Achernar

Achernar

Template:Location mark Location of Achernar (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Eridanus
Pronunciation	Template:IPAc-en
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	0.40–0.46
Characteristics
Spectral type	B3 Vpe + A1V-A3V
U−B Template:Engvar	−0.66
B−V Template:Engvar	−0.16
Variable type	Be
Astrometry
Radial velocity (Rv)	+16 km/s
Proper motion (μ)	RA: 87.00±0.58 mas/yr Dec.: −38.24±0.50 mas/yr
Parallax (π)	23.39±0.57 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)
Orbit
Period (P)	yr
Semi-major axis (a)	" ()
Eccentricity (e)
Inclination (i)	°
Longitude of the node (Ω)	°
Periastron epoch (T)
Argument of periastron (ω) (secondary)	°
Details
A
Mass	6.0 Template:Solar mass
Radius	6.78 – 9.16 Template:Solar radius
Luminosity	Template:Solar luminosity
Surface gravity (log g)	2.772 – 3.561 cgs
Temperature	12,673 – 17,124 K
Rotational velocity (v sin i)	250 km/s
Age	63 Myr
B
Mass	Template:Solar mass
Radius	Template:Solar radius
Luminosity	Template:Solar luminosity
Temperature	K
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Template:Odlist
Database references
SIMBAD	data

Achernar is the brightest star in the constellation of Eridanus and the ninth-brightest in the night sky. It has the Bayer designation Alpha Eridani, which is Latinized from α Eridani and abbreviated Alpha Eri or α Eri. The name Achernar applies to the primary component of a binary system. The two components are designated Alpha Eridani A (the primary) and B (the secondary), with the latter known informally as Achernar B. As determined by the Hipparcos astrometry satellite, this system is located at a distance of approximately from the Sun.

Of the ten brightest stars in the night-time sky by apparent magnitude, Alpha Eridani is the hottest and bluest in color because it is spectral type B. Achernar has an unusually rapid rotational velocity, causing it to become oblate in shape. The secondary is smaller, is spectral type A, and orbits Achernar at a distance of .

Nomenclature

α Eridani (Latinised to Alpha Eridani) is the system's Bayer designation. The designations of the two components—Alpha Eridani A and B—derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).

The system bears the traditional name of Achernar (sometimes spelled Achenar), derived from the Arabic , meaning "The End of the River". However, it seems that this name originally referred to Theta Eridani instead, which latterly was known by the similar traditional name Acamar, with the same etymology. The IAU Working Group on Star Names (WGSN) approved the name with the spelling Achernar for the component Alpha Eridani A on 30 June 2016 and it is now so included in the List of IAU-approved Star Names.

In Chinese caused by adaptation of the European Southern Hemisphere constellations into the Chinese system, (), meaning Crooked Running Water, refers to an asterism consisting of Achernar, ζ Phoenicis and η Phoenicis. Consequently, Achernar itself is known as (, Template:Langx).

The indigenous Boorong people of northwestern Victoria, Australia, named it Yerrerdetkurrk. The Wardaman name is Gawalyan, the echidna.

Namesake

USS Achernar (AKA-53) was a United States Navy attack cargo ship named after the star.

Properties

Achernar is in the deep southern sky and never rises above the horizon north of 33°N, roughly the latitude of Dallas, Texas. It is best seen from the Southern Hemisphere in November; it is circumpolar south of 33°S, roughly the latitude of Santiago. At this latitude—e.g., the south coast of South Africa (Cape Town to Port Elizabeth)—when at lower culmination it is only 1 degree above the horizon. Further south, it is visible at all times during night.

File:AlphaEriLightCurve.png

A light curve for Alpha Eridani, plotted from TESS data, with the 1.263-day period listed in the GCVS shown in red

Achernar is a bright, blue star about six to seven times the mass of the Sun. It has a stellar classification of B6 Vep, but despite appearing similar to a main sequence star, it is thought to have recently exhausted the hydrogen in its core and begun to evolve away from the main sequence. It has expanded to an average radius eight times the Sun's and is about 3,000 times more luminous. Infrared observations of the star using an adaptive optics system on the Very Large Telescope show that it has a companion star in a close orbit. This appears to be an A-type star in the stellar classification range A0V–A3V, which suggests a stellar mass of about double that of the Sun. The separation of the two stars is and their orbital period is 7 years.

The brightness of Achernar varies very slightly, by a maximum of 0.06 magnitudes or about 6%. A period of is given in the General Catalogue of Variable Stars, but several periods have been identified between about and . The longest periods are very similar to the rotation period of the star, although the exact period appears to vary as the rotational velocity of its upper atmosphere changes. The shortest periods may be harmonics of the longer periods. The variability type of Achernar is given only as a Be star and the exact causes of the brightness changes are unknown. The star itself appears to pulsate and the disk around it varies in size and shape as well as apparently disappearing at times.

File:Achernar.svg

Extreme rotation speed has flattened Achernar.

As of 2015, Achernar was the least spherical star known in the Milky Way. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 35% greater than its polar diameter. The oblateness of Achernar is comparable to that of the dwarf planet Haumea, and the stars of Altair and Regulus. The polar axis is inclined about 60.6° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse.

Because of the distorted shape of this star, there is a significant temperature variation by latitude. At the pole, the temperature is , while the equator is at . The average temperature of the star is about . The high polar temperatures are generating a fast polar wind that is ejecting matter from the star, creating a polar envelope of hot gas and plasma. The entire star is surrounded by an extended envelope that can be detected by its excess infrared emission, or by its polarization. The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this. The disk is not stable and periodically collapses back into the star. The maximum polarization for Achernar's disk was observed in September 2014, and it is now decreasing.

Co-moving companion

The red dwarf 2MASS J01375879−5645447 lies about half a degree north of Achernar. It has been identified as being at the same distance and sharing a common proper motion, as well as being of about the same age. The projected separation of the two is slightly over one light year and they would not be gravitationally bound, but it is proposed that both are part of the Tucana-Horologium association.

Historical visibility

Precession caused Achernar to lie much further south in ancient times than at present: 7.5 degrees from the south celestial pole around 3400 BCE (declination Template:DEC) and still as far south as declination Template:DEC by around 1500 BCE. Hence the Ancient Egyptians could not have known it. Even in 100 CE, its declination was around Template:DEC, meaning Ptolemy could not possibly have seen it from Alexandria. However, it was visible from Syene in the time of the Almagest.

The first star atlas to contain Achernar in the chart of Eridanus is Johann Bayer's Uranometria. Bayer did not observe it himself, and the first European knowledge of it is attributed to Pieter Dirkszoon Keyser on the first voyage of the Dutch to the East Indies ("Eerste Schipvaart"). Thus it was the only first-magnitude star not listed in Ptolemy's Almagest.

Alpha Eridani will continue to move north in the next few millennia, reaching its maximum northern declination between the 8th and 11th millennia, when it will be visible as far north as Germany and southern England.