Theta2 Sagittarii is a blue subgiant star that can be located in the constellation of Sagittarius. Theta2 Sagittarii 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.
Theta2 Sagittarii is the Bayer Classification for the star. HIP98421 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD189118. The Id of the star in the Gould Star Catalogue is 256. 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 Theta2 Sagittarii, the location is 19h 59m 51.28 and -34d41`51.5 .
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 -69.51 ± 0.18 miliarcseconds/year towards the north and 108.23 ± 0.28 miliarcseconds/year east if we saw them in the horizon.
Theta2 Sagittarii has a spectral type of A4/A5IV. This means the star is a blue subgiant star. The star is 7358.00000000 Parsecs from the Galactic Centre or terms of Light Years is 23999.0988515200000000s. The star has a B-V Colour Index of 0.17 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,952 Kelvin.
Theta2 Sagittarii Radius has been calculated as being 2.06 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,433,562.17.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 2.08. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.
Theta2 Sagittarii has an apparent magnitude of 5.30 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 1.89 If you used the 2007 Parallax value, you would get an absolute magnitude of 1.87. 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 20.81 which gave the calculated distance to Theta2 Sagittarii as 156.73 light years away from Earth or 48.05 parsecs. It would take a spaceship travelling at the speed of light, 156.73 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 20.62 which put Theta2 Sagittarii at a distance of 158.18 light years or 48.50 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.
The star's Galacto-Centric Distance is 7,358.00 Parsecs or 23,999.10 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.
|Bayer Designation||Theta2 Sagittarii|
|Hipparcos Library I.D.||98421|
|Henry Draper Designation||189118|
|Star Type||subgiant star|
|Absolute Magnitude||1.89 / 1.87|
|Visual / Apparent Magnitude||5.30|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||19h 59m 51.28|
|Galactic Latitude||-28.34 degrees|
|Galactic Longitude||6.17 degrees|
|1997 Distance from Earth||20.81 Parallax (milliarcseconds)|
|156.73 Light Years|
|2007 Revised Distance from Earth||20.62 Parallax (milliarcseconds)|
|158.18 Light Years|
|Galacto-Centric Distance||23,999.10 Light Years / 7,358.00 Parsecs|
|Proper Motion Dec.||-69.51 ± 0.18 milliarcseconds/year|
|Proper Motion RA.||108.23 ± 0.28 milliarcseconds/year|
|Radial Velocity||-17.60 ± 0.80 km/s|
|Calculated Effective Temperature||7,952 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|