Cygnus, the Swan
flies through the evening
skies of summer, and is one of the three constellations that host
the asterism of the Summer Triangle. This trio of first magnitude
stars is led by Vega, in the constellation Lyra, followed by Deneb
and Altair to complete the triangle. Deneb gets its name from
the Arabic word for “tail.” And indeed, it marks the
tail of the swan Cygnus as the fancied bird flies along the Milky
Way. The third vertex of the Summer Triangle, Altair, is in Aquila,
the eagle. Its name is Arabic, also meaning “eagle.”
Greek myth tells that
the gods rewarded Cycnus, brother of Phaethon, by placing him
in the sky as a swan. This was in light of Cycnus's devotion to
Phaethon after the foolhardy son of Apollo fell into the river
Po. You will remember the story of how Zeus struck the lad with
a thunderbolt to stop his mad, chaotic dash driving the chariot
of the Sun across the heavens. Cycnus repeatedly dove into the
river in an effort to return his brother for a proiper burial.
In so doing, he looked to the gods watching from Mount Olympus
much like a swan diving for food.
Greek legend also relates
that the swan is the form taken by Zeus to disguise his visits
to Leda, the wife of the king of Sparta, so that he would not
be recognized by his jealous wife Juno. Leda became the mother
of Helen of Troy and of the twins, Castor and Pollux.
Though the constellation
has been known as the Swan since ancient times, it has been at
various times and places thought of as the Hen, the Ibis, or simply
the Bird. In China and the Far East, it was associated with one
of the Magpies who went annually to make a bridge across the river
that was the Milky Way to allow the goddess to cross and meet
her lover. Now, officially, the constellation is Cygnus, the Swan,
while unofficially, to many it is the Northern Cross, really a
most appropriate name as well.
The bill of the Swan is
the very lovely double star Albireo, called by some the most beautiful
double star in the sky. If you wanst to excite your friends, show
them this fine example of astronomy's treasures with binoculars
or a small telescope. The pair of stars, gold and blue, is separated
by 35 seconds of arc and separated from us by 380 light years.
The individual stars shine at magnitudes of 3.1 and 5.1.
Many other double and
multiple stars are to be found among the stars of Cygnus, including
a noted one, 61 Cygni. The principal partner of this multiple
was, in 1838, the first star to have its parallax measured. The
determination showed that the star was only eleven light years
away. It is now counted among the 20 nearest stars to Earth. A
third companion, 61C, was inferred by its effect on the proper
motion of 61A. Accordingly, it may be a large planet of eight
Jupiter masses, or it may be a small, nonluminous star, a brown
dwarf. The A and B stars are red dwarfs of 5.2 and 6.0 magnitudes.
Worthy of note in this
part of the sky are two Messier objects, M29 and M39. Both are
open clusters of fairly bright stars, and they can be identified
with binoculars.
Conrad Jung has captured
an interesting scene in the central region of Cygnus. The yellowish-white
2nd-magnitude star, Gamma Cygni, about 750 light-years away and
nearly 6000 times more luminous than the Sun, is surrounded by
a network of bright emission and dark nebulae. Gamma Cygni, also
known as Sadr, is the central star in the Northern Cross asterism.
Moving from Deneb at the top of the Cross (the tail of the Swan),
to Albireo at the bottom (the bill of the Swan), one finds one
of the most celebrated double stars in the sky. If you want to
excite your friends, show them this fine example of astronomy's
treasures with binoculars or a small telescope. The stars, gold
and blue, shine at magnitudes of 3.1 and 5.1. They are separated
by 35 seconds of arc and separated from us by 300 light years.
Another beautiful object
in Cygnus is the North America Nebula. Although best appreciated
in astrophotographs, this nebula, NGC 7000, can be discerned with
large binoculars or a small telescope. The resemblance to a map
of the terrestrial continent is uncanny.