Eridanus constellation is located in the southern hemisphere. Its name is the Ancient Greek name for the river Po in Italy.
Eridanus represents the celestial river. In Sanskrit, it is called srotaswini, which means “stream,” “current,” or “torrent.” The constellation is associated with the Greek myth of Phaëton and usually depicted as a river flowing from the waters poured by Aquarius.
Eridanus was first catalogued by the Greek astronomer Ptolemy in the 2nd century. Notable features in the constellation include Achernar, one of the brightest stars in the sky, the nearby star Epsilon Eridani, the Eridanus Supervoid, the largest void known, the Eridanus Group of galaxies, and the Witch Head Nebula, a well-known reflection nebula located in the constellation.
FACTS, LOCATION & MAP
Eridanus is the sixth largest constellation in the night sky, occupying an area of 1138 square degrees. It is located in the first quadrant of the southern hemisphere (SQ1) and can be seen at latitudes between +32° and -90°. The neighboring constellations are Caelum, Cetus, Fornax, Horologium, Hydrus, Lepus, Orion, Phoenix, Taurus and Tucana.
Eridanus contains seven stars with known planets and no Messier objects. The brightest star in the constellation is Achernar, Alpha Eridani, with an apparent magnitude of 0.445. Achernar is also the ninth brightest star in the sky. There are no meteor showers associated with the constellation.
In Greek mythology, the constellation is associated with the story of Phaëton (or Phaëthon), the son of the Sun god Helios and the Oceanid Clymene. Phaëton wanted to drive his father’s chariot across the sky and kept begging for Helios’ permission to do so until the god agreed, advising Phaëton to follow the beaten track where he saw wheel marks.
Phaëton mounted the chariot and the horses, sensing that the driver was lighter, flew upwards into the sky, leaving the familiar track behind. The inexperienced driver could not control the horses and the reins slipped from his hands. The chariot plunged so close to the Earth that lands caught fire. It is said that this was how Libya became a desert, Ethiopians got dark skin, and the seas dried up. Zeus saw what was going on and had to intervene to prevent further disaster. He struck Phaëton down with a thunderbolt and, when Phaëton’s hair caught on fire, he leapt from the chariot and fell into the Eridanus. His father Helios, stricken with grief, did not drive his chariot for days, leaving the world in darkness.
The Greek poet Aratus called the constellation Eridanus, while a number of other sources including Ptolemy referred to it as Potamos, which means “the river.” Eratosthenes associated the celestial river with the Nile, the only river that runs from south to north. Hyginus, a Latin author, agreed, pointing out that the bright star Canopus in the constellation Carina was at the end of the river much like the island Canopus lies at the mouth of the great river in Egypt. The actual constellation, however, represents a river that runs from north to south. Later, the river came to be identified as the river Po in Italy by Greek and Latin authors.
The name Eridanus, according to a theory, comes from the name of a Babylonian constellation known as the Star of Eridu (MUL.NUN.KI). Eridu was a Babylonian city held sacred to the god Enki-Ea. Enki-Ea was the ruler of the cosmic domain of the Abyss, usually imagined as a reservoir of fresh water below the Earth’s surface.
MAJOR STARS IN ERIDANUS
Achernar – α Eridani (Alpha Eridani)
Achernar, Alpha Eridani, is the brightest star in Eridanus constellation and the ninth brightest star in the night sky. It is located in the southern corner of the constellation. The star has an apparent magnitude of 0.445 and is approximately 139 light years distant from Earth. It is the hottest, bluest star among the ten brightest stars in the sky. It never rises above the horizon north of 33°N and the best time to observe it in the southern hemisphere is November. The star never sets below the horizon south of 33°S.
Alpha Eridani has the stellar classification B6 Vep. It is a main sequence star six to eight times more massive than the Sun and about 3,150 times more luminous. It has a class A star twice as massive as the Sun as a companion in close orbit. The two stars are separated by about 12.3 astronomical units and orbit each other with a period of 14-15 years. The star is classified as a Lambda Eridani type variable.
Achernar is a very fast rotator. As a consequence, it is the least spherical star studied in the Milky Way galaxy. It has the shape of an oblate spheroid, and its diameter at the equator is 56% greater than its polar diameter. The star has a circumstellar disk of gas, also a consequence of fast spinning.
The name Achernar is derived from the Arabic ākhir an-nahr, which means “The End of the River.”
Cursa – β Eridani (Beta Eridani)
Beta Eridani is a giant star belonging to the spectral class A3 III. It is the second brightest star in the constellation, with an apparent magnitude of 2.796. It lies approximately 89 light years from the solar system and can be found near the border with Orion. The star has a visual companion with an apparent magnitude of 10.90, located 120 arc seconds away.
Like Achernar, Beta Eridani is a fast spinner, with a projected rotational velocity of 196 km/s. The star is classified as a variable. Its visual magnitude changes from magnitude 2.72 to 2.80.
Beta Eridani’s traditional name, Cursa, comes from the Arabic phrase Al Kursiyy al Jauzah, which means “the chair of the central one,” and refers to a star association that also includes Lambda Eridani, Psi Eridani, and Tau Orionis in Orion constellation.
