Constellation Eridanus star map

Abbreviation: Eri

Genitive: Eridani

English: The River

Size Ranking: 6th

Area: 1,138 square degrees.

This long straggling constellation represents a river in Greek mythology. The River Eridanus features in the story of Phaethon, who fell into it after a disastrous attempt to drive the chariot of his father, the sun-god, Helios, and Jupiter was forced to strike him down with a thunderbolt. But it also represents a real river. Early mythologists identified it with the Nile, but later Greek writers said it was the river Po in Italy.

This extensive constellation, the sixth largest in the heavens, is often overlooked because of its faintness. It meanders from Taurus and Orion in the north to Hydrus and Tucana in the south.

The two brightest stars in Eri are positioned at the extremes of the constellation. In the north, adjacent to Rigel, βOri, is Cursa, βEri, while in the south resides Achernar, αEri, which, I suggest, can be readily located by taking a line from Sirius, αCMa, to Rigel, then ‘turning right’ through 90° and extending to meet αEri, such that the distance from Rigel to Achernar, is three times the distance from Sirius to Rigel.

Similarly, Acamar, θEri, is located by taking a line from Mirzam, βCMa, to Canopus, αCar, then turning left through, again, 90° and moving the same distance to θEri.


Features of Interest:

αEri, Achernar, ‘the river’s end’, magnitude 0.46, is a blue white star 144ly away.

βEri, Cursa, ‘the foot stool’, referring to its position under the foot of Orion, magnitude 2.79, is also a blue white star 89ly away.

εEri, magnitude 3.73, an orange star 10.5ly distant, is among the most Sun-like of the nearby stars. It is orbited every seven years by a planet of similar mass to Jupiter.

θEri, Acamar, 161ly away, is a striking pair of blue white stars of magnitudes 3.20 and 4.30.

ο2Eri, 16ly away, is a remarkable triple star consisting of a 4.40 magnitude orange primary, a star similar to the Sun, with a 9.50 magnitude white dwarf companion. This white dwarf has an 11th magnitude companion which is red dwarf, thereby completing an interesting trio. The white and red dwarfs orbit each other every 250 years, and will remain easily split until the end of this century.

32Eri, 290ly distant, is a commendable double star consisting of a yellow giant, magnitude 4.80, and a magnitude 6.10 blue green companion.

39Eri, 206ly away, is an orange giant , magnitude 4.90, with an 8th magnitude companion divisible in small telescopes.



1. Redshift 2 – Maris Multimedia

2. The Guiness Book of Astronomy – Patrick Moore

3. Stars and Planets – Ian Ridpath and Wil Tirion

4. Where the Stars Are – Orion.



NGC pair 1531 - 1532



The larger galaxy (NGC1532) is a spiral, and from our point of view within the Milky Way it is seen as nearly edge-on. Intense reddish star-forming regions spatter the edges of the dusty arms silhouetted in the foreground against the galactic disk. The three-dimensional nature of the galaxy is revealed by the background spiral arms dimmed by intervening gas and dust.

The smaller of the galactic pair (NGC1531) is dwarfed by its larger companion in much the same fashion as the Large Magellanic Cloud is by our Milky Way. Hints of interaction between these two galaxies are seen in at least two stray associations of stars and glowing red clumps of hydrogen gas. A warp in a background spiral arm of NGC1532 and a possible bridge of matter that connect the pair suggest continued influences between the two galaxies.

This pair is located about 55 million light-years away toward the southern constellation of Eridanus. It is part of a larger group that includes at least one other dwarf galaxy outside the field of this image. The mass of NGC1532 is estimated to be slightly greater than that of our Milky Way.