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Tau4 Eridani, 16 Eridani, HD20720, HIP15474

Tau4 Eridani is a red pulsating giant star that can be located in the constellation of Eridanus. Tau4 Eridani is the brightest star in Eridanus based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Tau4 Eridani is the Bayer Classification for the star. HIP15474 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD20720. The Id of the star in the Gould Star Catalogue is 81. Stars in the southern hemisphere are more likely to have a Gould Id than the northern hemisphere. For example, there are no Gould classified stars in Ursa Major.

Tau4 Eridani has alternative name(s), tau04 Eri.

Location of Tau4 Eridani

The location of the star in the galaxy 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 Tau4 Eridani, the location is 03h 19m 30.97 and -21d45`28.6 .

Proper Motion of Tau4 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 32.92 ± 0.35 miliarcseconds/year towards the north and 51.89 ± 0.54 miliarcseconds/year east if we saw them in the horizon.

Tau4 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 10.0000000 that I have given is based on the Spectral Types page that I have found on the Internet. You might find a different figure, one that may have been calculated rather than generalised that I have done. The figure is always the amount times the luminosity of the Sun. It is an imprecise figure because of a number of factors including but not limited to whether the star is a variable star and distance.

Physical Properties (Colour, Temperature, Radius) of Tau4 Eridani

Tau4 Eridani has a spectral type of M3/M4III. This means the star is a red giant star. The star is 7444.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24279.5993273600000000s. The star has a B-V Colour Index of 1.61 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 3,754 Kelvin.

Tau4 Eridani Radius has been calculated as being 31.76 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 22,100,751.91.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 37.49. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Tau4 Eridani Apparent and Absolute Magnitudes

Tau4 Eridani has an apparent magnitude of 3.70 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 -0.79 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 Tau4 Eridani

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 12.63 which gave the calculated distance to Tau4 Eridani as 258.24 light years away from Earth or 79.18 parsecs. It would take a spaceship travelling at the speed of light, 258.24 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 10.71 which put Tau4 Eridani at a distance of 304.54 light years or 93.37 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 7,444.00 Parsecs or 24,279.60 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 Tau4 Eridani

The star is a pulsating Slow Irregular 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. Tau4 Eridani brightness ranges from a magnitude of 3.761 to a magnitude of 3.694 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.1 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.

Tau4 Eridani Facts

Alternative Names

Flamsteed Name16 Eridani
Flamsteed Short Name16 Eri
Short Nametau04 Eri
Bayer DesignationTau4 Eridani
Hipparcos Library I.D.15474
Bonner DurchmusterungBD-22 584
Gould I.D.81
Henry Draper Designation20720

Visual Facts

Star Typegiant star
Absolute Magnitude-0.79 / -1.15
Visual / Apparent Magnitude3.70
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)03h 19m 30.97
Declination (Dec.)-21d45`28.6
Galactic Latitude-56.00 degrees
Galactic Longitude212.09 degrees
1997 Distance from Earth12.63 Parallax (milliarcseconds)
 258.24 Light Years
 79.18 Parsecs
2007 Revised Distance from Earth10.71 Parallax (milliarcseconds)
 304.54 Light Years
 93.37 Parsecs
Galacto-Centric Distance24,279.60 Light Years / 7,444.00 Parsecs
Proper Motion Dec.32.92 ± 0.35 milliarcseconds/year
Proper Motion RA.51.89 ± 0.54 milliarcseconds/year
B-V Index1.61
Radial Velocity41.70 ± 0.70 km/s
Spectral TypeM3/M4III
Colour(M) Red

Variable Star Details

Variable Star ClassPulsating
Variable Star TypeSlow Irregular
Mean Variability Period in Days0.055
Variable Magnitude Range (Brighter - Dimmer)3.694 - 3.761

Estimated Facts

Luminosity (x the Sun)10.0000000
Calculated Effective Temperature3,754 Kelvin

Sources and Links

SIMBAD SourceLink

Related Stars

Multi-Star System

The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.


Proper Motion mas/yr
H.D. IdB.D. IdStar CodeMagnitudeR.A.Dec.SpectrumColourYear
20720-22 584.0A4.0000049.0000037.00000M2Red
B9.500001857
C10.800001955
D10.700001879
E10.700001879
F10.000001880

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