Universe Guide

Achernar (Alpha Eridani) Star Facts

Achernar Facts

Information on Achernar

Achernar Shape

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.

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.

Achernar's Alternative Names

Alpha Eridani (Alf Eri) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in 1603. The brightest star in the constellation is normally given the Alpha designation, there are exceptions such as Pollux which is Beta Geminorum.

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

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

Location of Achernar

The location of the main sequence 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 object is compared to the celestial equator and is expressed in degrees. For Achernar, the location is 01h 37m 42.75 and -57° 14` 12.0 .

Radial Velocity and Proper Motion of Achernar

Proper Motion

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 milliarcseconds/year towards the north and 87.00 ± 0.57 milliarcseconds/year east if we saw them in the horizon.

Radial Velocity

The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is 18.60 km/s with an error of about 3.00 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.

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 2,858.39 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 Achernar

Achernar Colour and Temperature

Achernar has a spectral type of B3Vp. This means the star is a blue main sequence star. The star has a B-V Colour Index of -0.15 which means the star's temperature is about 13,674 Kelvin. The temperature was calculated using information from Morgans @ Uni.edu at being .

Achernar Radius

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. 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 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. If you want that in miles, it is 96,441,377,416.

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.

Using the 2007 distance, the star is roughly 8,817,759.02 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 7,392.00 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*.

Travel Time to Achernar

The time it will take to travel to this star is dependent on how fast you are going. U.G. has done some calculations as to how long it will take going at differing speeds. A note about the calculations, when I'm talking about years, I'm talking non-leap years only (365 days).

The New Horizons space probe is the fastest probe that we've sent into space at the time of writing. Its primary mission was to visit Pluto which at the time of launch (2006), Pluto was still a planet.

DescriptionSpeed (m.p.h.)Time (years)
Airbus A380736127,061,805.59
Speed of Sound (Mach 1)767.269121,883,575.27
Concorde (Mach 2)1,534.5460,941,708.21
New Horizons Probe33,0002,833,863.30
Speed of Light670,616,629.00139.45

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.

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Additional Achernar Facts and Figures

Visual Facts

Primary / Proper / Traditional NameAchernar
Alternative NamesAlpha Eridani, Alf Eri, HD 10144, HIP 7588, HR 472, 2 G. Eridani
Spectral TypeB3Vp
Constellation's Main StarYes
Multiple Star SystemNo / Unknown
Star Type Main Sequence Dwarf Star
GalaxyMilky Way
Absolute Magnitude -2.77 / -2.70
Visual / Apparent Magnitude0.45
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)01h 37m 42.75
Declination (Dec.)-57° 14` 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 Distance from Earth23.39 Parallax (milliarcseconds)
 139.45 Light Years
 42.75 Parsecs
 8,817,759.02 Astronomical Units
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
Semi-Major Axis6734.00
Stellar Luminosity (Lsun)2,858.39
Brightest in Night Sky9th

Companions (Multi-Star and Exoplanets) Facts

Exoplanet CountNone/Unaware

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 Calculated Facts

Radius (x the Sun)5.77
Effective Temperature13,674 Kelvin

Sources and Links

SIMBAD SourceLink

Location of Achernar in Eridanus

Achernar Location in Eridanus

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

Eridanus Main Stars

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