vendredi 12 septembre 2008

Cygnus 4

The Swan, is one of the more obvious asterisms in the summer skies, which -- because of its shape -- is sometimes called the Northern Cross.
Swans occur throughout the Greek myths; often one of the principal gods has occasion to transform himself into a swan, usually to seduce some attractive nymph or even a queen. Zeus, for example, felt he had a better chance with Leda, the King of Sparta's wife, should he turn himself briefly into a swan, just on her wedding night. The result was Pollux, half-brother of Castor.
The swan commemorated in the night skies, at least as far as the Greeks are concerned, isn't precisely known. It may be Cycnus, son of Poseidon.
Zeus had two brothers, Hades and Poseidon. When the three of them overthrew their father Cronus, they drew lots for their realm. Hades won the Underworld, Zeus the Heavens, and Poseidon the Seas.
Poseidon soon wished to dominate the earth as well, and contested Athene for the right. All the gods cast their vote, and Athene won by a single vote.
Cycnus, Poseidon's son, had been exposed at birth, lain out on the seashore to die. However, a swan took pity and flew down to care for the newborn.
Cycnus became the king of Colonae, a city north of Troy, but he wasn't a particularly good king. He set his own children adrift on the sea when his new wife fell in love with one of his sons (Tenes). He then killed his wife when he found she had lied to him.
Cycnus defended Troy when Achilles' onslaught. However in their individual struggle, Achilles proved too strong, as he choked the life out of Cycnus. Poseidon grieved for his son and turned him into a swan.
There are another three obscure Greek gods named Cycnus, all of which have something to do with swans. The Greeks always linked names with their like-sounding counterparts. The Greek word for swan is "kuknos" which was close enough to "cycnus" to explain its etymology.
Despite the myths, this constellation was known simply as "Ornis" (Bird) to the Greeks. It was the Romans who named it Cygnus and who adopted the Greek myths to explain its name.
The Arabs (and other cultures since then) saw the constellation as a hen.

The constellation is quite bright, with the stars being generally third and fourth magnitude.
Alpha Cygni is known as Deneb, from Al Dhanab al Dajajah (the Hen's Tail). It marks the tail of the swan.
This is a supergiant (more than a hundred times the diameter of the Sun) with a very high luminosity. Since it is so far away (3200 light years) its real brilliance is lost in space.
Beta Cygni is called Albireo, which is really a mistake. The words written in a sixteen-century edition of Ptolemy's Almagest, had been "ab ireo" (the meaning of which rests a mystery). The Arabs called it "Al Minhar al Dajajah", the Hen's Beak.
This is a magnificent binary with a nice colour contrast (see below).
Gamma Cygni is "Sadr", from Al Sadr al Dajajah, "The Hen's Breast". Between gamma and beta Cygni is the Cygnus Star Cloud, a vast region of exceptional beauty.
Epsilon Cygni is "Gienah", from Al Janah, "The Wing".

The constellation has several superb visual binaries as well as one of the more intriguing Mira-type variables. Several faint deep sky objects are also found in Cygnus, but it seems surprising that, while the constellation lies in the heart of the Milky Way, it has no truly outstanding clusters, nebulae, or galaxies.

Double stars in Cygnus:
Beta1 and beta2 form an extraordinary binary: gold and blue (or perhaps yellow and blue-green).
The component is quite wide, making it a popular object for binoculars.
AB: 3.1, 5.1; PA 54 degrees, separation 34.3".
Delta Cygni is a visual binary with an orbit of 828 years. Presently the values are: 2.9, 6.3; 224º, 2.5".
Mu Cygni is another visual binary (4.8, 6.1) with a long orbit, 789 years. For the next fifty years the orbit will continue to appear to approach the primary (as seen from the earth). The 2000.0 values are: 309º, 1.85".
Tau Cygni is a visual binary with a 49.9 year orbit: 3.9, 6.8. The 2000.0 year values are PA 328º, separation 0.8".
30 Cygni and 31 Cygni [omicron1] form a wonderful triple, suitable for binoculars:
AB: 4.0, 5.0; 333º and separation 338" (orange and turquoise). C: 7.0; 173º, separation 107" (blue).
61 Cygni is another fine binary of two orange stars: 5.2, 6.0. The 2000.0 values are PA 150º, and separation 30.3".
61 Cygni also holds the distinction of being the first star to have its parallax measured. This occurred in 1838, by Friedrich Wilhelm Bessel, a German astronomer.

Aucun commentaire: