Achernar
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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<ref name="IAU-LSN"/> of a binary system.<ref name=aaa484_1_L13/> 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,<ref name=aaa323_L49/><ref name=GSM/> this system is located at a distance of approximately Template:Convert from the Sun.<ref name=aaa474_2_653/>
Of the ten brightest stars in the night-time sky by apparent magnitude,<ref group="nb">The ten brightest stars in the nighttime sky in terms of apparent magnitude are, from brightest to dimmest, Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar, and Betelgeuse</ref> 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 Template:Val.
NomenclatureEdit
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).<ref name="planetnaming"/>
The system bears the traditional name of Achernar (sometimes spelled Achenar), derived from the Arabic {{#invoke:Lang|lang}} Template:Transliteration, meaning "The End of the River".<ref group="nb">In modern Arabic {{#invoke:Lang|lang}} means "down the river".<ref name="reverso_context"/></ref> 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.<ref name=allen/> 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.<ref name="IAU-LSN"/><ref name="WGSN"/><ref name="TriRpt18"/>
In Chinese caused by adaptation of the European Southern Hemisphere constellations into the Chinese system, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Crooked Running Water, refers to an asterism consisting of Achernar, ζ Phoenicis and η Phoenicis. Consequently, Achernar itself is known as {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref name=aeea/>
The indigenous Boorong people of northwestern Victoria, Australia, named it Yerrerdetkurrk.<ref name=hamacher/>
NamesakeEdit
USS Achernar (AKA-53) was a United States Navy attack cargo ship named after the star.<ref name=danfs/>
PropertiesEdit
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.
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.<ref name=kervella/> 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 Template:Val and their orbital period is 7 years.<ref name=aaa484_1_L13/>
The brightness of Achernar varies very slightly, by a maximum of 0.06 magnitudes or about 6%. A period of Template:Val is given in the General Catalogue of Variable Stars,<ref name=gcvs/> but several periods have been identified between about Template:Val and Template:Val. 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.<ref name=balona2020/>
As of 2015, Achernar was the least spherical star known in the Milky Way.<ref name=kong2015/> 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.<ref name=souza2014/> 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 Template:Val, while the equator is at Template:Val. The average temperature of the star is about Template:Val. 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,<ref name=aaa493_3_L53/> or by its polarization.<ref name=apj671_1_L49/> The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this.<ref name=apj671_1_L49/> 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.<ref name=cotton/> Template:Clear left
Co-moving companionEdit
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.<ref name=kervella/>
Historical visibilityEdit
Template:More citations needed section 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)<ref name="Stellarium"/> 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.
Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars farthest from any other, their nearest neighbors being each other. Antares is now the most isolated first-magnitude star. Antares is located in a constellation (Scorpius) with many bright second-magnitude stars, whereas the stars surrounding Alpha Eridani and Fomalhaut are considerably fainter.Template:Citation needed
The first star atlas to contain Achernar in the chart of Eridanus is Johann Bayer's Uranometria.<ref name=ridpath/> 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.<ref name=ridpath/>
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.Template:Citation needed