Eridanus Constellation aka The River
Eridanus constellation is one of the earliest constellations recognized in the night sky. Ptolemy included it in his catalogue of 48 constellations in the second century and it remains one of the 88 modern constellations that we have today. While most constellations represent characters from mythology, animals, and things, Eridanus is a unique one. It depicts a river which is something from nature itself. And just like an overflowing river, it is long.
The celestial river is called srotaswini in Sanskrit, meaning “current,” “stream,” or “torrent.” Eridanus ranks as the sixth-largest constellation, having an area of 1,138 square degrees. The size of this constellation is reflected in one of its real-life river equivalents, the River Po. It is the longest river in Italy. Important rivers from different parts of the world are also associated with Eridanus like the Nile of Egypt, the Euphrates of Western Asia, the Rhine of Europe, and the Yellow River of China to name a few.
The constellation Eridanus belongs in the Heavenly Waters Family with eight other constellations. These constellations are Delphinus the Dolphin, Equuleus the Little Horse, Piscis Austrinus the Southern Fish, Columba the Dove, Pyxis the Compass, Carina the Keel, Puppis the Stern, and Vela the Sails.
The Heavenly Waters Family of constellations is grouped together because they all have water as a common theme. Most of them are not located near each other in the night sky. They are also not tied together by single mythology. The mythology related to Eridanus is the story of Phaeton (also Phaethon) and the sun chariot. The form and position of Eridanus in the night sky is said to be the path that Phaeton took when he drove the chariot. Others see Eridanus as the water poured by the Water Bearer Aquarius.
What Does Eridanus Look Like?
Eridanus forms a long winding river. By knowing what it looks like, we can easily see the connection of each star inside this constellation. It simply looks like this:
Where To See the Eridanus Constellation
Eridanus constellation is located in the Southern Sky or the southern celestial hemisphere. Specifically, it lies in the first quadrant (SQ1). It sits in 3.25-hour right ascension and -29o declination. It is near the celestial equator. From here on Earth, it is seen between latitudes +32o and -90o. Observers from places farther north than 32o cannot see it in the night sky.
Eridanus and Its Neighbors
We can easily see Eridanus using the constellations surrounding it. Since the celestial river flows across the night sky, it snakes through many constellations. It is bordered by Taurus the Bull, Orion the Hunter, Lepus the Hare, Caelum the Engraving Tool, Horologium the Clock, Tucana the Toucan, Hydrus the Male Water Snake, Phoenix the Phoenix, Fornax the Furnace, and Cetus the Sea Monster.
Another way to spot the celestial river is through Orion. It is one of the easiest constellations to identify because of the string of three stars forming Orion’s Belt asterism. We can see one end of Eridanus near Rigel, the brightest star in the Orion constellation. The southernmost part of Eridanus is marked by its brightest star, Achernar, near the Phoenix constellation.
When To See the Eridanus Constellation
Constellations can either be seasonal or circumpolar when we talk about the time we can see them in the night sky. What is the difference?
Seasonal constellations are constellations that we can only see some time of the year. Those we can see all year long are called circumpolar constellations.
The southern constellation of Eridanus is seasonal. Based on meteorological seasons, we can see it in the northern hemisphere in winter. During that time, it is summer in the southern hemisphere. We can see it best during the month of December at 9:00 pm.
Eridanus and Its Stars
Since Eridanus is one of the largest constellations in the night sky, it contains a lot of stars. These stars use the abbreviation “Eri” for short. Eridanus has 24 main stars, with only four of them considered as bright stars more visible than the fourth magnitude. But it does not make the celestial river any less interesting. Many stars in this constellation are known to have planets.
Also, the International Astronomical Union (IAU) has formally named 19 stars in the constellation. Some of these named stars are Ayeyarwady (HD 18742), Chaophraya (WASP-50), Koeia (HIP 12961), Montuno (WASP-79), Mouhoun (HD 30856), Sceptrum (53 Eridani A), Tojil (WASP-22), Theemin (υ2 Eridani) and Beemim (υ3 Eridani).
Let us dive into the stars that make up the celestial river!
1. Alpha Eridani (α Eridani)
Alpha Eridani is a binary star system. It has a hot blue appearance because of its primary component. It is located about 139 light-years away from our Sun. Achernar was the traditional name of the Alpha Eridani star system. Now, the name is formally designated to its primary, Alpha Eridani A.
