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Cygnus, 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.
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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.
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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.
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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).
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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".
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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.
Variable stars in Cygnus:
Cygnus has many variable stars, most of which are too slight to notice
without high-tech equipment.
Alpha Cygni is the prototype for a variable class of pulsating
supergiants. These variables have a spectral type of A or B and very
high absolute magnitudes.
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Some fifteen supergiants are members of this group (including kappa Cas). The period ranges from five to ten days and the amplitude is less than 0.1 magnitude. For alpha Cygni, the range is 1.21 to 1.29.
Tau Cygni is a delta Scuti type variable, ranging from 3.65 to
3.75.
Upsilon Cygni is a gamma Cas type variable: 4.28-4.50.
Chi Cygni is by far the most interesting variable of the
constellation. This is a Mira-type variable with period of 408.05 days.
It takes several months to reach its maximum, then several more before
it disappears from sight, at a minimum of 14.2.
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The maximum varies, mostly it winds up in the 4.3-4.5 range, although it
has been known to achieve third-magnitude status. Burnham has a finder's
chart, but if it is in the fifth or fourth magnitude range, you should
have no difficulty in finding it: it has a bright red colour, and is
located about two degrees SW of eta Cygni (or about one quarter the
distance from eta to beta Cygni).
The star's next appearance should be in mid September of 1997. In 1998
it should reach its maximum in the last week of October.
Deep Sky Objects in Cygnus:
Cygnus contains two rather uninteresting Messier objects and some faint
and difficult nebulae:
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M29 (NGC 6913) is an open cluster, quite a lackluster Messier of
about half a dozen eighth magnitude stars shaped like a square. The
cluster is found 1.5 degrees south of gamma Cygni (and a couple of arc
mintues to the east).
M39 (NGC 7092) is large and scattered and equally unspectacular;
a group of faint (seventh magnitude) stars forming a rough triangle.
It is nine degrees ENE of alpha Cygni.
NCC 7000 is called "North American Nebula" because of its shape. It's
a bright slightly greenish emission nebula.
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The nebula is described in most references as "bright" but in fact you'll
find it is extremely faint. It is best seen in binoculars, and is found between alpha Cygni and
xi Cygni.
The Veil Nebula West (NGC 6960) and The Veil Nebula East (NGC
6992/95) are fine filaments seemingly suspended in the cosmos. It
takes quite a large scope, perfect conditions, and plenty of patience
to appreciate their delicate lines.
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The nebulae are 2.5 to 3 degrees south of epsilon Cygni. The star
52 Cygni is in the same field as the western segment, and is the best
starting point to search for the elusive nebulae.
52 Cygni is three degrees due south of epsilon Cygni (and it's a
binary as well, Struve 2726: 4,9; 67º, 6.6").
Cygnus A is the second brightest source in the 'radio sky', after
the supernova remnant known as Cas A. This pecularly-shaped galaxy is
considered to be a billion light years distant, and is an object of
intense investigation. Two lobes of radio emission are fed by jets of
energetic particles from the galaxy core. (I thank Philip Blanco for this
description. Philip has a web page devoted to Cygnus A. Those interested
just set their search engine to 'Cygnus A'.)
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Cygnus A is found in a highly nebulous region of the constellation,
about three and a half degrees west of gamma Cyg.
For a closer appreciation of Cygnus, visit the Binocular Section.
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