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AK Hydrae, HD73844, HIP42502

Primary Facts on AK Hydrae

  • AK Hydrae's star type is pulsating giant star that can be located in the constellation of Hydra. The description is based on the spectral class.
  • AK Hydrae is not part of the constellation outline but is within the borders of the constellation.
  • Based on the spectral type (M6III) of the star, the star's colour is red .
  • The star can not be seen by the naked eye, you need a telescope to see it.
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 509.63 light years away from us.

AK Hydrae's Alternative Names

HIP42502 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD73844.

AK Hydrae has alternative name(s) :- , AK Hya.

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-16 2541.

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

Location of AK Hydrae

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 star is compared to the celestial equator and is expressed in degrees. For AK Hydrae, the location is 08h 39m 53.54 and -17° 18` 09.4 .

Radial Velocity and Proper Motion of AK 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 -152.22 ± 0.29 miliarcseconds/year towards the north and 0.10 ± 0.41 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 34.96 km/s with an error of about 1.02 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.

AK 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 99.67 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 AK Hydrae

AK Hydrae has a spectral type of M6III. This means the star is a red giant star. The star is 7,473.00 Parsecs from the Galactic Centre or in terms of Light Years is 24,374.19 s. The star has a B-V Colour Index of 1.33 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 4,273 Kelvin.

AK Hydrae Radius has been calculated as being 15.97 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 11,115,286.73.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 12.81. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

AK Hydrae Apparent and Absolute Magnitudes

AK Hydrae has an apparent magnitude of 6.59 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 0.14 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.62. 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 AK Hydrae

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 5.12 which gave the calculated distance to AK Hydrae as 637.04 light years away from Earth or 195.31 parsecs. It would take a spaceship travelling at the speed of light, 637.04 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 6.40 which put AK Hydrae at a distance of 509.63 light years or 156.25 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 32,228,651.40 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 7,473.00 Parsecs or 24,374.19 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Variable Type of AK Hydrae

The star is a pulsating Semiregular late- (M variable type which means that its size changes over time. The Variable Type is usually named after the first star of that type to be spotted. AK Hydrae brightness ranges from a magnitude of 6.523 to a magnitude of 5.841 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.6 days (variability).

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 AK Hydrae Facts and Figures

Visual Facts


Primary / Proper / Traditional NameAK Hydrae
Alternative NamesHD 73844, HIP 42502, BD-16 2541, AK Hya
Spectral TypeM6III
Constellation's Main StarNo
Multiple Star SystemNo / Unknown
Star TypeGiant Star
Colour red
GalaxyMilky Way
ConstellationHydra
Absolute Magnitude 0.14 / 0.62
Visual / Apparent Magnitude6.59
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)08h 39m 53.54
Declination (Dec.)-17° 18` 09.4
Galactic Latitude14.57 degrees
Galactic Longitude241.52 degrees
1997 Distance from Earth5.12 Parallax (milliarcseconds)
 637.04 Light Years
 195.31 Parsecs
2007 Distance from Earth6.40 Parallax (milliarcseconds)
 509.63 Light Years
 156.25 Parsecs
 32,228,651.40 Astronomical Units
Galacto-Centric Distance24,374.19 Light Years / 7,473.00 Parsecs
Proper Motion Dec.-152.22 ± 0.29 milliarcseconds/year
Proper Motion RA.0.10 ± 0.41 milliarcseconds/year
B-V Index 1.33
Radial Velocity34.96 ± 1.02 km/s
Eccentricity0.58
Semi-Major Axis4322.00
Stellar Luminosity (Lsun)99.67

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Variable Star ClassPulsating
Variable Star TypeSemiregular late- (M
Mean Variability Period in Days0.567
Variable Magnitude Range (Brighter - Dimmer)5.841 - 6.523

Estimated Facts


Calculated Effective Temperature4,273 Kelvin

Sources and Links


SIMBAD SourceLink

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