Achernar (Alpha Eridani): The Flattest Bright Star in the Sky

Achernar, Alpha Eridani (α Eri), is a hot blue main sequence star located approximately 139 light-years away in the constellation Eridanus (the River). With an apparent magnitude that varies between 0.40 and 0.46, it is the brightest star in Eridanus and the ninth brightest star in the sky. Largely invisible from the northern hemisphere, it marks the end of the celestial River.

Achernar forms a binary star system with a fainter companion, discovered in 2007. The bright star is notable for its unusually oblate shape, caused by the star’s exceptionally high rotational velocity. Resembling a spinning top more than a perfect sphere, the star’s shape was discovered in 2003. At the time, Achernar was the flattest star known.

What type of star is Achernar?

Achernar is a blue star of the spectral type B3-6Vpe. The spectral class indicates a chemically peculiar hot blue main sequence dwarf that is still fusing hydrogen into helium in its core. However, astronomers believe that Achernar has exhausted its supply of hydrogen and is now in the brief evolutionary phase that follows the main sequence turn-off. The star is estimated to have passed the turn-off around 70,000 years ago. It will soon begin fusing helium in a shell around the core.

Achernar has a mass six times that of the Sun and a radius of 6.19 – 9.16 solar radii. With an effective temperature of 12,673 – 17,124 K, it shines with a luminosity of 3,493 Suns. Achernar’s size and temperature are based on data obtained with the Very Large Telescope Interferometer (VLTI) in 2014. They are different at the equator and the poles because Achernar has an oblate shape. Its diameter and temperature vary by latitude.

Achernar (Alpha Eridani), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

A star spinning itself out of shape

Achernar is an exceptionally fast spinner. It has a projected rotational velocity of 250 km/s and takes 37.25 hours to complete a spin around its axis. In comparison, the Sun spins at 1.997 km/s and has an equatorial rotation period of about 25.7 days.

Achernar spins at about 95% of its critical rotation speed, the maximum speed at which it can spin before centrifugal force causes equatorial material to be ejected from the star. Below this limit, the outward centrifugal force is balanced by the star’s inward gravitational force.

Because of the rapid rotation rate, the star’s shape has been distorted into an oblate spheroid. The oblateness is common among rapidly rotating stars. Vega in the constellation Lyra, Altair in Aquila and Regulus in Leo are among the best known examples of this.

Model view of Achernar, based on the profile measured with the VLTI. Two different models are shown: in “A”, the polar axis is inclined 50° to the line-of-sight; in “B”, this angle is 90°. Image credit: ESO (CC BY 4.0)

The flattest first magnitude star

Achernar’s radius at the equator is 35% larger than its radius at the poles. This makes the luminary of Eridanus the flattest first magnitude star, rivalled only by Regulus.

Regulus, a B-type subgiant, is slightly less flat. Observations with the CHARA Array in 2005 showed that its equatorial radius was 32% larger than its polar radius. The hot blue star spins at 96.5% of its critical velocity.

Altair is a more modest example, with a 25% increase of the radius from pole to equator. Similarly, Vega’s equatorial radius is estimated to be 19% larger than its polar radius.

Achernar’s flattened shape was first observed by the European Southern Observatory’s Very Large Telescope (VLT) in 2003. The degree of flattening measured by the VLT Interferometer (VLTI) at the ESO Paranal Observatory was unprecedented at the time of discovery.

A team led by Domiciano de Souza et al. initially reported a flattening ratio of more than 3/2, which challenged theoretical models. In 2014, researchers refined the star’s physical parameters to an equatorial radius of 9.16 solar radii and an equatorial velocity of 298.8 km/s.

Achernar will not stay flat forever. Once it rapidly expands into a supergiant, the flattening will disappear.

Profile of the rapidly rotating star Achernar, as deduced from observations with the VLT Interferometer (VLTI). The size is indicated in units of 0.001 arcsec (milli-arcsec). Individual angular diameter measurements are indicated by pairs of small points with associated error bars on opposite sides of the centre. The fully drawn curve represents the best fitting ellipse. The ratio of the axes is 1.56 ± 0.05. The major axis of this ellipse is a measure of the “real” size of the star. Because of the projection effect, the minor axis shows the largest possible extension in the perpendicular direction. The axes ratio is therefore a minimal value; the star may be even more flattened than suggested by this ellipse. Image credit: ESO (CC BY 4.0)

Gravity darkening: Why the poles burn hotter

Achernar’s poles are inclined 65° to our line of sight. Because the star has a flattened shape, the poles are closer to the centre of mass and have a higher surface gravity than the equatorial region. Consequently, the poles also have a higher temperature and brightness. They are said to be “gravity brightened,” while the equator is “gravity darkened.”

