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.
Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John named the stars in the constellation with a number and its latin name, this star's Flamsteed designation is 21 Hydrae. The Flamsteed name can be shortened to 21 Hya.
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 106 G. Hydrae. 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-06 2845.
More details on objects' alternative names can be found at Star Names .
The location of the 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 21 Hydrae, the location is 09h 12m 26.05 and -07° 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 milliarcseconds/year towards the north and -19.03 ± 0.42 milliarcseconds/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 -4.30000 km/s with an error of about 2.90 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.
Based on the star's spectral type of A3m , 21 Hydrae's colour and type is blue - white star. The star has a B-V Colour Index of 0.23 which means the star's temperature is about 7,184 Kelvin. The temperature was calculated using information from Morgans @ Uni.edu.
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 22.56 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.
21 Hydrae estimated 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. 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 3.1488858385453702322302806432. The figure is derived at by using the formula from SDSS rather than peer reviewed papers. It 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 12.10000 which gave the calculated distance to 21 Hydrae as 269.56 light years away from Earth or 82.64 parsecs. If you want that in miles, it is about 1,584,642,255,595,373.91, based on 1 Ly = 5,878,625,373,183.61 miles.
In 2007, Hipparcos data was revised with a new parallax of 11.53000 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.
Using the 2007 distance, the star is roughly 17,889,221.99 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. 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 time it will take to travel to this star is dependent on how fast you are going. U.G. has done some calculations as to how long it will take going at differing speeds. A note about the calculations, when I'm talking about years, I'm talking non-leap years only (365 days).
The New Horizons space probe is the fastest probe that we've sent into space at the time of writing. Its primary mission was to visit Pluto which at the time of launch (2006), Pluto was still a planet.
|Description||Speed (m.p.h.)||Time (years)|
|Speed of Sound (Mach 1)||767.269||247,245,792.56|
|Concorde (Mach 2)||1,534.54||123,622,735.16|
|New Horizons Probe||33,000||5,748,607.03|
|Speed of Light||670,616,629.00||282.88|
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.
|Primary / Proper / Traditional Name||21 Hydrae|
|Alternative Names||HD 79193, HIP 45184, HR 3655, 106 G. Hydrae, 21 Hya, BD-06 2845|
|Constellation's Main Star||No|
|Multiple Star System||No / Unknown|
|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.)||-07° 06` 35.6|
|Galactic Latitude||26.95257361 degrees|
|Galactic Longitude||237.54819671 degrees|
|1997 Distance from Earth||12.10000 Parallax (milliarcseconds)|
|269.56 Light Years|
|2007 Distance from Earth||11.53000 Parallax (milliarcseconds)|
|282.88 Light Years|
|17,889,221.99 Astronomical Units|
|Galacto-Centric Distance||24,273.08 Light Years / 7,442.00 Parsecs|
|Proper Motion Dec.||21.45000 ± 0.39000 milliarcseconds/year|
|Proper Motion RA.||-19.03000 ± 0.42000 milliarcseconds/year|
|Radial Velocity||-4.30000 ± 2.90 km/s|
|Stellar Luminosity (Lsun)||22.5600000|
|Radius (x the Sun)||3.15|
|Effective Temperature||7,184 Kelvin|
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