Apparent and Absolute Magnitudes

Polis has an apparent magnitude of 3.84 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of -10.95 If you used the 2007 Parallax value, you would get an absolute magnitude of -11.39. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to Polis

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 0.11 which gave the calculated distance to Polis as 29651.21 light years away from Earth or 9090.91 parsecs. It would take a spaceship travelling at the speed of light, 29651.21 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 0.09 which put Polis at a distance of 36240.37 light years or 11111.11 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated.

Variable Type of Polis

The star is a eclipsing binary system Beta Persei (Algol) variable type which means that its size changes over time. The Variable Type is usually named after the first star of that type to be spotted. Polis brightness ranges from a magnitude of 4.000 to a magnitude of 4.000 over its variable period. The smaller the magnitude, the brighter the star.

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.