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Head of Hydrus (Alpha Hydri) Star Facts

Head of Hydrus Facts

Head of Hydrus's Alternative Names

Alpha Hydri (Alf Hyi) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in the early nineteenth century. The brightest star in the constellation is normally given the Alpha designation although there are exceptions such as Pollux which is Beta Geminorum.

The Id of the star in the Yale Bright Star Catalogue is HR591. HIP9236 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD12311. The Gliese ID of the star is GL 83. The star was part of the original catalogue devised by German Astronomer Wilheim Gliese of stars located within 20 parsecs of Earth. Star Names

More details on objects' alternative names can be found at Star Names .

Location of Head of Hydrus

The location of the main sequence 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 Head of Hydrus, the location is 01h 58m 45.87 and -61° 34` 11.7 .

Radial Velocity and Proper Motion of Head of Hydrus

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 26.77 ± 0.38 miliarcseconds/year towards the north and 263.66 ± 0.44 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 the Sun is 8.70 km/s with an error of about 3.10 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.

Head of Hydrus 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 28.79 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 Head of Hydrus

Head of Hydrus has a spectral type of F0V. This means the star is a blue to white main sequence star. The star is 7,396.00 Parsecs from the Galactic Centre or in terms of Light Years is 24,123.04 s. The star has a B-V Colour Index of 0.29 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 7,270 Kelvin.

Head of Hydrus Radius has been calculated as being 3.45 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 2,400,532.67.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 3.47. 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 0.07 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

Head of Hydrus Apparent and Absolute Magnitudes

Head of Hydrus has an apparent magnitude of 2.86 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 1.16 If you used the 2007 Parallax value, you would get an absolute magnitude of 1.15. 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 Head of Hydrus

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 45.74 which gave the calculated distance to Head of Hydrus as 71.31 light years away from Earth or 21.86 parsecs. It would take a spaceship travelling at the speed of light, 71.31 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 45.43 which put Head of Hydrus at a distance of 71.79 light years or 22.01 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 4,539,856.75 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 7,396.00 Parsecs or 24,123.04 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Meteor Showers Radiating from near Head of Hydrus

The Alpha Hydrids Meteor Shower radiants from a point near this star. The meteor shower runs typically between 15-30 Jan with a peak date of 20/21 Jan. The speed of a meteor in the shower is 43 Km/s.

Source of Information

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.

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Additional Head of Hydrus Facts and Figures

Visual Facts


Primary / Proper / Traditional NameHead of Hydrus
Alternative NamesAlpha Hydri, Alf Hyi, HD 12311, HIP 9236, HR 591, Gliese 83
Spectral TypeF0V
Constellation's Main StarYes
Multiple Star SystemYes
Star Type main sequence Dwarf Star
ColourYellow - White
GalaxyMilky Way
ConstellationHydrus
Absolute Magnitude 1.16 / 1.15
Visual / Apparent Magnitude2.86
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)01h 58m 45.87
Declination (Dec.)-61° 34` 11.7
Galactic Latitude-53.76 degrees
Galactic Longitude289.45 degrees
1997 Distance from Earth45.74 Parallax (milliarcseconds)
 71.31 Light Years
 21.86 Parsecs
2007 Distance from Earth45.43 Parallax (milliarcseconds)
 71.79 Light Years
 22.01 Parsecs
 4,539,856.75 Astronomical Units
Galacto-Centric Distance24,123.04 Light Years / 7,396.00 Parsecs
Proper Motion Dec.26.77 ± 0.38 milliarcseconds/year
Proper Motion RA.263.66 ± 0.44 milliarcseconds/year
B-V Index0.29
Radial Velocity8.70 ± 3.10 km/s
Iron Abundance0.07 ± 9.99 Fe/H
Eccentricity0.06
Inclination42.50
Semi-Major Axis6988.00
Stellar Luminosity (Lsun)28.79
Orbital Period (Days)605.96
Argument Of Periastron233.96

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Calculated Facts


Radius (x the Sun)3.47
Effective Temperature7,270 Kelvin

Sources and Links


SIMBAD SourceLink

Location of Head of Hydrus in Hydrus


Head of Hydrus Location in Hydrus

The map was generated using Night Vision, an awesome free application by Brian Simpson.

Hydrus Main Stars


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