21 Hydrae is a blue star that can be located in the constellation of Hydra. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
The Id of the star in the Yale Bright Star Catalogue is HR3655. HIP45184 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD79193. The Id of the star in the Gould Star Catalogue is 106. 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 Hydrae, the location is 09h 12m 26.05 and -07d 06` 35.6 .
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 21.45 ± 0.39 miliarcseconds/year towards the north and -19.03 ± 0.42 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 us is -4.30000 km/s with an error of about 2.90 km/s .
21 Hydrae has a spectral type of A3m. This means the star is a blue star. The star is 7442.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24273.0760604800000000s. The star has a B-V Colour Index of 0.23 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,184 Kelvin.
21 Hydrae Radius has been calculated as being 3.01 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 2,092,379.03.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 3.15. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.
21 Hydrae has an apparent magnitude of 6.10 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 1.51 If you used the 2007 Parallax value, you would get an absolute magnitude of 1.41. 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 12.10 which gave the calculated distance to 21 Hydrae as 269.56 light years away from Earth or 82.64 parsecs. It would take a spaceship travelling at the speed of light, 269.56 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 11.53 which put 21 Hydrae at a distance of 282.88 light years or 86.73 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 7,442.00 Parsecs or 24,273.08 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.
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 Hydrae|
|Flamsteed Name||21 Hydrae|
|Flamsteed Short Name||21 Hya|
|Hipparcos Library I.D.||45184|
|Yale Bright Star Catalogue (HR) Id||3655|
|Bonner Durchmusterung||BD-06 2845|
|Henry Draper Designation||79193|
|Absolute Magnitude||1.51 / 1.41|
|Visual / Apparent Magnitude||6.10|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||09h 12m 26.05|
|Declination (Dec.)||-07d 06` 35.6|
|Galactic Latitude||26.95 degrees|
|Galactic Longitude||237.55 degrees|
|1997 Distance from Earth||12.10 Parallax (milliarcseconds)|
|269.56 Light Years|
|2007 Revised Distance from Earth||11.53 Parallax (milliarcseconds)|
|282.88 Light Years|
|Galacto-Centric Distance||24,273.08 Light Years / 7,442.00 Parsecs|
|Proper Motion Dec.||21.45 ± 0.39 milliarcseconds/year|
|Proper Motion RA.||-19.03 ± 0.42 milliarcseconds/year|
|Radial Velocity||-4.30 ± 2.90 km/s|
|Calculated Effective Temperature||7,184 Kelvin|