21 G. Sgr is a blue very luminous supergiant star that can be located in the constellation of Sagittarius. 21 G. Sgr is the brightest star in Sagittarius based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
HIP88298 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD164402. The Id of the star in the Gould Star Catalogue is 21. 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.
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 21 G. Sgr, the location is 18h 01m 54.38 and -22d46`49.0 .
Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 320000.0000000 that I have given is based on the Spectral Types page that I have found on the Internet. You might find a different figure, one that may have been calculated rather than generalised that I have done. The figure is always the amount times the luminosity of the Sun. It is an imprecise figure because of a number of factors including but not limited to whether the star is a variable star and distance.
21 G. Sgr has a spectral type of B0Iab.... This means the star is a blue supergiant star. The star has a B-V Colour Index of -0.03 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 11,122 Kelvin. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.
21 G. Sgr has an apparent magnitude of 5.72 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 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.
Using the original Hipparcos data that was released in 1997, the parallax to the star was given as -0.14 which gave the calculated distance to 21 G. Sgr as -23297.38 light years away from Earth or -7142.86 parsecs. It would take a spaceship travelling at the speed of light, -23297.38 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.
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.
|Traditional/Proper Name||21 G. Sgr|
|Hipparcos Library I.D.||88298|
|Bonner Durchmusterung||BD-22 4503|
|Henry Draper Designation||164402|
|Star Type||supergiant star|
|Visual / Apparent Magnitude||5.72|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||18h 01m 54.38|
|Galactic Latitude||-0.03 degrees|
|Galactic Longitude||7.16 degrees|
|1997 Distance from Earth||-0.14 Parallax (milliarcseconds)|
|-23297.38 Light Years|
|Radial Velocity||4.20 ± 1.40 km/s|
|Luminosity (x the Sun)||320,000.0000000|
|Calculated Effective Temperature||11,122 Kelvin|
The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.
|Proper Motion mas/yr|
|H.D. Id||B.D. Id||Star Code||Magnitude||R.A.||Dec.||Spectrum||Colour||Year|