Costellazione Cigno, stelle principali e mitologia
La costellazione del Cigno è una delle più imponenti fra le 48 descritte da Tolomeo. Alcune delle stelle che la compongono sono molti brillanti e visibili ad occhio nudo, soprattutto d'estate. La figura del cigno, che si può stilizzare anche con la forma di una croce, si svolge lungo la Via Lattea ed è quindi ricca si elementi molto luminosi.
The constellation Cygnus: main stars
The animal’s tail is formed by the blue giant Deneb (Alpha Cigni), an extremely bright star that is easily visible despite its great distance from Earth (1800 light years). It has been calculated that if the star were as distant from us as Sirius, its brightness would be almost equal to that of the Moon. Deneb is 60,000 times brighter than our Sun. At the other end of the constellation Cygnus, representing the head of the animal, is Albireo (Beta Cigni). This is one of the best-known double stars, because the two celestial bodies are easily distinguishable with simple binoculars. The brightest star in Cygnus after Deneb is Sadr (Gamma Cigni), from which the animal’s wings unfold and, to the south, the Milky Way which constitutes its body.
Among the other stars that form the constellation of Cygnus there are 61 Cygnus, one of the closest stars to Earth (11.4 light years) and one of the first whose distance it was possible to measure, and the 16 Cigni complex. The latter is a double star made up of two orange stars, easily visible with a medium-power telescope. The star 16 Cigni B has a planetary system with a planet similar to our Jupiter. Several other stars belonging to the constellation have planetary systems, of which the best known is probably that of Gliese777. The latter is a fairly close red dwarf (about 52 light years) with 2 planets, one of which has a rocky conformation and a size between that of Jupiter and Earth.
The constellation Cygnus: mythology
Several myths are linked to the constellation Cygnus, generally connected to characters transformed into birds. Among them is Cycnus, one of Poseidon’s many sons. The latter was one of Priam’s allied kings during the Trojan War. After his death in battle, which occurred at the hands of Achilles, his father was so saddened that he decided to bring him back to life in the form of a swan.
According to another legend, Zeus fell in love with the nymph Nemesis, who did not return his attentions. This she managed to escape her god by transforming into different animals until he, in the form of a swan, managed to capture her. The nymph then gave birth to an egg, which she gave to Leda, queen of Sparta. When she hatched from her egg, the twins Castor and Pollux came out and, according to some versions, also Helen of Troy.
The names of the main stars, such as Deneb and Albiero, are of Arabic origin and indicate the various parts of the bird. The first, which indicates the star on the swan’s tail, means “tail”, while the second has a bizarre etymology. The term in fact derives from a medieval incorrect translation from Arabic and therefore, in fact, the word Albireo means nothing. Sadr, on the other hand, which constitutes the central part of the constellation, means “bust”.
The constellation Cygnus also resembles a cross and as such has been very important in Christian symbolism according to which it represents, in different interpretations, the Passion of Jesus or the cross of Saint Helena. The constellation Cygnus has recently been associated with the so-called Vision of Constantine, the first emperor to encourage the spread of Christianity in the Roman Empire. According to this new theory, the so-called vision would actually be an alignment of the main planets with the constellation of Cygnus, interpreted by the Emperor as a divine sign.
The constellation Cygnus: other asterisms
The central part of the constellation Cygnus, as mentioned, has the shape of a cross which includes the body of the bird and the first part of the wings. The vertical axis is formed by Deneb and Albireo, i.e. tail and head of the swan, while the horizontal axis is formed by the stars Gienah and Rukh, i.e. Epsilon and Delta Cigni. The two axes cross in the star Sadr (Gamma Cigni). The Northern Cross is so called in contrast to the Southern Cross, which obviously lies in the opposite direction.
The star Deneb is also part of another stellar configuration, the so-called Summer Triangle. This is visible in the Northern Hemisphere from June to January and is made up of 3 particularly bright stars that constitute its vertices. These are Deneb, Altair, of the constellation of Aquila and Vega, of the constellation of Lyra. This formation can be identified quite easily against the background of the Milky Way even when observing the sky with the naked eye. The triangle can be used by less experienced people to identify nearby constellations. During the first months of the year the Summer Triangle is not visible, although it is possible to identify a very similar structure in the sky. This is the Winter Triangle, formed by Sirius, Procyon and Betelgeuse. Since the seasons are reversed, in the southern hemisphere the names of the two triangles are also reversed.