In 2014, astronomers estimated an effective temperature of 17,124 K at the poles and 12,673 K at the equator for Achernar. They measured a polar radius of 6.78 solar radii and an equatorial radius of 9.16 solar radii.

A Be star with a circumstellar disk

Achernar is the nearest Be star to the Sun. The Be nature of Achernar was discovered by Andrews & Breger in 1966 through the detection of hydrogen lines in emission.

Be stars are non-supergiant B-type stars that show emission lines in their spectra. The emission lines come from circumstellar material expelled from the star because of its rapid rotation. The material forms a disk that varies in shape in size and occasionally disappears. The disk can be detected because it emits an infrared excess.

Like many other Be stars, Alpha Eridani shows small, but regular variations in brightness and belongs to a class of variable stars known as Lambda Eridani variables.

Lambda Eridani stars are Be stars that exhibit amplitude variations of only a few hundredths of a magnitude with periods that range from 0.5 to 2 days.

Achernar’s polar winds

The high temperatures at Achernar’s poles are creating a powerful polar wind which is expelling matter from the star’s surface. As a result, a shell of hot gas forms at the poles. An extended disk of ionized gas surrounds the entire star, seen in the star’s excess emission in infrared wavelengths.

Achernar’s polar wind was resolved interferometrically by Kervella & Domiciano de Souza in 2006 and Kervella et al. in 2009.

The Achernar binary system

Achernar has a faint companion, Alpha Eridani B, discovered in 2007.

The companion star was discovered by Kervella & Domiciano de Souza using the VLT spectrometer and imager for the mid-infrared (VISIR) in 2007. Follow-up observations indicated that Achernar B was most likely an A1V-A3V star separated by less than 0.15 arcseconds from Achernar. Initially, astronomers suspected that interaction between the two stars at their closest approach (periastron) may have caused the Be episodes in the primary component.

However, in 2022, Kervella et al. determined the system’s orbital parameters and found that there had not been any significant interaction between the components. This shows that Be stars can form through single-star evolution, without mass transfer in a binary system.

Achernar is the primary component of the binary system Alpha Eridani 140 ly away in the constellation of Eridanus. Credit: Pablo Carlos Budassi (CC BY-SA 4.0)

Orbital elements

The Alpha Eridani system has an orbital period of 7.0389 ± 0.0015 years. Alpha Eridani A and B have an orbital separation of 7.35 astronomical units (AU).

The system has an orbital eccentricity of 0.7258. The highly eccentric orbit brings the two components within 2 astronomical units of each other and takes them as far apart as 12.7 astronomical units.

Even though they come very close to each other at periastron, astronomers believe that mass transfer between the stars is unlikely.

Achernar B

Achernar B is an A2V-A3V-type star with a mass of 2.02 solar masses and a radius of 1.70 solar radii. A-type stars typically have a fast rotation rate so the star itself may be a fast spinner.

The companion is much smaller and cooler than Achernar A, but considerably larger and hotter than the Sun. It shines with 17.5 solar luminosities and has an effective temperature of 9,064 K.

Future evolution of Achernar

With six times the mass of the Sun, Achernar follows the evolutionary path of intermediate-mass stars. It is not massive enough to go out as a supernova when it reaches the end of its life cycle.

Astronomers estimate that Achernar will enter the instability strip in around 400,000 years. At this point, it will expand to a size of around 50 solar radii and become a classical Cepheid variable.

Classical Cepheids are young stars several times more massive than the Sun whose brightness varies due to pulsations. Their luminosity is directly related to their variation period, which makes them excellent standard candles, objects used to measure distance by comparing their luminosity to their apparent brightness. Relatively bright Cepheid variables include Polaris (the North Star) in Ursa Minor, l Carinae in Carina, Beta Doradus in Dorado, Eta Aquilae in Aquila, Mekbuda in Gemini, and the class prototype Delta Cephei in Cepheus.

