The star-forming regions of Cassiopeia are an extensive portion of the sky rich in giant molecular clouds and highly luminous associations of blue stars; the name derives from the Cassiopeia constellation, in whose direction they are located when observed from Earth. The galactic structures observable in this celestial sector do not form a single complex but are instead several distinct complexes separated by thousands of light-years, appearing aligned along our line of sight.
The region closest to Earth lies on the outer edge of the Orion Arm, the secondary spiral arm that also contains the Solar System: it consists mainly of large concentrations of dark nebulae connected to the , situated in a very northern position relative to the galactic plane and first observed by Edwin Hubble.
The more visible and extensive areas are located in the Perseus Arm, the spiral arm immediately exterior to ours, at a distance of over 7000 light-years; unlike the former, this region is not obscured, as it lies almost exactly on the galactic plane, where the line of sight is clearer in this direction. Here, some particularly bright OB associations are found, some associated with well-known open clusters such as M103 and NGC 457, as well as large nebular complexes, particularly visible on the eastern side of the constellation and connected to the famous Double Cluster in Perseus.
Observation
The star-forming regions of Cassiopeia are located in the direction of the northernmost section of the Milky Way, deep in the Northern Hemisphere, within the eponymous constellation; however, despite their large extent, even their brightest structures are not visible to the naked eye or with small instruments: in this section, bright stars are scarce, and the background stellar fields are less rich compared to other areas of the galactic plane; even the luminous band of the Milky Way appears highly irregular and crossed by large dark bands due to the presence of extensive dark dust clouds that obscure the light behind them.
Located at a declination of approximately 65°N, the Cassiopeia constellation (and its nebular regions) is circumpolar from much of the northern hemisphere; Cassiopeia is one of the classic figures of boreal autumn evenings, appearing at the zenith in Canada, Northern Europe, and Russia. From the Southern Hemisphere, however, visibility is limited, and most of its regions remain below the horizon, never becoming visible.A declination of 60°N corresponds to an angular distance of 30° from the south celestial pole; this means that north of 30°N, the object is circumpolar, while south of 30°S, the object never rises.
The star-forming regions in Cassiopeia closest to us are located a few degrees north of the galactic equator. None of their objects are visible without the aid of a telescope: they are mostly concentrations of dark nebulae, occasionally highlighted by bright cocoons that shine by reflection due to one or more nearby stars; the associations of young stars are similarly obscured, to the point that the section of the sky where they should be visible appears as if it were far from the bright band of the Milky Way. In contrast, all the stellar regions in the Perseus Arm, over 8000 light-years away, are easily observable even with binoculars or an amateur telescope, thanks to their position on the galactic equator, which is much less obscured: thus, most of the open clusters visible in Cassiopeia, such as the well-known M103, NGC 457, and NGC 663, are located in this spiral arm, as are some of the best-known nebulae in the northern sky, the Heart Nebula and the Soul Nebula, connected to an extensive star-forming region.
Precessional epochs
Due to 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 pole and south pole of the ecliptic.
The section of the Milky Way in the Cassiopeia constellation is currently located at approximately 0h of right ascension, which corresponds to the point where the ecliptic intersects the celestial equator (equinox); the intersection at 18h with the ecliptic corresponds to the solstice of December 22, and that at 6h to the solstice of June 21.
In this precessional phase, the Cassiopeia complex tends to assume increasingly northern declinations.To determine this, it is sufficient to analyze the two coordinates provided by SIMBAD for the vernal equinoxes of 1950 and 2000, for any object belonging to this constellation. In about 5000 years, when the complex reaches 6h of right ascension, it will attain its northernmost point: at that time, as shown in the image, it will be a few degrees from the north celestial pole, as the latter will be located in the direction of the nearby constellation of Cepheus.
Galactic environment and line of sight The section of the Milky Way in the direction of Cassiopeia shows clear evidence of obscuration caused by large dust clouds, particularly in its northernmost part; this cloud system is the same as that visible in the adjacent constellation of Cepheus, representing its natural eastward extension. The closest nebular system in this direction, also the primary cause of obscuration, is located just over 900 light-years away and has an actual extent of about 260 light-years. This structure is connected to another, slightly more distant and larger complex known as the Cepheus Cloud; within it are several substructures, including a well-known Bok globule cataloged as Sh2-136 (in Cepheus): this is a dark cocoon visible against a faintly nebulous background, approximately 2 light-years in size, containing forming young stellar objects.
The relationship of these two objects with the Gould Belt, the band of giant stars visible from Perseus to the group of southern constellations forming part of the Argo Navis constellation, has not yet been confirmed: although the radial velocity suggests a connection to an expanding superbubble associated with the Belt, its position, relatively detached from the plane where the Belt lies, suggests it is a separate and independent structure.
