Star Constellation Facts: Eridanus

Star Facts: Achernar
Eridanus photographed above Sinai seashore in northeast Egypt by Stephan Seip

Eridanus, like the river it depicts, is a long winding constellation that starts in the northern sky near Rigel in Orion, before eventually winding its way far south close to Hydrus. It is the 6th largest of the 88 constellations, taking up an area of 1,138 square degrees of the night sky, and is a southern hemisphere constellation visible for observers situated between latitudes +32° and -90°.


The “Celestial River”, as Eridanus is known, has represented many different rivers to various civilizations over the centuries. In early times, Eridanus is believed to have depicted the river Tigris or Euphrates, while to others it was seen as the Nile. By Greek and Roman times, the constellation referred to the river Po (“Eridanos” in Greek), Italy’s longest river which runs 405 miles (652 km) from northwestern Italy to the Adriatic Sea on the east. Similarly, in classical Sanskrit, the constellation was known as Srotaswini, which roughly translates into “stream,” “current,” or even “torrent.”

Heavenly Waters Family of Constellations

Eridanus is part of the ‘Heavenly Waters’ family of constellations, together with Carina, Columba, Delphinus, Equuleus, Piscis Austrinus, Puppis, Pyxis, and Vela. Eridanus also borders on the constellations Orion, Lepus, Taurus, Cetus, Fornax, Caelum, Horologium, Phoenix, Tucana, and Hydrus,

Notable Stars

The constellation Eridanus contains no Messier objects, but it does have some notable stars and deep sky objects, among which is Achernar (9th most luminous stars in the sky), seven known stars with planets, and the mysterious Eridanus Supervoid, an empty region of space that defies explanation.

Star Constellation Facts: Eridanus– Achernar (Alpha Eridani), located about 139 light years away, is the most luminous star in the constellation, and the ninth most luminous in the entire sky (0.445 mag). Classed as B6 Vep, Alpha Eridani is about eight times as massive as the Sun, but it is a spectacular 1,350 times as luminous. It is also one component of a binary system, the other companion star being twice as massive as the Sun, and having an orbital period of about 15 years, and a separation of only about 12.3 astronomical units (AU). The name Achernar derives from the Arabic akhir an-nahr, which translates into “End of the River”, and is best time seen during November in the southern hemisphere. It never rises above 33 degrees north of the horizon, and never sets below 33 degrees south of the horizon, which means that far northern observers may never see it.

– Cursa (Beta Eridani), located about 89 light years away, is the second most luminous star in the constellation, and has an average apparent magnitude of 2.796, although being a variable star its magnitude varies between 2.72 and 2.80. Classed as a A3 III star, Beta Eridani is orbited by a visual companion with an apparent magnitude of 10.90. Beta Eridani also has a high spin rate, with an estimated rotation of at least 196 km/second. The name, Cursa, derives from the Arabic Al Kursiyy al Jauzah, which roughly translates into “the chair of the central one,” that is taken to refer to a star association that among others, include Lambda Eridani, Psi Eridani, and Tau Orionis in Orion.

– Acamar (Theta Eridani), located about 161 light years away, and with a magnitude of 3.2, is a binary system consisting of an A4 star  that is orbited by a A1 star. The name, Acamar derives from the Arabic Akhir an-nahr, which means “the end of the River”, but since Achernar is not visible from Greece, both Ptolemy and Hipparchus chose Acamar to denote the termination of Eridanus, the River in the Heavens.

Notable Deep Sky Objects

Eridanus Supervoid (CBM Cold Spot/WMAP Cold Spot), supervoids are huge regions in space that are completely devoid of galaxies, and while several others are known, the Eridanus Supervoid stretching over about a one billion light years makes it the biggest yet discovered. Since the cosmic microwave background radiation is relatively uniform, with only very slight variations in temperature, the void was exposed by linking the discovery of a huge cold spot with the observed absence of radio galaxies. At present, there are no explanations as to how such a huge area of space could be empty, but some investigators are inclining to the controversial idea that the area represents a type of “quantum entanglement” between our Universe and another, neighboring universe.

In simple terms, the area is thought by some to be a “point of contact” between different universes, and despite the best efforts of dozens of investigators, no amount of computer modelling has ever been able to explain the void in terms of any of the current, accepted theoretical models of the origin of the universe.

Eridanus Group (Eridanus Cloud); also known as the Eridanus Cloud, this major grouping of around 200 galaxies is about 75 million light years away. Of the group, about 70% are spiral and irregular galaxies, while the remaining 30% are either irregular, or lenticular (lens shaped) galaxies. However, the Eridanus Cloud is a much larger structure that consists of several sub-groups, of which the Eridanus Group is but one, with only 31 galaxies. Nine of the 31 members are listed in the NGC, while two others are listed in the IC, or Index catalogue. The most luminous member of the Eridanus Group is NGC 1395, a large elliptical galaxy. Two other prominent members of the Eridanus Group are the giant elliptical galaxy NGC 1407, and spiral galaxy NGC 1332, both of which are surrounded by smaller sub-groups within the Eridanus Group.

Witch Head Nebula (IC 2118); with an apparent visual magnitude of 13, and about 1,000 light years away, the Witch Head Nebula is an exceedingly faint reflection nebula that is assumed to be associated with the star Rigel, in the neighboring constellation of Orion. It is also suspected that the nebula is the remains of a very old supernova, but there is no certainty on either notion.

NGC 1187; while Eridanus contains several prominent galaxies, the 11.4 magnitude spiral galaxy NGC 1187 is the location of two recently observed supernovas. The first, SN 1982R, was discovered in October of 1982, and he second, SN 2007Y, during 2007.