The Cygnus Molecular Nebular Complex (also known simply as the Swan Complex) is a giant molecular cloud located in the heart of the boreal Milky Way, in the direction of the constellation Cygnus. It is one of the most turbulent star-forming areas in the Milky Way Galaxy and its largest known molecular nebula complex. Within it are several H II regions, vast and brilliant stellar associations, open clusters, and a large number of some of the brightest stars in the galaxy.
The most notable structure in the complex is known by the catalog abbreviation Sh2-109; it is a vast ensemble, spanning hundreds of light-years, of H II regions, ionized by very bright stars, concentrated in the various OB Association found in this area of the sky. Sh2-109 is also the brightest and most prominent part of the huge molecular nebula complex known as Cygnus X; the total mass of gas and dust in this region is between 10,000 and 100,000 solar masss.
The region lies on the boundary between the Orion Arm, in which the Solar System is located, and the Perseus Arm, at a distance estimated at 5,000 light-years; the complex would still be in an early stage of its evolution, as would be evidenced by the presence of some extremely young and concentrated open clusters with bright and massive stellar components. In the furthest part of the region, connected to one of the OB associations in the area, is the well-known Cygnus X-1 object, an X-ray source thought by many to be a black hole sucking matter from its companion star, a blue supergiant.
Observation
The Cygnus complex is located in the direction of the stretch of sky between the bright star Deneb and Sadr, the heart of the Swan; however, neither with the naked eye nor with binoculars or a small telescope can detect it; what appears with a small instrument is a collection of aggregates of stars and small open clusters, which form a fairly obvious flare to the point that the stretch of the Milky Way of which they are a part is one of the brightest in the sky.
Being in the Northern Hemisphere at a declination of about +40°, the area of the nebula complex is perfectly observable from all the northern regions of the Earth, from which it appears visible for most of the year and, north of the 50th parallel north, even circumpolar; this branch of the Milky Way completely dominates the evening sky of the boreal summer and autumn, where it appears practically at the zenith at the northern mid-latitudes. From the Southern Hemisphere, on the other hand, this section of the sky is observable with great difficulty, especially from the mid-latitudes southward; in the southern tropical belt, however, it is fairly observable.A declination of 40°N is equivalent to an angular distance from the celestial south pole of 50°; which is equivalent to saying that north of 50°N the object occurs circumpolar, while south of 50°S the object never rises.
A large telescope is also needed to be able to observe the associated nebulosity, and is more readily detectable via astrophotography; it should also be borne in mind that the part of the sky in which the cloud is condensed is largely obscured by a large complex of dark nebulae, known in the northern hemisphere as the Cygnus Fissure or "Boreal Coal Sack," which stands in the way of observer's line of sight.
In the precessional epochs
Because of the phenomenon known as the precession of the equinoxes, the celestial coordinates of stars and constellations can vary significantly, depending on their distance from the north and south poles of the ecliptic.
Deneb, Sadr, and the Milky Way section of the Cygnus complex are at about 20h right ascension, not far from 18h, which is equivalent to the point at which, except the area around the north pole of the ecliptic, celestial objects reach their southernmost declination.Just consider that the intersection of the ecliptic with the 18h right ascension equals the point where the Sun is at the winter solstice on December 22.
At present, having passed 18h right ascension some 2,500 years ago, the complex tends to assume increasingly northern declinations. When, about 11,000 years from now, the complex is at six h right ascension, it will reach its northernmost point: at that time, it will be, as seen in the image opposite, a few degrees from the celestial north pole.To determine this, it is sufficient to analyze the two coordinates provided by SIMBAD for the vernal equinoxes of 1950 and 2000, whose coordinates are +19°52' and +19°41', respectively.
Galactic environment and line of sight The area of our Galaxy visible in the direction of the Swan constellation is dominated in an absolute sense by the expanse of dark nebulae known as the Swan Fissure; this is a low-velocity cloud system that appears, from our point of view, to cross longitudinally across the entire Milky Way south of Sadr for a length of as much as 86°. This cloud is at an average distance of about 700 parsecs (equal to 2300 light-years) and extends for about 1000 light-years. On the edge of this cloud complex are some open clusters, such as NGC 6940, about 2,400 light-years away, and some Wolf-Rayet stars, including the bright WR 147, whose brightness is strongly obscured (it appears to be of 15th apparent magnitude, although its absolute magnitude is -4.7) at a distance of 630 parsecs (2050 light-years).
Observing from Earth in a northerly direction concerning the Cygnus Fissure, two nebulae are spotted: the North America Nebula and the Pelican Nebula; both are at a distance of about 800 parsecs (2600 light-years), thus a short distance from the dark complex of the Fissure. This complex, together with that of the Fenditura itself, is part of the same very extensive giant molecular cloud system that separates Gould's Belt the branch of bright stars in which our Sun is also immersed, from that of the large stellar and nebulous Swan complexes located beyond this cloud.
