Universe Guide


Achernar, Alpha Eridani, HD10144, HIP7588, HR472

Achernar (Alpha Eridani) is a blue eruptive main sequence dwarf star that can be located in the constellation of Eridanus. Achernar is the 10th brightest star in the night sky and 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.

Alpha Eridani is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR472. HIP7588 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD10144. The Id of the star in the Gould Star Catalogue is 2. 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. In Arabic, it is known as Akhir an-Nahr.

Achernar is one of those rare stars that is not entirely spherical like the majority of stars, in fact, its more egg-shaped. Other egg-shaped stars include Regulus and Vega. Its shape is caused by its spin and also by probably its companion star pulling on it. Below is an artists impression of the star. It is located at the southern end of its constellation and therefore is not visible by a lot of northern hemispheric countries. However, given its proper motion, one day it will be seen as far north as England. At the other end of the constellation is the second brightest star Cursa, Beta Eridani.

Artists impression of what Achernar looks like.

Location of Achernar

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 Achernar, the location is 01h 37m 42.75 and -57d14`12.0 .

Proper Motion of Achernar

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 -38.24 ± 0.43 miliarcseconds/year towards the north and 87.00 ± 0.57 miliarcseconds/year east if we saw them in the horizon.

Achernar 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 1060.0000000 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 Achernar

Achernar has a spectral type of B3Vp. This means the star is a blue main sequence dwarf star. The star is 7392.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24109.9943884800000000s. The star has a B-V Colour Index of -0.15 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 13,674 Kelvin.

Achernar Radius has been calculated as being 5.96 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 4,145,948.22.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 5.77. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Achernar Apparent and Absolute Magnitudes

Achernar has an apparent magnitude of 0.45 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 -2.77 If you used the 2007 Parallax value, you would get an absolute magnitude of -2.70. 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 Achernar

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 22.68 which gave the calculated distance to Achernar as 143.81 light years away from Earth or 44.09 parsecs. It would take a spaceship travelling at the speed of light, 143.81 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 23.39 which put Achernar at a distance of 139.45 light years or 42.75 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 7,392.00 Parsecs or 24,109.99 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 Achernar

The star is a eruptive 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. Achernar brightness ranges from a magnitude of 0.459 to a magnitude of 0.398 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.

Achernar Facts

Alternative Names

Traditional/Proper NameAchernar
Arabic NameAkhir an-Nahr
English MeaningEnd of the river
Bayer DesignationAlpha Eridani
Hipparcos Library I.D.7588
Yale Bright Star Catalogue (HR) Id472
Gould I.D.2
Henry Draper Designation10144

Visual Facts

Star Typemain sequence dwarf star
Absolute Magnitude-2.77 / -2.70
Visual / Apparent Magnitude0.45
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)01h 37m 42.75
Declination (Dec.)-57d14`12.0
Galactic Latitude-58.79 degrees
Galactic Longitude290.84 degrees
1997 Distance from Earth22.68 Parallax (milliarcseconds)
 143.81 Light Years
 44.09 Parsecs
2007 Revised Distance from Earth23.39 Parallax (milliarcseconds)
 139.45 Light Years
 42.75 Parsecs
Galacto-Centric Distance24,109.99 Light Years / 7,392.00 Parsecs
Proper Motion Dec.-38.24 ± 0.43 milliarcseconds/year
Proper Motion RA.87.00 ± 0.57 milliarcseconds/year
B-V Index-0.15
Radial Velocity18.60 ± 3.00 km/s
Spectral TypeB3Vp
Brightest in Night Sky10th
Colour(B) blue

Variable Star Details

Variable Star ClassEruptive
Variable Star TypeIrregular
Mean Variability Period in Days0.076
Variable Magnitude Range (Brighter - Dimmer)0.398 - 0.459

Estimated Facts

Luminosity (x the Sun)1,060.0000000
Calculated Effective Temperature13,674 Kelvin

Sources and Links

SIMBAD SourceLink

Related Stars

Location of Achernar in Eridanus

Achernar (Alpha Eridani) Location in Eridanus

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

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