Achernar (Alpha Eridani) is a blue eruptive main sequence dwarf star that can be located in the constellation of Eridanus. The description is based on the spectral class. Achernar is the 9th 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.
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.
Alpha Eridani (Alf Eri) 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 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 star alternative names can be found at Star Names .
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 Achernar, the location is 01h 37m 42.75 and -57° 14` 12.0 .
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. The Radial Velocity, that is the speed at which the star is moving away/towards us is 18.60000 km/s with an error of about 3.00 km/s .
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.
Achernar has a spectral type of B3Vp. This means the star is a blue main sequence dwarf star. The star is 7,392.00 Parsecs from the Galactic Centre or terms of Light Years is 24,109.99 s. 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. 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 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.
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*.
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).
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.
|Alternative Names||Alpha Eridani, Alf Eri, HD 10144, HIP 7588, HR 472, 2 G. Eridani|
|Multiple Star System||No / Unknown|
|Star Type||main sequence Dwarf Star|
|Absolute Magnitude||-2.77 / -2.70|
|Visual / Apparent Magnitude||0.45|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||01h 37m 42.75|
|Declination (Dec.)||-57° 14` 12.0|
|Galactic Latitude||-58.79 degrees|
|Galactic Longitude||290.84 degrees|
|1997 Distance from Earth||22.68 Parallax (milliarcseconds)|
|143.81 Light Years|
|2007 Distance from Earth||23.39 Parallax (milliarcseconds)|
|139.45 Light Years|
|Galacto-Centric Distance||24,109.99 Light Years / 7,392.00 Parsecs|
|Proper Motion Dec.||-38.24 ± 0.43 milliarcseconds/year|
|Proper Motion RA.||87.00 ± 0.57 milliarcseconds/year|
|Radial Velocity||18.60 ± 3.00 km/s|
|Stellar Luminosity (Lsun)||2,858.39|
|Brightest in Night Sky||9th|
|Variable Star Class||Eruptive|
|Variable Star Type||Irregular|
|Mean Variability Period in Days||0.076|
|Variable Magnitude Range (Brighter - Dimmer)||0.398 - 0.459|
|Calculated Effective Temperature||13,674 Kelvin|
The map was generated using Night Vision, an awesome free application by Brian Simpson.