Beyond this dark barrage lie the large OB associations Cygnus OB7 and Cygnus OB4; at the extreme periphery of the large Swan nebular complexes lie a fair number of open clusters, some of which are observable without undue difficulty even with small instruments, such as NGC 6910 and the well-known M29; both are more than 5,000 light-years away. The molecular complex proper is formed by Sh2-109, which constitutes the largest portion and extends its offshoots almost to these clusters, and Cygnus X, a vast complex that is not fully illuminated but emits strong X-ray radiation; Sh2-109 and Cygnus X together occupy almost 450 parsecs in diameter, equal to a good 1400 light-years.
On the edge that from Earth appears to be the "southern" edge of the complex, at a distance of 5100 light-years, lies another easily observable open cluster, NGC 6871; In the sector of the complex located in the opposite direction of our line of sight, however, lie some of the brightest OB associations: This is the case with the "southernmost" Cygnus OB1 association, Cygnus OB9, and especially the very bright Cygnus OB2, which contains some of the brightest known stars within the Milky Way Galaxy, most notably Cygnus OB2-12.
Continuing further, thus moving beyond the complex and into the more remote areas known in this Galactic sector, there are two more brilliant OB associations, Cygnus OB3 and Cygnus OB8, to which is added an open cluster observable with difficulty, NGC 6819, all of which are somewhat detached from the line of sight of the complex; the distance of all these objects is around 7700 light-years. At a similar distance but in a different direction, just northwest of Deneb as seen from Earth, there is finally another H II region, known as Sh2-115, ionized by the bright stars of the Berkeley 90 cluster, which contains 4400 solar masses of gas and dust in about 110 light-years in diameter.
Structure
The molecular complex, as seen, is located at a distance of about 5,000 light-years from the Earth, in the direction of a very rich section of the Milky Way; it is possible to distinguish a few different areas, all part of the same complex: the two main ones are the vast extension called Cygnus X and the set of H II regions known as Sh2-109. The former comprises the largest structure, which permeates the large OB associations found in this area, while the latter is a system of dense nebulae in which star formation is active. The Cygnus X area is heavily obscured by the dark complex of the Fissure, which overlaps our line of sight and almost completely masks both the large H II regions and the very bright star fields of the various young star associations.
A total of as many as 159 distinct clouds have been identified overall, of which various characteristics such as density, size, and mass are known; in addition, there are seven large H II regions, three supernova remnants, 45 T Tauri stars, 18 molecular jets, and as many as 215 infrared radiation sources, coincident with young stellar objects and protostars, probably associated with the molecular clouds.
IC 1318 One of the densest and most easily observable structures is formed by the nebula system of IC 1318 (Sh2-108); in photographs it shows itself as a collection of nebulae more or less surrounding the star Sadr and more or less detached from each other, to the point that they have been classified as separate nebulae: in fact, they are numbered from IC 1318a to IC 1318e, going from west to east. What makes the nebula bright is not, as it may seem, the star Sadr since it is not related to the molecular complex: although it is a very distant star, located around 1,500 light-years from Earth, it is the foreground compared to the nebulous field.
The most intense directly observable part is precisely the section of IC 1318 visible near the LDN 889 dark band; the westernmost parts, on the other hand, appear more rarefied and filamentary, which would suggest that this part was formed by one or more supernovae explosions.
Sh2-109 Sh2-109 is a vast and complex system of H II regions, dark nebulae, bright columns of gas, and associations of young stars. The area of sky in which it is located is visible a few degrees south of Sadr, just north of the open cluster NGC 6871; its apparent extent is as much as 17°, which at a distance of 5000 light-years is equivalent to a real diameter of as much as 4600 light-years.
Cygnus X Cygnus X has long been considered one of the most structurally complex regions lying on the Galactic plane; it is a vast nebulosity, including within its minor nebula structures and a large number of OB associations. Originally known as a single, prominent radio wave source, it was given the acronym X to distinguish it from the extragalactic source Cygnus A. With the development of observational techniques in the various wavelengths, several hundred radio sources were discovered, up to several 800 in the 1980s; these studies also showed that the central regions of the complex are also the most strongly obscured.
Although there has long been some consensus among scholars on the large-scale galactic structures found in the Cygnus X region, there remains some difficulty in determining the kinematic distances of star-forming regions, as the difference between the various radial velocities is consistent with the dispersion of galactic clouds. The arm of Perseus can be traced by mapping the distribution of the various H II regions and the intrinsically brighter stars; according to these studies, the Cygnus X complex is located in conjunction with our arm and that of Perseus. According to other studies, however, the arm would be that of the Cygnus.
About seventy young stellar objects of spectral class A and B at distances up to 2000 parsecs (equivalent to about 6500 light-years) have been identified in the Cygnus X complex based on the study of the CO, it was found that most of these objects are located within the extreme limits of the Orion Arm.
H II regions and stellar formation phenomena