Achernar is the brightest
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
of
Eridanus, and the
ninth-brightest in the night sky. It has the
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...
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
''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
'' astrometry
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
His ...
satellite,[ this system is located at a distance of approximately from the Sun.][
Of the ten apparent brightest stars in the night-time sky, Alpha Eridani is the hottest and bluest in color, due to Achernar being of spectral type B. Achernar has an unusually rapid ]rotational velocity
In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
, causing it to become oblate
In Christianity (especially in the Roman Catholic, Orthodox, Anglican and Methodist traditions), an oblate is a person who is specifically dedicated to God or to God's service.
Oblates are individuals, either laypersons or clergy, normally livi ...
in shape. The secondary is smaller, of spectral type A, and orbits Achernar at a distance of .
Nomenclature
''α Eridani'' ( Latinised to ''Alpha Eridani'') is the system's Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...
. 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 system
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...
s, 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
Theta Eridani, Latinized from θ Eridani, is a binary system in the constellation of Eridanus with a combined apparent magnitude of 2.88. Its two components are designated θ1 Eridani, formally named Acamar (the traditional name of th ...
instead, which latterly was known by the similar traditional name ''Acamar'', with the same etymology.[ The ]IAU Working Group on Star Names
The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...
(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
Chinese can refer to:
* Something related to China
* Chinese people, people of Chinese nationality, citizenship, and/or ethnicity
**''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation
** List of ethnic groups in China, people of va ...
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 (, en, the First Star of Crooked Running Water).[
The indigenous ]Boorong
The Wergaia or Werrigia people are an Aboriginal Australian group in the Mallee and Wimmera regions of north-Western Victoria, made up of a number of clans. The people were also known as the Maligundidj (in the Wotjobaluk language) which means ...
people of northwestern Victoria, Australia, named it ''Yerrerdetkurrk''.[
]
Namesake
USS ''Achernar'' (AKA-53) was a United States Navy attack cargo ship.
Properties
Achernar is in the deep southern sky and never rises above the horizon beyond 33°N, roughly the latitude of Dallas, Texas. It is best seen from the Southern Hemisphere in November; it is circumpolar above (i.e. south of) 33°S, roughly the latitude of Santiago. On this latitude, e.g. the south coast of South Africa ( Cape Town to Port Elizabeth
Gqeberha (), formerly Port Elizabeth and colloquially often referred to as P.E., is a major seaport and the most populous city in the Eastern Cape province of South Africa. It is the seat of the Nelson Mandela Bay Metropolitan Municipality, Sou ...
) when in lower culmination it is barely visible to the naked eye as it is only 1 degree above the horizon, but still circumpolar. Further south, it is well visible at all times during night.
Achernar is a bright, blue star with about six times the mass of the Sun
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
. It has a stellar classification
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
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.][
]
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
A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circ ...
with an equatorial diameter 35% greater than its polar diameter. 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. A similar situation occurs for the star ]Regulus
Regulus is the brightest object in the constellation Leo and one of the brightest stars in the night sky. It has the Bayer designation designated α Leonis, which is Latinized to Alpha Leonis, and abbreviated Alpha Leo or α Leo. Re ...
.
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 decretes 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
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
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
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outsid ...
, 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
The Tucana-Horologium association (Tuc-Hor), or Tucana Horologium moving group, is a stellar association with an age of 45 ± 4 Myr and it is one of the largest stellar associations within . The association has a similar size to the Beta Pictoris ...
.[
]
Historical visibility
Due to precession, Achernar lay much farther south in ancient times than at present, being 7.5 degrees off the south pole around 3400 BCE (declination ) and still lying at declination by around 1500 BCE. Hence the Ancient Egyptians could not have known it. Even in 100 CE its declination was around , meaning Ptolemy could not possibly have seen it from Alexandria—whereas Theta Eridani was visible as far north as Crete. So, Ptolemy's "End of the River" was certainly Theta Eridani. Alpha Eridani was not visible from Alexandria until about 1600.
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, although Antares is located in a constellation ( Scorpius) with many bright second-magnitude stars, whereas the stars surrounding Alpha Eridani and Fomalhaut are considerably fainter.
The first star catalogue to contain Achernar in the chart of Eridanus is Johann Bayer
Johann Bayer (1572 – 7 March 1625) was a German lawyer and uranographer (celestial cartographer). He was born in Rain, Lower Bavaria, in 1572. At twenty, in 1592 he began his study of philosophy and law at the University of Ingolstadt, a ...
's '' Uranometria''.[ Bayer did not observe it himself, and it is attributed to ]Pieter Dirkszoon Keyser
Pieter Dirkszoon Keyser (occasionally Petrus Theodorus; – 11 September 1596) was a Dutch navigator and celestial cartographer who mapped several constellations on the southern celestial hemisphere.
Voyages and star observation
Little is ...
and the first voyages of the Dutch to the East Indies ("Eerste Schipvaart
The First Dutch Expedition to East Indies (Dutch: ''Eerste Schipvaart'') was an expedition that took place from 1595 to 1597. It was instrumental in opening up the Indonesian spice trade to the merchants that eventually formed the Dutch E ...
"). Thus it was the only first-magnitude star not listed in Ptolemy's ''Almagest
The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it canoni ...
''.[
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.
]
Notes
References
Further reading
*
{{DEFAULTSORT:Achernar
B-type main-sequence stars
Be stars
Eridanus (constellation)
Eridani, Alpha
0472
CD-57 00334
010144
007588
Southern pole stars
Achernar
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 A ...