Eridanus

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Eridanus is a constellation in the southern hemisphere. It is the sixth largest constellation in the sky. Its name is derived from Eridanos, the ancient Greek name for the river Po in northern Italy. The constellation was created by the Greek astronomer Ptolemy in the 2nd century.

Eridanus is associated with the myth of Phaëton, the son of the sun god Helios, who one day took the reins of his father’s chariot, but was not able to control it and ended up burning both heaven and earth. Zeus saved the day by killing him with a thunderbolt. The constellation Eridanos represents the path along which Phaëton drove. It is also said that, once cast to earth, his body was extinguished in the river Po. The celestial river originates near the star [5971] Rigel in Orion and ends at [3679] alpha Eridani, the brightest star in the constellation.

The constellation Eridanus occupies an area of 1138 square degrees and contains six stars with known planets. It can be seen at latitudes between +32° and -90° and is best visible at 9 p.m. during the month of December.

Despite being among the largest constellations, Eridanus does not have many bright stars or visible deep sky objects. The most notable stars are [3679] alpha Eridani or Achernar ("end of the river") and [3688] epsilon Eridani, a star similar to our Sun.

[3679] Achernar is a first magnitude star notable for its flat shape, caused by the star's rapid spinning. Its equatorial radius is 50 percent larger than the distance between the poles. Lying about 144 light-years away, Achernar is the ninth brightest star in the sky, and also the bluest and hottest among the 10 brightest stars in the sky.

[3688] epsilon Eridani, a main sequence star, is the third closest star that can be seen by the naked eye. It has at least one extrasolar planet, believed to be a gas giant similar to Jupiter. Being the nearest star similar to the Sun, only 10.5 light-years away, it is often featured in works of science fiction, both written and televised (the television series Babylon 5 and Space: Above and Beyond being among the best known examples).

Other notable stars in Eridanus include [3682] theta Eridani or Acamar ("the last of the river"), a binary pair of blue-white stars 120 light-years distant, [3681] gamma Eridani or Zaurak ("the boat"), a yellow star with an apparent magnitude of 2.95, and [3683] delta Eridani or Rana ("the frog"), a subgiant and suspected variable star only 29 light-years away from the Sun.

Eridanus also contains one of the largest supervoids (areas devoid of galaxies) ever discovered. The Eridanus Supervoid has a diameter of approximately one billion light-years across and has given rise to a number of theories related to the origins of the universe.

Eridanus is also home to several notable deep sky objects. The Witch Head Nebula, IC2118, lies close to [3680] beta Eridani and is thought to be either a gas cloud reflecting the light of [5971] Rigel, a supergiant star in the constellation Orion, or an ancient supernova remnant. The nebula is blue in colour and lies approximately 900 light-years away from Earth.

Other notable objects are NGC 1535, a planetary nebula that appears as a blue-white disc with two rings, and NGC 1234, a barred spiral galaxy with an apparent magnitude of 14.2.