Cygnus Constellation

Mythology

Cygnus, the Swan, flies along the plane of the Milky Way with outstretched wings. In Greek mythology the swan was most commonly Zeus in disguise — the god transformed himself into a swan to seduce (or according to some accounts assault) Leda, the queen of Sparta, resulting in the birth of the twin heroes Castor and Pollux (Gemini) and the beautiful Helen of Troy. In another tradition the swan is Orpheus, the legendary musician, who was placed in the sky near his lyre (Lyra) after his death. A third myth identifies it with Cycnus, the son of Ares and friend of Phaethon, who mourned so inconsolably after Phaethon's fatal chariot crash into the river Eridanus that the gods transformed him into a swan to ease his grief. Deneb (Alpha Cygni), the tail of the swan and the 19th brightest star in the sky, is a blue-white supergiant approximately 2,600 light-years from Earth and one of the most intrinsically luminous stars visible to the naked eye — roughly 100,000 times as luminous as the Sun. Deneb forms one vertex of the Summer Triangle asterism along with Vega and Altair. Cygnus X-1 in this constellation was the first black hole candidate identified, in 1971.

Sources: Ovid Metamorphoses 2.367-380; Hyginus Astronomica 2.8; Aratus Phaenomena 275-282.

Overview

Cygnus is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 804 square degrees of sky in the northern hemisphere sky, best visible at latitudes between +90 and -60 degrees in September. The constellation belongs to the Hercules family of constellations. Its Latin genitive is Cygni, the form used when naming stars within the constellation — for example, the brightest star Deneb may appear in catalogues as Alpha Cygni or similar. The IAU standardised the boundaries of all 88 constellations in 1930 under the direction of Eugène Delporte, using straight lines of right ascension and declination to eliminate the ambiguities of earlier variable-boundary systems.

Notable stars

The brightest star in Cygnus is Deneb. Individual stars within Cygnus are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Cygni) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Cygnus) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Cygnus boundary are catalogued by the IAU with the abbreviation Cyg.

History and mythology

The figure of Cygnus appears in the classical astronomical tradition. Ptolemy included this constellation in the Almagest (c. 150 CE), the definitive ancient catalogue of 48 constellations, which formed the foundation for Islamic, medieval European, and Renaissance astronomy. The constellation's figure and mythology were transmitted through works such as Aratus's Phaenomena, Eratosthenes's Catasterismi, and Hyginus's Astronomica. See the Mythology section above for the full narrative.