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 constellation of the southern sky that is 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 Mesopotamia, for instance, Eridanus is believed to have depicted the river Tigris or Euphrates, while to the Egyptians 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.

Meteor Showers

There are at least three meteor showers associated with the constellation Eridanus, the most prominent being the Eta Eridanids, which runs from late July to August 14th, with its peak on the 8th/9th when up to 6 meteors an hour can be seen. The other two weak showers includes the Nu Eridanids which runs from August 30th to September 12th; and the Omicron Eridanids which occurs between November 1st to 10th when less than one meteor per hour may be observed.

Notable Stars

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

– 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 with a visual magnitude of 0.445. This hot blue star (B6 Vep) is about eight times as massive as our sun, but it is a spectacular 1,350 times more luminous.  It is also the main component of a binary system seperated by 12.3 astronomical units (AU), with the other companion star being twice as massive as the Sun, and having an orbital period of about 15 years. The name Achernar derives from the Arabic for “End of the River”, and is best seen during November in the southern hemisphere. The star never rises above 33 degrees north of the horizon, and never sets below 33 degrees south, meaning that far northern observers may never see it.

Star Constellation Facts: Eridanus– Cursa (Beta Eridani), the constellation’s second most luminous star, is a blue-white giant situated 89 light years from our solar system with an apparent magnitude of 2.796, although being a variable star this can vary from between 2.72 and 2.80. Beta Eridani has a high spin rate, with an estimated rotation of at least 196 km/second, compared to say 2 km/ second for the Sun.

The name Cursa derives from the Arabic Al Kursiyy al Jauzah, which roughly translates into “the chair of the central one,” which is taken to refer to a star association that it forms together with Lambda Eridani, Psi Eridani, and Tau Orionis in Orion.

– Zaurak (Gamma Eridani), the third most luminous star in Eridanus, is a red giant found 150 light year away. It is about 36 times larger than the Sun, and has a visual magnitude of 2.91 as seen from Earth. Zaurak derives from the Arabic for “the boat”.

– Acamar (Theta Eridani) is a binary system located about 120 light years from the Sun with a visual magnitude of 3.2. Its main component is an A4 star that is orbited by an A1 star. The name Acamar derives from the Arabic for “the end of the river” , which is actually marked by the star Achernar, but since it was not visible from ancient Greece, both Ptolemy and Hipparchus chose Acamar to denote the termination of Eridanus the River, instead.

Other stars of interest in Eridanus includes the blue-white subgiants Nu Eridani and Kappa Eridani; the yellow stars Chi Eridani, Theemim, and Beid; the orange stars Rana, Sadira, Azha, Sceptrum, and Angetenar; and the red giants Iota Eridani, Pi Eridani, and Upsilon-1 Eridani.

Notable Deep Sky Objects

Eridanus has no Messier Objects, but it does contain a number of notable, but faint, deep sky objects, including the following:

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

– NGC 1187 is a spiral galaxy located 60 million light-years away of magnitude 11.4. It is also the location of two recently observed supernovas, namely SN 1982R discovered in 1982, and SN 2007Y discovered in 2007.

– The Eridanus Supervoid (CBM Cold Spot/WMAP Cold Spot), or supervoids in general, are huge regions in space that are completely devoid of galaxies, and while several others are known, the Eridanus Supervoid stretches over about a billion light years, making 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) is a major grouping of around 200 galaxies found about 75 million light years from Earth, of which about 70% are spiral and irregular galaxies, and the remaining 30% are either irregular or lenticular (lens shaped). The Eridanus Group consists of several sub-groups, including one which contains around 31 galaxies, 9 of which are listed in the New General Catalogue (NGC), and 2 others in Index Catalogue (IC). This includes the bright elliptical galaxy NGC 1395; as well as the giant elliptical galaxy NGC 1407, and the spiral galaxy NGC 1332, both of which are surrounded by smaller sub-groups within the Eridanus Group.