jeudi 18 septembre 2008

Eridanus 5

Is a very long, winding river that starts at the left foot of Orion in the north, sweeps south of Taurus, west to the edge of Cetus, then doubles back east to Caelum, and eventually ends far to the south, at the border with Hydrus.
It's difficult now to identify which river the constellation represents. Some writers claimed it was the Tigris or Euphrates, others the Nile. Homer called it an "ocean stream". Since Roman times the river Po was even taken to be the locale.
In fact it may originally have been a mythic river, not meant to be identified with any geographical site.
In any event the story goes that Phaethon, son of Helios the Sun God, pestered his father to allow him to drive the celestial chariot across the skies one day. (The sun was seen as carried daily on a chariot driven by Helios.) Phaethon was encouraged by his sisters and even his own mother. But Helios always refused, knowing that his son was far from ready to assume such an awesome responsibility.
However under the constant pleading by Phaethon, Helios eventually relented. So one day Phaethon climbed into the chariot, drawn by two white horses. He grasped the reins and set off across the skies. But it was soon apparent that Phaethon was incapable of controlling the horses, which galloped so high in the sky that the earth was close to freezing, then plunged so close to the earth that the fields were burnt.
Zeus quickly had enough of this nonsense and sent a thunderbolt, killing the young driver. Phaethon fell into the sacred river Eridanus while his sisters, for having encouraged him in this foolhardy adventure, were changed into poplar trees which stood along its banks.

Eridanus' stars are fairly faint, except for alpha (Achernar, "the end of the river"). This bright star is only visible in latitudes south of 33 degrees, which eliminates most of North America and all of Europe.
Curiously theta Eridani bears nearly the same name, Acamar. This is a variation on Achernar since this star was once the river's terminus and it then bore the name now carried by alpha Eridani.

Double stars:
Theta1 and theta2 Eridani form an attractive pair: 3.4, 4.5; PA 88º, separation 8.2".
Omicron2 Eridani is an interesting triple system only 15.9 light years away.
AB form a very wide pair, with orbit of perhaps 8000-9000 years: 4.5, 9.7; PA 105º, separation 82.8".
BC form a close visual binary with orbit of 252 years. The primary is a white dwarf, with about twice the diameter of the earth. The component a red dwarf.
In fact omicronB is considered the easiest white dwarf for amateur telescopes. The component C has an extremely small mass, considered to be about 0.2 solar mass.
9.7, 10.8; presently PA 337º and 9.3" separation.
32 Eridani is another attractive pair, with colour contrast - yellow and blue: 5.0, 6.3; 347º, 6.9" separation.
32 Eri is near the northern boundary, 10º west of nu Eri (or about 10º north of gamma Eri).
p Eridani is a visual binary near the southern boundary of the constellation. It has an orbit of 483.7 years: 5.8, 5.8; currently the component is at PA 191º and separation 11.5".
The binary is found one degree north of Achernar (alpha Eri).

Variable stars:
Eridanus has a variety of variables, most of which are too small magnitude for amateur observation. Below are only a few examples.
Gamma Eri is an Lb type variable: 2.88-2.96.
Delta Eri is an RS CVn type variable: 3.51-3.56.
Lambda Eri is a beta Cep variable: 4.22-4.34, it has a period of 16h 50m.
Nu Eri is also a beta Cep variable: 3.4-3.6 with period of 4h 16m.

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