● Achernar (α Eridani A)
Achernar is the brightest star in the entire constellation of the celestial river. Overall, it is also the ninth brightest star in the entire sky! It has an apparent magnitude of 0.40. The stellar classification of this main-sequence star is B6 Vep. It is one of the bluest stars in the night sky.
Achernar has a mass seven times that of our Sun’s and is more luminous than it by about 3,150 times. It has an oblate shape because it rotates rapidly. In fact, it is considered the flattest star known in the Milky Way galaxy. The unique shape of Achernar affects the distribution of temperature in different areas. It has an average temperature of roughly 15,000 K, but it is hotter at the pole than in its equator area.
The name Achernar has an Arabic origin. It is from ākhir an-nahr, which means “The End of the River”. This name is a description of its position in the southern end of Eridanus.
● Alpha Eridani B (α Eridani B)
Alpha Eridani B is also called Achernar B sometimes. It is an A-type star. It orbits the primary star, Achernar. They have an orbit period of about 14-15 years.
2. Cursa (β Eridani)
Cursa is the formal name of the second brightest star in the Eridanus constellation, Beta Eridani. We can see it in the naked eye because it has an apparent magnitude of 2.796. This star is not difficult to spot because it marks the northern end of the celestial river. Also, it lies close to the border of Orion the Hunter which is easy to identify.
This giant star has a stellar classification of A3 III. It has an estimated temperature of about 8,104 K. It is a rapid rotator, having a rotational velocity of 196km/second. Cursa has 2.0 solar mass and a radius 2.4 times that of the Sun’s. It is about 89 light-years away from our planet. A 10th magnitude optical companion was discovered with this star.
3. Zaurak (γ Eridani)
Gamma Eridani is formally named Zaurak. The name means “boat” in Arabic. Zaurak is an M-type giant that has already evolved. It is a variable star and its apparent magnitude varies between 2.88 to 2.96. We can see it in the naked eye. It is about 203 light-years away from our Sun.
4. Delta Eridani (δ Eridani)
Delta Eridani is traditionally called Rana. This traditional name has its origin in Latin, meaning “the frog”. Delta Eridani is a variable K-type subgiant that is about to become a red giant star. Its apparent magnitude varies between 3.51 to 3.56. It is about 29 light-years away from the Sun.
5. Ran (ε Eridani)
Ran is the formal name given to Epsilon Eridani. It is a relatively young star, estimated to be about 200 to 800 million years old. It has an apparent magnitude of 3.73 and a distance of about 10.5 light-years away. With that, it is considered one of the closest star or star systems that we can see with the naked eye. In fact, it ranks third after the brightest star, Sirius, and Alpha Centauri.
This is a K-type main-sequence star with a surface temperature of about 5,000K. Ran is smaller and less massive than the Sun but has more magnetic activity than it. The formal name of this star is from the goddess of the sea Rán in Norse Mythology. Even if its designation is Epsilon, the fifth letter of the Greek alphabet, it is actually the tenth brightest star in the constellation of the celestial river. This star was already recognized by astronomers as early as the 2nd century. It was the thirteenth star of the Eridanus constellation in the Almagest.
Ran has an exoplanet called Epsilon Eridani b. We can see an artist’s impression of it in the photo above. This Jupiter-mass planet was given the name Ægir, from the name of Rán’s husband in Norse Mythology. According to NASA, the Epsilon Eridani planetary system is just 10 light-years away, making it the closest planetary system to our solar system.
6. Zeta Eridani (ζ Eridani)
Zeta Eridani is a binary star system that belongs to a multiple star system called WDS J03158-0849. In this multiple star system, Zeta Eridani is the primary and designated “A” while 14 Eridani is the secondary star, designated “B”.
Zeta Eridani is a spectroscopic binary itself. It has an apparent magnitude of 4.80 and is about 110 light-years away from the Sun. Since Zeta Eridani is designated J03158-0849 A, its components are J03158-0849 Aa and J03158-0849 Ab.
● J03158-0849 Aa
J03158-0849 Aa is formally named Zibal. This was the traditional name of the Zeta Eridani star system. It is an A-type main-sequence star with strong absorption lines of metals. Zibal is said to be 800 million years old.
