Polis (Mu Sagittarii, 13 Sagittarii)

Polis Facts

• Polis's star type is eclipsing binary sys giant star that can be located in the constellation of Sagittarius. The description is based on the spectral class.
• Polis is not part of the constellation outline but is within the borders of the constellation.
• Based on the spectral type (B2III:) of the star, the star's colour is blue .
• Mu Sagittarii is the Bayer name for the star. It was assigned this name by Johann Bayer in 1603. The closer to the start of the Greek Alphabet the name, the brighter the star is. Alpha stars tend to be the brightest in the constellation. A notable exception is Pollux (Beta Geminorum) which is the brighest star in the Gemini constellation.
• The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
• Using the most recent figures given by the 2007 Hipparcos data, the star is 36240.37 light years away from us.

Polis's Alternative Names

Mu Sagittarii (Mu. Sgr) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR6812. 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.

Polis has alternative name(s) :- , mu. Sgr.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 13 Sagittarii with it shortened to 13 Sgr.

The Gould star designation is one that was designed by American astronomer, Benjamin Apthorp Gould. Gould stars are predominantly in the Southern and Equatorial constellations but do appear in northern constellations such as Bootes and Orion. The star has the designation 41 G. Sagittarii. There are no stars with a Gould designation in Ursa Major for example.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD-21 4908.

More details on objects' alternative names can be found at Star Names .

Location of Polis

The location of the giant star in the night sky 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 object is compared to the celestial equator and is expressed in degrees. For Polis, the location is 18h 13m 45.81 and -21° 03` 31.8 .

Radial Velocity and 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.

The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is -2.70 km/s with an error of about 2.70 km/s . When the value is negative then the star and the Sun are getting closer to one another, likewise, a positive number means that two stars are moving away. Its nothing to fear as the stars are so far apart, they won't collide in our life-time, if ever.

Physical Properties (Colour, Temperature) 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. If you need the diameter of the star, you just need to multiple the radius by 2. 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.

Using the 2007 distance, the star is roughly 2,291,814,981.75 Astronomical Units from the Earth/Sun give or take a few. An Astronomical Unit is the distance between Earth and the Sun. The number of A.U. is the number of times that the star is from the Earth compared to the Sun.

Variable Type of Polis

The star is a eclipsing binary sys 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 3.961 to a magnitude of 3.836 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.1 days (variability).

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.