A prominent constellation during summer in the northern hemisphere, Cygnus represents a Swan flying along the length of the Milky Way. In the centre of this region is a dark cloud of obscuring matter, the Cygnus Rift, that marks the beginning of the long dark band of the Great Rift.
The association of this formation of stars with a swan goes back to classical times. There are numerous ancient stories to explain the connection, and it is far from clear where Cygnus' mythical origins lie. As part of the tale of Orpheus, for example, the hero was transformed into a celestial swan and placed among the stars with his lyre, and this explains not only the swan shape of Cygnus, but also the bright neighbouring constellation of Lyra the Lyre.
Cygnus in the Sky
The distinctive shape of Cygnus, and the brightstarDeneb that marks the Swan's tail, make it one of the more easily located constellations of the northern sky during summer and autumn. Deneb is one of three stars that form the Summer Triangle, along with Vega in Lyra and Altair in Aquila. Together these three stars form a large southward-pointing triangle, with Deneb and Cygnus lying on the triangle's northeastern corner.
Running southwestward from Deneb, the starsSadr and Albireo form a nearly straight line, representing the body, neck and head of the Swan. Another line of stars runs across this perpendicularly from Aljanah in the southeast to Fawaris in the northwest. The resulting asterism is a cross-shaped formation of brightstars that is commonly known as the Northern Cross.
Among the stars of Cygnus, one stands out against all the others: the whitesupergiantAlpha Cygni, also known as Deneb (from the Arabic for the 'tail' of the Swan). This is the twentieth brighteststar in the sky, despite lying an immense distance from the Solar System. Its precise distance is a matter of debate: though conventionally considered to be about 3,000 light years from the Sun, this value has recently been challenged, and in fact it may be rather nearer, at a distance of perhaps 1,500 light years. It is one of the most luminouswhitestars known, and if it lay at a distance of 10 parsecs from the Sun, it would shine more brightly than Venus in the skies of Earth.
Deneb lies at the northern end of a line of three stars that run southwestwards to form the body and neck of the flying Swan. The centre of the Swan's body is marked by bright yellowstar named Sadr, another supergiant that lies closer to the Sun than Deneb but is rather less luminous, and so appears fainter in the sky. Sadr shines against a remarkable backdrop, standing out against a wide field of nebulosity known as the Sadr Region.
On the main line of the Swan's body, about halfway between Sadr and Albireo, lies a faint orangestar designated Eta Cygni. On almost the same line of sight as Eta Cygni, but far too faint and distant to be seen with the naked eye, is a bluesupergiantstar, HDE 226868. In 1964 an extremely intense X-ray source was discovered in orbit around this star and given the designation Cygnus X-1. This was the first, albeit indirect, evidence of an actual black hole, an object which until that time had been considered merely theoretical in nature. The X-rays of Cygnus X-1 are generated by the intense energies of matter being drawn from the supergiant into the black hole's accretion disc.
Cygnus is home to a vast array of dark clouds running across the band of the Milky Way. Lying some 300 light years from the Solar System, these form a wide empty region in the central parts of Cygnus known as the Cygnus Rift, from which a dark band extends southwestward along the Milky Way. This is the beginning of the Great Rift, which runs along the middle of the Milky Way for about a third of its length, finally coming to an end in the constellation of Centaurus far to the south. These 'rifts' are not actual breaks or divisions in the Milky Way, but rather clouds and trails of obscuring dust blocking out the light from the wider Galaxy beyond.
Apart from its rift clouds, Cygnus is dense with nebulae of many kinds. A particularly rich region of nebulosity forms the backdrop to the central starSadr, a wide star-forming region laned with patterns of dust and filled with newly-hatched clusters of young stars. Further north, beyond Deneb, is another dense star-forming area of ionised hydrogen. This Deneb complex is largely hidden behind obscuring dark dust, but to the east and west lie more visible zones. One of these lobes forms the Pelican Nebula, while the other creates the pattern known as the North America Nebula, named for the remarkable similarity of its shape to that continent on Earth.
A few degrees southwestward of Sadr, rather more distant than either that star or the Sadr Region beyond, lies a ball of matter expelled from a highly active central star. This is a 'bubble nebula', in which forces from a central star of the volatile Wolf-Rayet type send a shockwave outward through the star's surrounding material, creating an irregular bubble in space. In this case one curving edge of the bubble stands out, giving this formation its common name, the Crescent Nebula. Cygnus in fact contains two significant formations of this kind: further south from the Crescent Nebula lies Campbell's Hydrogen Star, a small Wolf-Rayet star which has a surrounding bubble of its own.
Along Cygnus' southeastern wing, in the general direction of the starAljanah, lies a vast and ancient supernova remnant. This is visible as a broken ring of material extending across a region of space some ninety light years across. The material appears in the form of tenuous wisps, from which it takes its name, the Veil Nebula. Two great arcs of the remnant, extending in opposite directions from the original supernova explosion, stand out particularly strongly, and are distinguished by subdividing the entire Veil into Western and EasternNebulae.
In the northern parts of the constellation is a small nebula known as the Blinking Planetary Nebula. This is a planetary nebula with a somewhat elliptical aspect, and a prominent pair of lobes on either side. These lobes are structures known as FLIERS (Fast Low-Ionization Emission Regions) - streams of gases moving rapidly through the material of the nebula. Despite its name (and its vaguely eye-like appearance) this nebula does not in fact blink. Rather, it gained its name from an optical effect, whereby the object can be seen more easily when observed obliquely than directly. Shifting the focus of gaze at the nebula through a telescope can therefore cause it to apparently 'blink', suddenly becoming more or less visible. (This is actually true of telescopic observations in general, but is particularly evident with this Blinking Planetary).
Away to to the east, where Cygnus borders the faint constellation of Lacerta, lies another of the region's many nebulae. This is a shroud of nebulous material some 3,300 light years from the Sun, within which is a clump of stars forming a cluster known as the Collinder 470. The light from these stars illuminates the gas and dust around them, forming a structure about 15 light years across known as the Cocoon Nebula. Out westward from the bright nebula runs a trail of dark material, Barnard 168, obscuring the background stars in a dark and ragged line.