After the first crossing of the instability strip, Achernar will continue to increase in size, reaching 120 solar radii in about 600,000 years. At this time, it may begin interacting with its binary companion at their closest approach. The interaction may modify both stars’ evolutionary paths.

The ninth brightest star in the sky

With an apparent magnitude of 0.40 – 0.46, Achernar is the ninth brightest star in the sky. It is only slightly fainter than Procyon in Canis Minor and a little brighter than Betelgeuse in Orion (most of the time) and Hadar in Centaurus.

Achernar is the brightest Be star in the sky and the second brightest B-type star, after Rigel. Like Rigel, it appears so bright because it is intrinsically luminous and not because it lies in the Sun’s neighbourhood like Alpha Centauri, Sirius, and Procyon.

Achernar is the brighter of the two stars in the constellation Eridanus selected for use navigation. The other one is Acamar, Theta Eridani. Both are listed among the 58 navigational stars. These bright stars have a special status in the field of celestial navigation because they are among the most recognizable stars in the sky.

Achernar and Acamar are among the 18 southern navigational stars with a declination between 30° S and 90° S. Other stars in this group are Rigil Kentaurus, Hadar and Menkent in the constellation Centaurus, Fomalhaut in Piscis Austrinus, Canopus, Avior and Miaplacidus in Carina, Kaus Australis in Sagittarius, Alnair in Grus, Suhail in Vela, Atria in Triangulum Australe, Shaula in Scorpius, Ankaa in Phoenix, Peacock in Pavo, and Acrux and Gacrux in Crux.

Facts

Alpha Eridani is the hottest and bluest of the ten brightest stars in the sky. It is the second brightest B-type star in the sky, after the supergiant Rigel in Orion. Several other first magnitude stars of the spectral type B – Hadar, Acrux, Spica, and Mimosa – are much hotter than Achernar and Rigel. They have surface temperatures in the range from 25,000 to 30,000 K. These massive, luminous stars appear fainter than Achernar because they lie at much greater distances.

In 2022, Kervella et al. suggested that Achernar is a member of the Tucana-Horologium moving group, a young stellar association that also includes the hot blue stars Peacock (Alpha Pavonis) in the constellation Pavo, Phi Eridani in Eridanus, Epsilon Hydri in Hydrus, and Eta Tucanae in Tucana.

The team identified a star comoving with Achernar, a low-mass red dwarf catalogued as 2MASS J01375879-5645447. The small, faint star was identified as a member of the Tucana-Horologium group. It lies 73,000 astronomical units from Achernar and is not a gravitationally bound companion. However, the two stars have very low relative velocities, indicating that Achernar too may be a member of the group. The association has an estimated age of 45 million years, which is comparable to Achernar’s age (63 million years).

Even if most northern observers will never see Achernar, many know the star from one of the many works of fiction that use or mention it. Notable uses include M. A. R. Barker’s Tékumel novels and games, Jack Vance’s novel The Eyes of the Overworld (1966), Edmond Hamilton’s Star Wolf (1967-1968) trilogy, and Joe Haldeman’s novel Mindbridge (1976).

Achernar, Canopus and Fomalhaut are usually identified as the Tre Facelle (three torches or three stars) mentioned in Dante’s Purgatorio. In the poem, the stars represent three virtues: faith, hope, and charity.

Achernar through history

Achernar’s location in the far southern sky made the bright star unknown to the Greeks and Romans. In ancient times, it was the fainter Acamar (Theta Eridani) that marked the end of Eridanus (the River).

Why Ptolemy never listed Achernar

Achernar is the only first-magnitude star not mentioned in Ptolemy’s Almagest. In antiquity, it was located even further south than it is today. Around 3400 BCE, its declination was 82° 40’, placing it within 7.5 degrees of the south celestial pole.

Around 1500 BCE, it was at declination -76°, invisible to ancient Egyptians. By Ptolemy’s time (100 CE) it moved to declination -67° and was still invisible from Alexandria, where Ptolemy lived.

Johann Bayer and the Age of Discovery

Achernar remained unknown in the northern hemisphere until the Age of Discovery. The first star catalogue to include the star on the map of Eridanus is Johann Bayer’s Uranometria (1603).

The German uranographer did not see Achernar himself, but based his depictions of the southern constellations on the observations of Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman, who had travelled to the East Indies in the late 16th century.

