Eridanus

By BrightK self-checked Published 2026-09-04 Updated 2026-10-18 Editorial policy

• Constellations
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What is Eridanus?

Constellations· Winter evenings (N) · a river winding from Rigel to the southern horizon. The river of the sky, winding from beside Rigel in Orion down past the southern horizon: the sixth-largest constellation. Achernar, at the river's end, spins so fast that it is among the most flattened stars known.

Stars of the figure

The 2 brightest of the 6 stars used in the figure (5 lines). Lower magnitude means brighter.

Deep-sky objects near this constellation

Constellation basics

Figures are representative approximations. Source: compiled from NASA and IAU public data.

The first white dwarf recognised — 40 Eridani B

Halfway along the river lies 40 Eridani, a 4th-magnitude star. To the eye it is one star, but it is really a group of three. William Herschel discovered component B in 1783. In 1910 an analysis of its light revealed something odd: it was hot enough to glow white, yet extremely faint.

A star that is hot but faint must be very small. That is how the first white dwarf was recognised. A white dwarf is the small, hot core left behind when a star like the Sun reaches the end of its life. 40 Eridani B packs about half the Sun's mass into a body roughly the size of Earth. The companion of Sirius had been discovered earlier, but it was confirmed as a white dwarf only in 1915.

40 Eridani is 16 light-years away. Star B sits well apart from the bright A star, which makes it one of the easiest white dwarfs to see in a small telescope. A small red star, C, orbits beside it. Books and films often make this system the home of alien civilisations, but for astronomy it matters more as the first specimen of a dead star.

The constellation in depth

Eridanus is the river into which Phaethon fell in Greek myth. Phaethon tried to drive the chariot of his father, the Sun god, lost control of the horses and scorched the Earth, until Zeus struck him with a thunderbolt and he fell into this river. People identified it with the Po in Italy or the Nile in Egypt. Cursa, the star at the river's source, means ‘footstool of Orion’.

Achernar, the star at the far end, is Arabic for ‘end of the river’. In ancient times the river stopped at the star now called Acamar; it was extended once the far southern sky became known. Achernar shines about three thousand times as brightly as the Sun and spins so fast that its equator moves at 250 km per second. As a result it is not round: its equator is 35 percent wider than its poles.

Epsilon Eridani is a close neighbour, 10.5 light-years away. It is a star of a similar kind to the Sun, but much younger. A Jupiter-like planet and a disk of dust have been found around it. Being so near and so Sun-like, it often turns up in science fiction.

On winter evenings, start at Cursa, just west of Rigel in Orion, and follow the faint stars southwest as the river bends back and forth. From mid-northern latitudes only the upper stretch is visible, and Achernar stays below the horizon. Set the Sky view to a January evening with constellation lines on to trace the channel, and move the observer to the southern hemisphere to follow it all the way to Achernar.

Frequently asked

When can I see Eridanus?

Eridanus is best seen in winter evenings, December–January; its southern end, with Achernar, never rises from London or New York.

How do I find Eridanus?

To find Eridanus, look for start at Cursa just west of Rigel and follow the faint bends southwest.

What is the brightest star in Eridanus?

The brightest star in Eridanus is Achernar, at magnitude 0.5.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.