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Tau9 Eridani, 36 Eridani, HD25267, HIP18673, HR1240

Tau9 Eridani is a blue rotating variable star that can be located in the constellation of Eridanus. The description is based on the spectral class. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Tau9 Eridani's Alternative Names

Tau9 Eridani (Tau09 Eri) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR1240. HIP18673 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD25267.

Tau9 Eridani has alternative name(s) :- , tau09 Eri.

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 36 Eridani with it shortened to 36 Eri.

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 161 G. Eridani. There are no stars with a Gould designation in Ursa Major for example.

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

Location of Tau9 Eridani

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 Tau9 Eridani, the location is 03h 59m 55.48 and -24° 00` 58.5 .

Proper Motion of Tau9 Eridani

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 16.48 ± 0.14 miliarcseconds/year towards the north and 12.12 ± 0.22 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 25.50000 km/s with an error of about 0.50 km/s .

Tau9 Eridani 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 264.59 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 Tau9 Eridani

Tau9 Eridani has a spectral type of Ap Si. This means the star is a blue variable star. The star is 7,452.00 Parsecs from the Galactic Centre or terms of Light Years is 24,305.69 s. The star has a B-V Colour Index of -0.12 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 14,942 Kelvin.

Tau9 Eridani Radius has been calculated as being 1.68 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,171,047.18.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 1.67. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Tau9 Eridani Apparent and Absolute Magnitudes

Tau9 Eridani has an apparent magnitude of 4.62 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 -0.41 If you used the 2007 Parallax value, you would get an absolute magnitude of -0.39. 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 Tau9 Eridani

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 9.85 which gave the calculated distance to Tau9 Eridani as 331.13 light years away from Earth or 101.52 parsecs. It would take a spaceship travelling at the speed of light, 331.13 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 9.96 which put Tau9 Eridani at a distance of 327.47 light years or 100.40 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 7,452.00 Parsecs or 24,305.69 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 Tau9 Eridani

The star is a rotating Alpha2 Canum Venaticorum 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. Tau9 Eridani brightness ranges from a magnitude of 4.605 to a magnitude of 4.578 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 1.2 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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Tau9 Eridani Facts

Visual Facts


Primary / Proper / Traditional NameTau9 Eridani
Alternative NamesTau09 Eri, HD 25267, HIP 18673, HR 1240, 161 G. Eridani, 36 Eridani, 36 Eri, tau09 Eri
Spectral TypeAp Si
Multiple Star SystemYes
Star TypeVariable Star
Colour blue
GalaxyMilky Way
ConstellationEridanus
Absolute Magnitude-0.41 / -0.39
Visual / Apparent Magnitude4.62
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)03h 59m 55.48
Declination (Dec.)-24° 00` 58.5
Galactic Latitude-47.62 degrees
Galactic Longitude219.65 degrees
1997 Distance from Earth9.85 Parallax (milliarcseconds)
 331.13 Light Years
 101.52 Parsecs
2007 Distance from Earth9.96 Parallax (milliarcseconds)
 327.47 Light Years
 100.40 Parsecs
Galacto-Centric Distance24,305.69 Light Years / 7,452.00 Parsecs
Proper Motion Dec.16.48 ± 0.14 milliarcseconds/year
Proper Motion RA.12.12 ± 0.22 milliarcseconds/year
B-V Index-0.12
Radial Velocity25.50 ± 0.50 km/s
Eccentricity0.06
Semi-Major Axis7877.00
Stellar Luminosity (Lsun)264.59
Associated / Clustered StarsTau1 Eridani
Tau3 Eridani
Tau4 Eridani
Tau5 Eridani
Tau6 Eridani
Tau7 Eridani
Tau8 Eridani

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Variable Star ClassRotating
Variable Star TypeAlpha2 Canum Venaticorum
Mean Variability Period in Days1.210
Variable Magnitude Range (Brighter - Dimmer)4.578 - 4.605

Estimated Facts


Calculated Effective Temperature14,942 Kelvin

Sources and Links


SIMBAD SourceLink

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