A prominent cross-shaped constellation of the northern sky, Cygnus represents a swan with outstretched wings flying southward along the band of the Milky Way. This is one of the sky's larger constellations, comparable with Sagittarius or Taurus in terms of area.
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 bright star Deneb 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 stars Sadr 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 bright stars that is commonly known as the Northern Cross.
As one of the sky's larger constellations, Cygnus is bordered by several other star-groups. Notable among these are Draco and Cepheus directly to the north, and Vulpecula to the south. Directly westward lies to small but distinctive parallelogram of Lyra the Lyre, while southeastward is Pegasus with its distinctive Square. The Square of Pegasus can be used as a 'pointer' to Cygnus in the sky: a line through the stars Algenib and Scheat (the southeastern and northwestern corners of the Square) points almost exactly towards Cygnus' brightest star Deneb.
Among the stars of Cygnus, one stands out against all the others: the white supergiant Alpha Cygni, also known as Deneb (from the Arabic for the 'tail' of the Swan). This is the twentieth brightest star 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 luminous white stars 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 is also a variable star, showing waves of non-periodic pulsations that can change its visual magnitude by a value of some 0.15. It is the prototype example of stars that show similar patterns of variability, which are designated 'Alpha Cygni variables'.
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 yellow star 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.
The line of stars runs on down to Albireo, the beak of the Swan on the borders of Vulpecula. This is a binary system with distinctly differing component stars: one is a golden colour, while the other is bright blue. Albireo A - the golden orange member of the pair - also appears to have another star in a much closer orbit than blue Albireo B.
Spreading out to either side of this main line of three stars are chains of fainter stars that form the outstretched wings of the Swan. The two brighter of these wing stars lie either side of the Swan's body, and combine with Deneb, Sadr and Albireo to form the group known as the Northern Cross. To the southeast is Epsilon Cygni or Aljanah (sometimes called Gienah or Gienah Cygni to distinguish it from another star of the same name in Corvus). This is a triple system, an orange giant with two companion stars about eighty light years from the Sun. Opposite Aljanah to the northwest is Delta Cygni or Fawaris, a blue subgiant star about twice as distant as Aljanah.
These brighter stars define the core shape of Cygnus the Swan, but the boundaries of the constellation encompass many stars that are fainter but no less significant. One of these lies off the northeast quadrant of the Northern Cross, a binary pair of two orange dwarf stars whose magnitude of +5.2 means that it is barely visible to the naked eye. This is the system designated 61 Cygni, important as one of the nearest star systems to the Sun. At a distance of just 11.4 light years, it is the seventeenth closest neighbour of the Solar System, lying only marginally more distant than Procyon in Canis Minor.
On the main line of the Swan's body, about halfway between Sadr and Albireo, lies a faint orange star 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 blue supergiant star, 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.
A wide western segment of Cygnus, around the star Fawaris and westward, lies within the target field of the Kepler planet-detecting spacecraft and, because of this, planetary systems have been discovered around many stars within the constellation. One notable example is Kepler-186, a faint red dwarf star some five hundred light years from the Sun with several planets in orbit. One of these planets, Kepler-186 f, is closely comparable to Earth in terms of both size and temperature.
Deep Sky Objects
The area of the sky described by Cygnus looks down along the length of the Galactic arm containing the Solar System, known as the Orion Arm. This lengthwise view down a dense region of the Milky Way looks out on a region packed with nebulae of different kinds. There are, however, few bright star clusters within Cygnus, with the main exception being the open cluster M39 which lies towards the constellation's eastern edge.
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 star Sadr, 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 star Aljanah, 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 Eastern Nebulae.
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.