Acamar – θ Eridani (Theta Eridani)
Acamar, Theta Eridani, is a binary star that is possibly a part of a multiple star system. The primary component in the system belongs to the spectral class A4 and has a visual magnitude of 3.2. The secondary component is a class A1 star with an apparent magnitude of 4.3. The two stars are separated by 8.3 seconds of arc.
The star’s traditional name, Acamar, comes from the Arabic Ākhir an-nahr, which means “the end of the river.” Acamar used to represent the end of the celestial river, Eridanus. Now it is the brighter star Achernar that holds that distinction. The stars’ names share the same etymology. Since Achernar is not visible from Greece, Acamar was chosen to mark the river’s end by Hipparchus and Ptolemy.
Acamar has an apparent magnitude of 3.2 and is about 161 light years distant from the Sun.
Zaurak – γ Eridani (Gamma Eridani)
Gamma Eridani has the stellar classification M1IIIb. It has an apparent magnitude of 2.95 and is approximately 150 light years distant from the solar system.
The star’s proper name, Zaurak, means “the boat” in Arabic.
Rana – δ Eridani (Delta Eridani)
Delta Eridani is a subgiant star of the spectral type K0 IV. It has a visual magnitude of 3.54 and is only 29.49 light years distant. Its traditional name, Rana, means “the frog” in Latin.
τ4 Eridani (Tau-4 Eridani)
Tau-4 Eridani is a binary star with the stellar classification M3.5III. It has an apparent magnitude of 3.70 and is approximately 258 light years distant. The primary component in the system is a class M giant star with a visual magnitude of 3.66.
ε Eridani (Epsilon Eridani)
Epsilon Eridani is the ninth closest star to the Sun, the third closest individual star or star system that can be seen without binoculars, and it used to be the closest star with a confirmed planet in its orbit until Alpha Centauri Bb was discovered in the Alpha Centauri system in Centaurus.
Epsilon Eridani is believed to about 440 million years old, or at least somewhere between 200 and 800 million years. It has a large outer debris disk.
The star is difficult to observe without binoculars from urban areas because of the light pollution.
As one of the nearest stars similar to the Sun, Epsilon Eridani has been the target of SETI (search for extraterrestrial intelligence) searches and often appears in works of science fiction.
Epsilon Eridani has an apparent magnitude of 3.736 and is only 10.489 light years distant from Earth.
It is a main sequence star that belongs to the spectral class K2V. It is smaller and less massive than the Sun, with 82% the Sun’s mass and 74% of the solar radius.
A giant planet, Epsilon Eridani b, was announced in 2000. It orbits the star with a period of seven years.
Epsilon Eridani is classified as a BY Draconis variable, with variations in luminosity caused by regions of higher magnetic activity moving in and out of sight as the star rotates.
Keid – 40 Eridani
40 Eridani, or Omicron-2 Eridani, is a triple star in the Eridanus constellation. The primary component, 40 Eridani A, is a main sequence dwarf belonging to the spectral class K1. It has an apparent magnitude of 4.43 and can be seen without binoculars.
The pair 40 Eridani b and 40 Eridani C are a white (spectral class DA4) and red dwarf (M4.5e) that have visual magnitudes of 9.52 and 11.17. They were discovered by William Herschel on January 31, 1783. The 11th magnitude red dwarf, 40 Eridani C, is a flare star and also has the designation DY Eridani.
The traditional name for 40 Eridani, Keid, comes from the Arabic word qayd, which means “(egg) shells.”
Of note to Star Trek fans: the Vulcan home world, the planet Vulcan, was mentioned in the original series (in “The City on the Edge of Forever”) as orbiting the star Alnitak in Orion constellation, but later in the script-adaptation anthology Star Trek II, author and literary critic James Blish placed the planet in the orbit of 40 Eridani A, a location Gene Roddenberry later accepted.
82 G. Eridani (HD 20794, e Eridani)
82 G. Eridani is a main sequence star belonging to the spectral class G8V. It has an apparent magnitude of 4.254 and is only 19.71 light years distant from the solar system.
The star is smaller and less massive than the Sun, as well as older. Its estimated age is between 6 to 12 billion years. It is a high velocity star, which means that it is moving quickly compared to the average speed of nearby stars.
Three planets were discovered orbiting the star on August 17, 2011. The planets have only a few times the Earth’s mass and are classified as super-Earths.
EF Eridani is an AM Herculis type variable, or a polar, which is a type of cataclysmic variable binary system that has a very strong magnetic field. It is located approximately 300 light years from Earth. The system’s visual magnitude varies between 14.5 and 17.3.
EF Eridani consists of a white dwarf star and a former star with a substellar mass orbiting the white dwarf. The substellar mass used to be a star which then lost all of its gas to the white dwarf. Now it is a ball with only 0.05 solar masses.
DEEP SKY OBJECTS IN ERIDANUS
Witch Head Nebula – IC 2118
The Witch Head Nebula is a faint reflection nebula in Eridanus. It is thought to be a remnant of an ancient supernova or possibly a gas cloud illuminated by the bright star Rigel in the neighbouring Orion constellation.