7. Azha (η Eridani)
Azha is the official name given to Eta Eridani. It is a suspected variable having an apparent magnitude between 3.81 to 3.90. No companion was discovered in this star. It is a red giant of the K3 spectral class. This star has already evolved and has the size ten times that of the Sun. It moves across the sky faster than the others. This b star is about 137 light-years from our Sun.
8. Theta Eridani (θ Eridani)
Theta Eridani is a binary star system. It is 120 light-years away from the Sun. Theta Eridani was traditionally named Acamar but the name is now formally designated to its primary component. The name Acamar is from the Arabic Ākhir an-nahr meaning “the end of the river”. Historically, it was thought that Theta Eridani is the end of the celestial river because the rest of its southern part is not visible from the rest of the northern hemisphere.
The two components of the Theta Eridani system are θ¹ and θ² Eridani. They have an angular separation of 8.3 arcseconds.
● θ¹ Eridani
θ¹ Eridani is formally named Acamar. This star belongs in the A4 spectral class. It has an apparent magnitude of +3.2.
● θ² Eridani
θ² Eridani is dimmer than its primary. It is also an A-type star having an apparent magnitude of +4.3.
9. Iota Eridani (ι Eridani)
Iota Eridani is one of the stars that make up the long outline of the celestial river. It is a K-type red giant star. It has a radius 11 times that of the Sun and shines at a brightness of about 57.5 Suns. We can see this solitary star in the unaided eye, having an apparent magnitude of 4.11. Iota Eridani is estimated to be four billion years old. It is about 151 light-years away from us.
10. Beid (ο¹ Eridani)
Beid is the official name given to Omicron¹ Eridani. The name is from the Arabic bayḍ which translates to “eggs” in English. It is a variable star having an apparent magnitude of 4.04 on average. It is a giant star of F0 III stellar classification. This star rotates rapidly, having a rotation period of only 1.9 days or less. Because of its fast projected rotational velocity, Beid has an equatorial bulge. It has twice the mass of our Sun and radiates at about 27 solar luminosities. The estimated distance of this star is 122 light-years from the Sun.
11. 40 Eridani (ο² Eridani)
Omicron² Eridani is better known as 40 Eridani. It is a triple star system that is less than 17 light-years away from our Sun. The components of this system are 40 Eridani A, B, and C.
● 40 Eridani A
40 Eridani A is formally named Keid, meaning “eggshells” in Arabic. It is a K-type main-sequence star. Eridani B and C orbit each other around this primary star.
○ 40 Eridani B
40 Eridani B is a white dwarf star. We cannot see it in the naked eye because it is a 9th magnitude star.
○ 40 Eridani C
40 Eridani C has a stellar classification of M4.5e. This red dwarf is an 11th magnitude star.
12. 1 Eridani (τ1 Eridani)
Tau1 Eridani is a spectroscopic binary having an orbital period of about 958 days. The primary is an F-type main-sequence. The apparent magnitude of this star is 4.46.
The system is 46 light-years away from our planet.
Nine stars and star systems share the Tau Eridani Bayer designation. They are designated τ1 Eridani to τ9 Eridani, encircled in the constellation chart above.
13. Angetenar (τ2 Eridani)
Angetenar is the standard name designated to Tau2 Eridani. It is a giant star that has already evolved. It has a stellar classification of K0 III. The apparent magnitude of this star is 4.78 so we can see it in the naked eye in a dark sky. It shines at a luminosity of 43 Suns. It is also more massive than our Sun by about 2.4 times. Angetenar lies about 187 light-years away from us.
The name Angetenar has an Arabic origin. It is from Al Ḥināyat an-Nahr which translated to “the Bend in the River”. This is a description of the star’s position in the Eridanus constellation.
Deep-sky Objects in Eridanus Constellation
There are plenty of interesting deep-sky objects in Eridanus. Though there are no Messier objects, it has many nebulae and galaxies.
NGC 1535 is a planetary nebula having an apparent magnitude of 10.5. It is also called Cleopatra’s Eye Nebula. We can see it using small telescopes. It was discovered by Willaim Herschel in 1785.
