Drift Along in Eridanus, the River Constellation – Sky & Telescope

Eridanus is an impressive stellar pattern. It boasts the longest north-south extent of all the constellations, spanning from the celestial equator all the way to –57° declination. While it starts off wide, it gradually narrows as it makes its long, winding way south toward the pole. Mythology connects it in turn with the Nile River in Africa and the Po River in Italy, and it’s one of the original 48 constellations listed in Ptolemy’s famous work on astronomy and mathematics, the Almagest.

The Stars of Eridanus

Eridanus is anchored at its southern end by its brightest star system: Achernar, Alpha (α) Eridani, is a binary made of a hot, blue B-type star and a smaller A-type secondary orbiting about 7.5 astronomical units away from each other — if the primary were the Sun in our solar system, the secondary would be about halfway between Jupiter and Saturn. Achernar’s total magnitude is 0.4, making it the ninth-brightest star system in the night sky.

Interestingly, though, Achernar was not originally the southernmost point of the river nor did it always carry that name. Because ancient Northern Hemisphere astronomers could only see so far south, the star now known as Acamar stood at the limit of their view and so was called Achernar. Once European explorers began making their way further south, however, they spotted another bright star and it became the new southern end of the now-extended constellation. The old Achernar was renamed Acamar, and the new star became Achernar. (Ironically, both names derive from the Arabic for “river’s end.”)

By contrast with Achernar, Eridanus’ second-brightest star, 2.8-magnitude Beta (β) Eridani or Cursa, holds down the extreme northern end of the constellation, lying just 3° north-northwest of Rigel in Orion.

Between Cursa and Achernar, we find two groupings of stars, including — about a third of the way along from the northern end — nine stars that Johann Bayer named Tau (τ): τ¹ to τ⁹. He did something similar with Upsilon (υ), with superscripts denoting seven different stars, although only four now retain those designations (two are plotted on the chart above).

Of the Taus, τ⁴ Eri is worth a look, using apertures of 4.5 inches or greater in order to split its two stars. This double (possibly binary) system comprises an orange M3/4-class star that varies between about magnitude 3.6 to 3.7 and a whitish companion of magnitude 9.5.

The Galaxies of Eridanus

Turning to Eridanus’s deep-sky delights, we’ll take a look at three galaxies.

The first is NGC 1232, nicknamed the “Eye of God” Galaxy. This majestic, face-on, intermediate spiral has multiple, swirling arms visible in photographs and large telescopes. Located around 60 million light-years away, it has an apparent magnitude of around 10 and is therefore within reach of 4-inch telescopes. Don’t expect to see much with that aperture, though; it’ll look like a small, fuzzy, slightly elliptical blob.

NGC 1232 forms an almost a perfect right-angle triangle with τ³ Eri and τ⁴ Eri, with the galaxy at the right-angle vertex. There aren’t any useful stars with which to hop from either τ³ or τ⁴ to NGC 1232, so I’d suggest just starting at τ⁴ and begin heading in a straight line for τ¹ Eri in the north-northwest direction. About 2° 40’ along the way and a little to the north, you’ll find the galaxy.

Our next target, the barred spiral NGC 1300, isn’t too far away from NGC 1232, both on the sky and in terms of distance (61 million light-years). It forms an almost perfect equilateral triangle with NGC 1232 and τ⁴ Eri. Start at NGC 1232 and head northeast; if about halfway along (1° 24’), you encounter a magnitude-7.0 star, you’ll know you’re on the right track. Keep going another 1° 13’ and you’re there. Be warned, however — NGC 1300 looks nice in photos, but through the eyepiece of a 6-inch telescope it is an 11.4-magnitude oval fuzz with dimensions of around only 3’ × 2’. It requires much larger apertures to make out the bar and the spiral arms.

Another face-on galaxy example in Eridanus is NGC 1291, a 9th-magnitude ring galaxy located about 33 million light-years away. Photos show it to have a bright core, circled at quite some distance by a thin broken ring of stars and nebulosity.

To find it, start at Acamar (θ Eri) and head in a straight line directly east toward γ Eri. Once you’re halfway along, stop, and then just drop 40′ directly south and you’ll have 1291 in your field of view. You can just make out NGC 1291 with 10×50 binoculars if you use averted vision. It’ll look pretty much like a star but with perhaps a tiny bit of extended glow. A 6-inch telescope better reveals its non-stellar nature; you should be able to just make out the core, with the galaxy’s ring contributing to what looks like a small halo.

That’s it for our cosmic river cruise along the length of Eridanus. If you take some time to dive into its depths for its stars and galaxies, you won’t be disappointed.