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Tau4 Eridani, 16 Eridani, HD20720, HIP15474, HR1003

Tau4 Eridani is a red pulsating giant 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.

Tau4 Eridani's Alternative Names

Tau4 Eridani (Tau04 Eri) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR1003. 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.

Tau4 Eridani has alternative name(s) :- , tau04 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 16 Eridani with it shortened to 16 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 81 G. Eridani. There are no stars with a Gould designation in Ursa Major for example.

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-22 584.

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

Location of Tau4 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 Tau4 Eridani, the location is 03h 19m 30.97 and -21° 45` 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. The Radial Velocity, that is the speed at which the star is moving away/towards us is 41.70000 km/s with an error of about 0.70 km/s .

Physical Properties (Colour, Temperature) of Tau4 Eridani

Tau4 Eridani has a spectral type of M3/M4III. This means the star is a red giant star. The star is 7,444.00 Parsecs from the Galactic Centre or terms of Light Years is 24,279.60 s. 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. 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 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.

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Tau4 Eridani Facts

Visual Facts


Primary / Proper / Traditional NameTau4 Eridani
Alternative NamesTau04 Eri, HD 20720, HIP 15474, HR 1003, 81 G. Eridani, 16 Eridani, 16 Eri, BD-22 584, tau04 Eri
Spectral TypeM3/M4III
Multiple Star SystemYes
Star TypeGiant Star
Colour red
GalaxyMilky Way
ConstellationEridanus
Absolute Magnitude-0.79 / -1.15
Visual / Apparent Magnitude3.70
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)03h 19m 30.97
Declination (Dec.)-21° 45` 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 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
Eccentricity0.13
Semi-Major Axis7368.00
Associated / Clustered StarsTau1 Eridani
Tau3 Eridani
Tau5 Eridani
Tau6 Eridani
Tau7 Eridani
Tau8 Eridani
Tau9 Eridani

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

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


Calculated Effective Temperature3,754 Kelvin

Sources and Links


SIMBAD SourceLink

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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