IC 2118 is also called Witch Head Nebula, which is a description of its amazing appearance. It is a reflection nebula illuminated by Rigel, a nearby supergiant star in the constellation Orion. Astronomers believe that IC 2118 is a remnant of a supernova from long ago. Carbon monoxide emissions in this nebula indicate star formation. It lies at a distance of 1,000 light-years from us.
The Eridanus Group, also called Eridanus Cloud, contains many interesting galaxies. Here are some of them:
NGC 1232 is an intermediate spiral galaxy, 60 million light-years away. There are many open clusters in its spiral arms that contain bright and blue stars. The size of this galaxy is about 200,000 light-years across. NGC 1232 has a satellite galaxy called NGC 1232A. We can see this smaller galaxy on the lower left side of the photo above.
NGC 1532 and NGC 1531
NGC 1532 is a barred spiral galaxy. It lies at a distance of about 55 million light-years in the Eridanus constellation. It is also called Haley’s Coronet. In the photo above, we can see the interaction of NGC 1532 (bigger) with the dwarf galaxy NGC 1531 (smaller). Because of their close distance, the two galaxies influence each other’s gravity. This gravitational influence has resulted in the formation of stars in NGC 1532.
NGC 1291 is a ring galaxy located 33 million light-years away. We can easily identify this galaxy because of its unusual inner bar and the prominent outer ring surrounding it. This galaxy is also called NGC 1269.
NGC 1300 is a barred spiral galaxy in the Eridanus constellation. It is 70 light-years away. It is more than 100,000 light-years across. There is a spiral region in the nucleus of NGC 1300 that spans 3,000 light-years. No black hole is discovered in this galaxy.
The Eridanus Supervoid is located at a distance of about 6 to 10 billion light-years. Remember that a void is a large empty area in space, meaning it does not have galaxies or anything. The Great Void of Eridanus is one of the largest voids ever known as it is said to be one billion light-years across.
There is still a lot of mystery surrounding the Eridanus Supervoid. One of the theories surrounding it speculates that the void is the product of a quantum entanglement involving our universe and another. The possibility is really endless!
Meteor Showers Related to Eridanus
Recent discoveries indicate that there are two meteor showers associated with the Eridanus constellation. Nu Eridanids radiate from Eridanus around August 30 and September 12 every year. Another meteor shower, Omicron Eridanids, peak between November 1 and 2.
Mythology Related to Eridanus Constellation
Phaeton was the son of the nymph Clymene and Helios the sun god. One day, he was challenged by Epaphus and other friends about his parents. He wanted to prove to them that his father is indeed the sun god. He asked Helios for evidence about their relationship. As a father trying to comfort his son, Helios assured Phaeton by telling him that he will grant anything he asked for.
Upon hearing this, Phaeton thought that the best way for him to prove his relationship to Helios is to drive the sun chariot. Helios hesitated with his son’s request. But he kept his promise, so he reminded Phaeton to be careful since the horses are difficult to control. He let Phaeton drive the sun chariot for a day.
Young Phaeton stepped into the chariot and took hold of the reins. Since the horses were more used to Helio’s weight, they thought that the chariot was empty and up they went into the sky, being totally out of control. The Earth caught on fire because the sun chariot has wreaked havoc! To prevent further damage, Zeus intervened and struck a thunderbolt straight to the chariot. Phaeton fell from the sky into the river Eridanus.
The Fall of Phaeton as a Recurring Theme
This myth has inspired creatives in many ways. Flemish artist Peter Paul Rubens’s famous painting called “The Fall of Phaeton” illustrated the mythology right at its climax, as seen above. We can see the horses getting out of control and Phaeton falling down the chariot.
The same theme also inspired Michelangelo Buonarroti in one of his artworks. Below is his piece using black chalk:
We can see three distinct parts of Michelangelo’s version. In the upper part is Zeus riding his eagle and is about to strike the lightning bolt towards the poor Phaeton. In the middle of the artwork, we can see Phaeton falling off the chariot. The third part of Michelangelo’s drawing shows many characters. The three women standing are the crying sisters of Phaeton. They are also called the Heliads. Their relative, Cycnus (Cygnus), was turned into a swan.
The character in the lower left is the god of the river where Phaeton fell, in River Eridanus. We can see a smaller water-bearer with him in between two of the sisters. Phaeton’s sisters were later turned into poplar trees near the banks of Eridanus.