The Witch Head Nebula had a notable mention in the TV show Andromeda as the site of the last major battle between the Commonwealth and the Nietzschean Alliance.
The nebula has an apparent magnitude of 13 and is approximately 1,000 light years distant from the solar system.
Eridanus Group (Eridanus Cloud)
The Eridanus Group, also known as the Eridanus Cloud, is a group of galaxies roughly 75 million light years distant in Eridanus. The group consists of about 200 galaxies, and about 70% of them are categorized as spiral and irregular galaxies. The other 30% are lenticular and elliptical galaxies. The brightest galaxy in the Eridanus Cloud is NGC 1407.
The galaxy group has several subgroups. The Eridanus Group (the term here applies to a subgroup) has 31 member galaxies, nine of which are listed in the New General Catalogue (NGC) and two in the Index Catalogue (IC). The brightest member is the elliptical galaxy NGC 1395. The other two subgroups are centred on the galaxies NGC 1407 (giant elliptical galaxy) and NGC 1332 (spiral galaxy).
NGC 1300 is a barred spiral galaxy, about 110,000 light years across, with a visual magnitude of 11.4. It is approximately 61.3 million light years distant. The galaxy belongs to the Eridanus cluster of galaxies.
The central region of the galaxy has a grand design spiral structure some 3,300 light years long, a spiral within a spiral.
NGC 1332 is a spiral galaxy discovered by William Herschel on December 9, 1784. It has an apparent magnitude of 10.3.
NGC 1395 is a bright, large elliptical galaxy in Eridanus. It has a visual magnitude of 9.8.
The galaxy was discovered by William Herschel on November 17, 1784.
NGC 1232 is an intermediate spiral galaxy. It has an apparent magnitude of 10.9 and is approximately 61 million light years distant. It is a member of the Eridanus cluster.
The nearby NGC 1232A is a satellite galaxy of NGC 1232. It is believed to be the cause of the unusual shape of the larger galaxy’s spiral arms.
NGC 1234 is another barred spiral galaxy. It has an apparent magnitude of 15.3.
The galaxy was discovered by the American astronomer Francis Preserved Leavenworth in 1886.
NGC 1535 is a planetary nebula in Eridanus. It is similar in structure and colour to the Eskimo Nebula in the constellation Gemini, except for the fact that the central star in NGC 1535 is hard to observe.
The nebula is approximately 1,500 light years distant.
NGC 1531 and NGC 1532
NGC 1531 is a dwarf galaxy in Eridanus. It has a visual magnitude of 12.9. The galaxy is interacting with the larger spiral galaxy NGC 1532.
NGC 1532 is a barred spiral galaxy seen edge-on. It has an apparent magnitude of 10.7. It has several dwarf companion galaxies and is clearly interacting with one of them, NGC 1531.
Eridanus Supervoid (CBM Cold Spot/WMAP Cold Spot)
The Eridanus Supervoid is the largest supervoid ever discovered. (A supervoid is an area of space that contains no galaxies.) It is about one billion light years in diameter.
Current theories of the origins of the universe cannot explain the supervoid, but it has been speculated that the supervoid may be the result of quantum entanglement between our universe and another.
The Eridanus Supervoid was discovered by linking a cold spot in the cosmic microwave background (CMB) to an absence of radio galaxies. Cosmic microwave background radiation refers to thermal radiation that fills the entire observable universe almost uniformly.
The CMB Cold Spot is a region of the universe seen in microwaves that is exceptionally large and cold relative to the average CMB radiation. A cold spot as large as the Eridanus Supervoid seems very unlikely in the accepted theoretical models.
NGC 1427A is an irregular galaxy in Eridanus. It has an apparent magnitude of 13.4 and is approximately 51.9 million light years distant.
NGC 1427A is the brightest dwarf irregular galaxy in the Fornax Cluster of galaxies, and lies in front NGC 1399, the central galaxy in the cluster, located in Fornax constellation.
NGC 1309 is another spiral galaxy in Eridanus, seen face-on, and spanning some 30,000 light years. The galaxy is approximately 120 million light years distant from the solar system.
A Type Ia supernova, SN 2002fk, was observed in the galaxy in September 2002.
The galaxy belongs to the Eridanus Group. It has an apparent magnitude of 12.0.
NGC 1291, sometimes also referred to as NGC 1269, is a ring galaxy in Eridanus. It is notable for the unusual structure of its outer ring and inner bar.
The galaxy was discovered by the Scottish astronomer James Dunlop in 1826, and included in the New General Catalogue as NGC 1291. John Herschel independently discovered the galaxy in 1836 and entered it as NGC 1269, not realizing that it already had a NGC number.
NGC 1291 is approximately 33 million light years distant. It has an apparent magnitude of 9.39.
NGC 1187 is another spiral galaxy in Eridanus, seen almost face-on. It has a visual magnitude of 11.4 and is approximately 60 million light years distant from the solar system. It was discovered by William Herschel in 1784.
The galaxy was the site of two observed supernova explosions in the last few decades. One (SN 1982R) was discovered in October 1982 and the other (SN 2007Y) in 2007.