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20 Hydrae

20 Hydrae Facts

20 Hydrae's Alternative Names

The Id of the star in the Yale Bright Star Catalogue is HR3641. HIP44961 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD78732.

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 20 Hydrae with it shortened to 20 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 103 G. Hydrae. There are no stars with a Gould designation in Ursa Major for example.



Location of 20 Hydrae


Radial Velocity and Proper Motion of 20 Hydrae

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 -6.03 ± 0.30 miliarcseconds/year towards the north and -17.50 ± 0.32 miliarcseconds/year east if we saw them in the horizon.


20 Hydrae Luminosity

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 171.12 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Physical Properties (Colour, Temperature) of 20 Hydrae

20 Hydrae has a spectral type of G8II. This means the star is a white to yellow luminous giant star. The star is 7,470.00 Parsecs from the Galactic Centre or in terms of Light Years is 24,364.40 s. The star has a B-V Colour Index of 1 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 4,973 Kelvin.

20 Hydrae Radius has been calculated as being 19.94 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 13,872,691.52.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 14.99. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's Iron Abundance is 0.24 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

20 Hydrae Apparent and Absolute Magnitudes

20 Hydrae has an apparent magnitude of 5.47 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 -1.00 If you used the 2007 Parallax value, you would get an absolute magnitude of -0.38. 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 20 Hydrae

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 5.08 which gave the calculated distance to 20 Hydrae as 642.05 light years away from Earth or 196.85 parsecs. It would take a spaceship travelling at the speed of light, 642.05 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 6.77 which put 20 Hydrae at a distance of 481.78 light years or 147.71 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 30,467,162.23 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 7,470.00 Parsecs or 24,364.40 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Source of Information


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Additional 20 Hydrae Facts and Figures

Visual Facts

Primary / Proper / Traditional Name20 Hydrae
Alternative NamesHD 78732, HIP 44961, HR 3641, 103 G. Hydrae, 20 Hya, BD-08 2593
Spectral TypeG8II
Constellation's Main StarNo
Multiple Star SystemNo / Unknown
Star TypeLuminous Giant Star
ColourWhite to Yellow
GalaxyMilky Way
Absolute Magnitude -1.00 / -0.38
Visual / Apparent Magnitude5.47
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)09h 09m 35.58
Declination (Dec.)-08° 47` 15.5
Galactic Latitude25.40 degrees
Galactic Longitude238.61 degrees
1997 Distance from Earth5.08 Parallax (milliarcseconds)
 642.05 Light Years
 196.85 Parsecs
2007 Distance from Earth6.77 Parallax (milliarcseconds)
 481.78 Light Years
 147.71 Parsecs
 30,467,162.23 Astronomical Units
Galacto-Centric Distance24,364.40 Light Years / 7,470.00 Parsecs
Proper Motion Dec.-6.03 ± 0.30 milliarcseconds/year
Proper Motion RA.-17.50 ± 0.32 milliarcseconds/year
B-V Index1.00
Radial Velocity26.00 ± 2.80 km/s
Iron Abundance0.24 ± 9.99 Fe/H
Semi-Major Axis6999.00
Stellar Luminosity (Lsun)171.12

Companions (Multi-Star and Exoplanets) Facts

Exoplanet CountNone/Unaware

Estimated Calculated Facts

Radius (x the Sun)14.99
Effective Temperature4,973 Kelvin

Sources and Links

SIMBAD SourceLink

Related Stars

Comments and Questions


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