jeudi 4 septembre 2008

Cassiopeia 4

Cassiopeia was the wife of Cepheus, the Ethiopian king of Joppa (now known as Jaffa, in Israel), and the mother of Andromeda. The queen was both beautiful and vain, and the story of how her vanity caused great distress is told in relation to the constellation Andromeda.
After promising her daughter in marriage to Perseus, Cassiopeia had second thoughts. She convinced one of Poseidon's sons, Agenor, to disrupt the ceremony by claiming Andromeda for himself. Agenor arrived with an entire army, and a fierce struggle ensued.
In the battle Cassiopeia is said to have cried "Perseus must die". At any rate it was Perseus who was victorious, with the help of the Gorgon's head.
Perseus had recently slain Medusa, the Gorgon, and had put its head in a bed of coral. He retrieved the head and waved it in midst of the warring wedding party, instantly turning them all to stone. In the group was both Cepheus and Cassiopeia.
A contrite Poseidon put both father and mother in the heavens. But because of Cassiopeia's vanity, he placed her in a chair which revolves around the Pole Star, so half the time she's obliged to sit upside down.

The asterism clearly shows the chair upon which Cepheus's queen sits. The Bayer stars are generally third and fourth magnitude, with the exception of the first four stars which make up the "chair".
Cassiopeia has many fine binaries, a few variables of note, and several interesting deep sky objects.

Double stars:
Gamma Cassiopeiae has a faint companion, made doubly-difficult to see because of the brightness of the primary: 2.5, 11; PA 252º and separation 2.3".
Eta Cas is a fine binary with colour contrast, yellow and red. Some observers see them as more gold and purple.
The companion orbits every 480 years. Present values are: 3.4, 7.5, PA 315º, separation 12.7".
Lambda Cas has two nearly equal stars: 5.5, 6; PA 179º, 0.5".
Iota Cas is a triple system, with AB a visual binary with an orbit of 840 years.
AB: 4.6, 6.9; presently at PA 231º and separation 2.5". C: 8, PA 114º, 7.3".
Omicron Cas has a faint companion: 4, 11; 302º, 33.6".
Phi Cas is another multiple system, with rather wide components. The binary lies on the edge of NGC 457 (see below).
AB: 5, 12; 208º, separation of 48.6".C: 7; 231º, 134".
Sigma Cas: 5.0, 7.1; 326º, 3".
Struve 3062 is a visual binary with orbit of 106.8 years: 6.4, 7.5; presently 322º and separation 1.5".

Variable stars:
Beta Cas is a delta Sct: 2.25-2.31 with period of 0.104 days (2h 30m 11.5s).
Gamma Cas is a prototype of an important class of variable.
"Gamma Cas" variables are B stars, very rapidly rotating subgiants or even dwarfs with emission spectra. The variation in magnitude is typically quite small.
The biggest exception is gamma Cas itself, which has a range of 1.5 to 3.0 with a sporadic period, roughly every 0.7 days.
Other stars in this class include zeta Tau and BU Tau ("Pleione"), mu Cen, lambda Pav, and epsilon Cap.
Iota Cas is an alpha CVn type variable: 4.45 to 4.53 every 1.74 days.
Omicron Cas is a gamma Cas type variable, ranging from 4.5 to 4.62.
R Cas is a Mira type variable with a period of 430.46 days, ranging from 4.7 to 13.5. A maximum should occur in the last week of August in the year 2000.

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