The name Achernar: The end of the river

The name Achernar (pronunciation: /ˈeɪkərnɑːr/) comes from the Arabic phrase ākhir an-nahr (Al Ahir al Nahr), meaning “the end of the river.” It was historically also spelled Acharnar, Acharnaar, Acharnarim, Acharnarin, Achironnahri, Acamar, and Enar.

The name was originally used for Theta Eridani, which now has the proper name Acamar, derived from the same phrase. Eridanus is one of the Greek constellations and, since the Greeks could not see Achernar, it was Acamar that marked the celestial River’s end.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) officially approved the name Achernar for Alpha Eridani A on June 30, 2016. The name applies only to the primary component but is informally used for the whole star system. The companion, Alpha Eridani B, is commonly referred to as Achernar B.

Chinese and Indigenous Australian names

The Chinese know Achernar as Shuǐ Wěi yī (水委一), or the First Star of Crooked Running Water. In traditional Chinese astronomy, Achernar formed an asterism known as Crooked Running Water with Wurren (Zeta Phoenicis) and Eta Phoenicis. The asterism was introduced in the early 17th century, after the arrival of western star maps.

The Boorong people of northwest Victoria in Australia called the star Yerrerdetkurrk. In local mythology, Yerrerdetkurrk was the mother-in-law of Totyarguil, represented by the constellation Aquila, who never let her son-in-law see her, heeding the local cultural taboo.

In the Wardaman culture, Alpha Eridani is known as Gawalyan (the echidna). Gawalyan was associated with Wurren (Little Fish), who gave it water from a great waterfall. In local ceremonies, water was carried by Earthly initiates in bowls and seen as symbolically transforming into clouds that bring the rains during the monsoon season. The ceremonies were held in late December, when Achernar and Wurren (Zeta Phoenicis) appear high above the horizon in the evening.

How to find Achernar

Achernar lies roughly halfway between Canopus in the constellation Carina and Fomalhaut in Piscis Austrinus. It appears near the constellation figure of Phoenix.

The star lies in the same region in sky as the somewhat fainter Alnair (Alpha Gruis), Ankaa (Alpha Phoenicis) and Alpha Tucanae, the brightest stars in the Southern Bird constellations Grus (the Crane), Phoenix and Tucana (the Toucan).

Achernar location, image: Stellarium (annotated for this article)

Where Achernar is visible

Achernar is located in the far southern sky and is best seen from the southern hemisphere. It never appears above the horizon from locations north of the latitude 33° N. The bright star is circumpolar (it never sets) for observers south of the latitude 33° S and can be seen throughout the year.

The best time of the year to observe Achernar is during the month of November, when it appears higher in the sky in the evening.

Using Achernar to find the south celestial pole

Achernar and the brighter Hadar (Beta Centauri) can be used to find the south celestial pole, which lies roughly halfway between the two stars.

The pole can also be located by drawing an imaginary equilateral triangle with Achernar and Canopus as two of the vertices. The third vertex of the triangle, located in the direction of the Southern Pointers (Alpha and Beta Centauri), is the south celestial pole. Another method is to draw a smaller triangle with the two Magellanic Clouds, also in the direction of the bright Alpha and Beta Centauri. The third corner will be the pole.

Sigma Octantis (Polaris Australis), the star that marks true south, is the closest visible star to the south celestial pole. However, shining at magnitude 5.47, it is barely visible even on a clear night and it is not part of any recognizable asterism that would make it more useful as a marker of the pole. For this reason, Achernar, Hadar and the stars of the Southern Cross are commonly used to find south instead.

Hadar, Achernar and south celestial pole, image: Stellarium (annotated for this article)

Achernar and the Magellanic Clouds

For observers south of latitude 33° N, Achernar is easy to find, both because it is very bright and because it lies in the same area of the sky as two bright galaxies, the Large and Small Magellanic Clouds.

Located in the constellations Dorado and Mensa, the Large Magellanic Cloud (LMC) is one of the closest galaxies to Earth and the fourth largest member of the Local Group, after the Andromeda Galaxy (M31), the Milky Way and the Triangulum Galaxy (M33). To observers south of the latitude 20° N, the galaxy appears as a faint cloud. In perfect observing conditions, it covers an area more than 20 times the diameter of the full Moon. With an apparent magnitude of 0.9, it is one of the brightest astronomical objects in the sky.

The Large Magellanic Cloud forms a pair with the Small Magellanic Cloud (SMC), another bright Milky Way satellite. With a visual magnitude of 2.7, the smaller galaxy is also brighter than most visible stars, but as its brightness is spread over an area of 2.5 by 5 degrees, it is not as easy to spot as the LMC from light-polluted areas. The SMC is located about 15 degrees below Achernar, while the LMC lies about 20 degrees to the east of the SMC. 47 Tucanae, the second brightest globular cluster in the sky (after Omega Centauri) can be seen next to the Small Magellanic Cloud.

One of the major enemies of astronomers is the Earth’s atmosphere, which makes celestial objects appear blurry when observed by ground-based telescopes. To counteract this, astronomers use a technique called adaptive optics, in which computer-controlled deformable mirrors are adjusted hundreds of times per second to correct for the distortion of the atmosphere. This spectacular image shows Yepun, the fourth 8.2-metre Unit Telescope of ESO’s Very Large Telescope (VLT) facility, launching a powerful yellow laser beam into the sky. The beam creates a glowing spot — an artificial star — in the Earth’s atmosphere by exciting a layer of sodium atoms at an altitude of 90 km. This Laser Guide Star (LGS) is part of the VLT’s adaptive optics system. The light coming back from the artificial star is used as a reference to control the deformable mirrors and remove the effects of atmospheric distortions, producing astronomical images almost as sharp as if the telescope were in space. Yepun’s laser is not the only thing glowing brightly in the sky. The Large and Small Magellanic Clouds can be seen, to the left and to the right of the laser beam, respectively. These nearby irregular dwarf galaxies are conspicuous objects in the southern hemisphere, and can be easily observed with the unaided eye. The prominent bright star to the left of the Large Magellanic Cloud is Canopus, the brightest star in the constellation Carina (The Keel), while the one towards the top-right of the image is Achernar, the brightest in the constellation Eridanus (The River). Image credit: ESO/B. Tafreshi (CC BY 4.0)

Constellation

Achernar lies at the southern end of the constellation Eridanus, marking the end of the River. The celestial River is represented by a string of faint stars that begins with Cursa (Beta Eridani) just slightly northwest of Rigel, at Orion’s foot. The northern end of the constellation lies on the celestial equator and is easily visible from the northern hemisphere, but the entire constellation can only be seen from locations south of the latitude 32° N.

Eridanus is the sixth largest constellation in the sky. Even though it stretches across 1,138 square degrees of the southern sky, it is not particularly prominent. It contains four stars brighter than magnitude 3.0 – Achernar, Cursa, Acamar, and Zaurak – but these stars are scattered across the constellation and do not form conspicuous asterisms.

Cursa (Beta Eridani), the constellation’s second brightest star, is a blue-white giant that shines at third magnitude from a distance of 90 light-years. Acamar (Theta Eridani) is a triple star system dominated by two A-type components, located 164 light-years away, and Zaurak (Gamma Eridani) is a variable red giant located 192 light years away.

Eridanus constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Eridanus is also home to Epsilon Eridani, known by the proper name Ran, the third nearest visible star or star system to the Sun (after Alpha Centauri and Sirius). With a visual magnitude of 3.736, Epsilon Eridani is visible even from areas with some light pollution.

Eridanus contains several notable deep sky objects. These include the reflection nebula IC 2118, also known as the Witch Head Nebula due to its striking appearance, the planetary nebula NGC 1535 (Cleopatra’s Eye Nebula), the spiral galaxy NGC 1232, the grand design spiral galaxy NGC 1300, and the ring galaxy NGC 1291.

The best time of the year to see the stars and deep sky objects in Eridanus is during the month of December, when the constellation appears higher above the horizon in the early evening.

The 10 brightest stars in Eridanus are Achernar (Alpha Eri, mag. 0.40 – 0.46), Cursa (Beta Eri, mag. 2.796), Acamar (Theta¹ Eri, mag. 2.88), Zaurak (Gamma Eri, mag. 2.88 – 2.96), Rana (Delta Eri, mag. 3.51 – 3.56), Upsilon⁴ Eridani (mag. 3.55), Phi Eridani (mag. 3.55), Chi Eridani (3.70), Tau⁴ Eridani (mag. 3.57 – 3.72), and Ran (Epsilon Eri, mag. 3.736).

Achernar – Alpha Eridani

Alpha Eridani B