Eridanus - The River

Eridanus is one of the largest constellations in the sky. It starts near the celestial equator and extends southwest of Orion. It winds under the feet of Orion, and then past the head of Cetus. It ends at the brilliant star Achernar.

History and Mythology

Eridanus was one of Ptolemy's original 44 constellations. It has been associated with the Nile and the Euphrates rivers since Biblical times. It is the second-longest constellation in the sky. According to the Aratos, this is the river created by Phaeton, the son of the sun god Helios. Legend tells us that Phaeton fell into the river Eridanus after his runaway chariot was struck by Jove's thunderbolt. His hair was on fire, and he plunged down from the heavens into the depths of the river, and was killed. Eridanus has also been called River of Heaven.

Notable Stars

Achernar, meaning "the river's end", is the brightest star in Eridanus, at magnitude 0.5. It is the 9th brightest star in the sky, but is not visible to observers in northern latitudes. Observers in southern Florida and Texas may be able to glimpse this blue giant star in October near the southern horizon. Achernar is a hot class B star 144 light years away, about 5,000 times the Sun's luminosity. It rotates rapidly, with a period of 2.2 days; this rapid rotation flattens it, and makes it the least spherical star studied in the Milky Way to date.

Cursa, meaning "foot stool of Orion", is a magnitude 2.8 star located 89 light-years away. Zaurak, "the bright star of the boat on the river", is a third magnitude star 220 light-years away. Theta Eridani, also called Acamar, "the last of the river", is a very nice double star, with white components of magnitude 3.2 and 4.4 separated by 8.3 arcseconds.

Epsilon Eridani is a 4th magnitude orange dwarf. While not particularly conspicuous, it is one of the nearest stars, only 10.5 light years away, that can be seen with the naked eye. Epsilon Eridani is believed to be a very young star, surrounded by a dust disk from which at least one Jupiter-sized planet has formed. It was one of the first stars for which an attempt to listen for signals from extraterrestrial intelligence was made (to date, none have been found, however).

Another nearby star system in Eridanus is 40 or Omicron Eridani. This is a triple star system, about 16 light years away. It consists of an orange dwarf, similar to Epsilon Eridani, orbited by a 11th-magnitude red dwarf and a 10th-magnitude white dwarf; the red and white dwarf appear 80 arcseconds away. The latter component is the easiest white dwarf to see in 8-inch and larger telescopes. In Star Trek, the 40 Eridani system is the home star system of the Vulcans.

Clusters, Nebulae, and Galaxies

IC 2118 is very large, extremely faint reflection nebula, also known as Witch Head Nebula due to its suggestive shape. It lies the east of Rigel in Orion, and appears 160 by 60 arcminutes in size. IC 2118 is an interstellar cloud of fine dust grains, about 900 light-years from Earth. It is illuminated by the nearby supergiant Rigel. Its bluish hue is caused not only by Rigel's color, but also because dust grains in the nebula reflect blue light more efficiently than they do red.

There are no prominent star clusters in Eridanus. NGC 1535 is a 9th magnitude planetary nebula nicknamed Cleopatra's Eye, located east of Zaurak. It appears somewhat blue in color, with a diameter of about 30 arcseconds.

Eridanus includes several faint galaxies visible in a medium size telescope. NGC 1300 is an 11th magnitude barred spiral galaxy. In larger telescopes, it has a visible bar passing through the nucleus and connected to a pair of prominent arms. It is a classic example of a barred spiral galaxy.

NGC 1232 is a spiral galaxy approximately 70 million light years away, and is part of the Eridanus cluster of galaxies, along with NGC 1300.