vendredi 12 septembre 2008

Cygnus 6

A popular name for Cygnus is the Northern Cross, and indeed its shape is far larger and more distinctive than the famous Southern Cross. In its cruciform shape the Greeks visualized the long neck, outstretched wings and stubby tail of a swan flying along the Milky Way, in which it is embedded. The mythographers tell us that the swan is Zeus in disguise, on his way to one of his innumerable love affairs, but his exact quarry is a subject of some disagreement.
The version of the tale that goes back to Eratosthenes says that Zeus one day took a fancy to the nymph Nemesis, who lived at Rhamnus, some way north-east of Athens. To escape his unwelcome advances she assumed the form of various animals, first jumping into a river, then fleeing across land before finally taking flight as a goose. Not to be outdone, Zeus pursued her through all these transformations, at each step turning himself into a larger and swifter animal, until he finally became a swan in which form he caught and raped her. Hyginus tells a similar story, but does not mention the metamorphoses of Nemesis. Rather, he says that Zeus pretended to be a swan escaping from an eagle and that Nemesis gave the swan sanctuary. Only after she had gone to sleep with the swan in her lap did she discover her mistake.
In both versions the outcome was that Nemesis produced an egg which was then given to Queen Leda of Sparta, some say by Hermes and others say by a passing shepherd who found the egg in a wood. Out of the egg hatched the beautiful Helen, later to become famous as Helen of Troy.
A simpler alternative says that Zeus seduced Leda in the form of a swan by the banks of the river Eurotas; with this story in mind, Germanicus Caesar refers to the swan as the ‘winged adulterer’. Leda was the wife of King Tyndareus of Sparta, which considerably complicated the outcome because she also slept with her husband later the same night.
According to one interpretation, she gave birth to a single egg from which hatched the twins Castor and Polydeuces as well as Helen. The shell of this egg was said to have been put on display at a temple in Sparta, hanging by ribbons from the ceiling. A rival account says that Leda produced two eggs, from one of which emerged Castor and Polydeuces while from the other came Helen and her sister Clytemnestra. To add to the confusion, Polydeuces and Helen were reputedly the children of Zeus, while Castor and Clytemnestra were fathered by Tyndareus. Castor and Polydeuces are commemorated by the constellation Gemini, where Polydeuces is better known to astronomers by his Latin name, Pollux.
Cygnus’s brightest star, Deneb, marks the tail of the swan; its name comes from dhanab, the Arabic word for ‘tail’. The Greeks had no name for this prominent star. Deneb is a highly luminous supergiant, over 3000 light years away, the most distant of all first-magnitude stars. It forms one corner of the so-called Summer Triangle of stars completed by Vega in the constellation Lyra and Altair in Aquila.
The beak of the swan is marked by a star named Albireo, revealed by small telescopes to be a beautiful coloured double star of green and amber, like a celestial traffic light. The German historian Paul Kunitzsch has traced the tortuous history of the name Albireo. It started with an Arabic translation of the Greek word for ‘bird’, ornis, the name by which both Aratus and Ptolemy knew the constellation. In the Middle Ages this Arabic name was mistranslated back into Latin, where it was described as ab ireo, meaning that it was thought to come from the name of a certain herb. This phrase was itself mistaken for an Arabic name and was rewritten as albireo. Hence the name Albireo, although it looks Arabic, is completely meaningless.
Cygnus lies in the Milky Way and hence contains many attractive star fields for sweeping with binoculars. Its most celebrated object cannot be seen by optical means at all: a black hole, called Cygnus X-1 because it is a strong source of X-rays, which lies halfway along the swan’s neck.

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