Beyond this system of obscuring clouds lies a region less rich in nebular complexes, but south of the Cepheus line of sight, there is a rather sparse OB association known as Lacerta OB1; at a distance of 2600 light-years (800 parsecs), one of the largest giant molecular clouds in our spiral arm is located: a long dust belt stretching thousands of light-years transversely across the arm, formed by the Cygnus Rift and the Aquila Rift, which connects to another dark band visible toward Cepheus, with extensions reaching into Cassiopeia.
On the outermost edge of the Orion Arm, the core Cassiopeia complex is dominated by an OB association cataloged as Cassiopeia OB14; the most notable open clusters in the area are and ASCC 4, the former easily observable with an amateur telescope, while the latter is much fainter and more dispersed.
Beyond the Orion Arm lies the Perseus Arm, one of the major spiral arms of the Milky Way; some of the largest nebulae, such as the Heart and Soul complex, and stellar associations visible in Cassiopeia belong to this spiral arm. In this region, far from the nebular complexes of our spiral arm, some of the brightest OB associations in the entire Galaxy are found: Cassiopeia OB1, Cassiopeia OB2, Cassiopeia OB4, Cassiopeia OB5, Cassiopeia OB7, and especially Cassiopeia OB6.
Structure
The Cassiopeia region does not have a uniform structure: as noted, the H II regions visible in this sector of the sky are located at various distances; within our spiral arm, two main areas can be distinguished: the closest, which is also the smallest and most obscured, at a distance of 180–600 pc, and the more extensive and richer region behind it, located on the outer edge of the Orion Arm, at 600–800 pc.
The most prominent region closest to us in this direction, excluding small clouds within a 200-parsec radius, is formed by the natural extension of the Cepheus complex. The term Cepheus Flare was coined by Edwin Hubble to describe the central area of Cepheus and northern Cassiopeia devoid of extragalactic objects, extending from the galactic plane to high galactic latitude regions where the Milky Way's glow becomes visible again, indicating the presence of a large amount of dust obscuring our Galaxy; its extent ranges between 100° and 120° of galactic longitude.
A study on the distribution of neutral hydrogen in the region detected the presence, at about 300-500 parsecs (approximately 1000-1600 light-years), of two dynamically distinct interstellar gas structures at galactic latitudes between +13° and +17°, moving at a relative velocity of about 1.5 km/s; these are likely expanding or colliding regions. Additionally, a large area of radio continuum emission, later called Loop III, was discovered, centered at galactic coordinates l=124±2°; b=+15±3° and extending over 65°, possibly created by a series of supernova explosions; this moving bubble structure indicates that the interstellar medium is subject to vigorous energetic dynamics: the wide range of different motions observed may reflect the action of multiple shock waves.
The regions beyond our spiral arm are much more extensive and complex, as well as less well-known due to their great distance (over 7000 light-years) and the obscuration of some areas; the spiral arm they belong to, the Perseus Arm, is, along with the Scutum–Centaurus Arm, one of the two major spiral arms of the Milky Way. The most prominent structures are by far the extensive OB associations, the result of intense ongoing star formation processes, and the Herbig Ae/Be star MWC1080, one of the most well-known and studied stars of this class. The most notable complex in this arm in the direction of Cassiopeia is that of the Heart and Soul nebulae, also known as W3/W4/W5, associated with two massive and luminous OB associations: Cassiopeia OB6 and Perseus OB1.
Regions near the Sun At a distance of 180±20 parsecs (590±65 light-years), the cloud LDN 1333 is a small dark nebula with an opacity class of 6, located at galactic coordinates l=128.88° and b=+13.71°. Its distance was estimated in the late 1990s, when it was also mapped at the wavelength of 13CO and C18O.The first is carbon monoxide with the isotope 13 of carbon; the second is carbon monoxide with the isotope 18 of oxygen. According to data from these studies, LDN 1333 is a dark nebula devoid of stars, part of a long filamentary cloud structure extending approximately 30 parsecs. These studies also refer to this complex as a "molecular cloud," with evidence of ongoing star formation processes, including the presence of several infrared sources, such as IRAS 02086+7600, emitted by protostars and coinciding with Hα emission stars; comparisons with sources in nearby complexes, such as the Taurus-Auriga and Chamaeleon complexes, suggest that LDN 1333 is the smallest among known molecular cloud complexes where new star formation occurs. Additional evidence comes from the presence of several young T Tauri stars in the region between LDN 1333 and the high longitudesGalactic longitudes, i.e., the region further east, toward Cassiopeia. l=116–124°; b=-17°. of the Cepheus Flare, in an area free of clouds. Studies of the dynamics and distribution of interstellar matter in this area revealed that the Cepheus Flare and the Cassiopeia region form an expanding superbubble enclosing an ancient supernova remnant; assuming a central distance of 300 parsecs, the bubble's radius would be approximately 50 parsecs, with an expansion velocity of 0.4 km/s. The total mass of the structure is estimated at about 720 M☉; the average mass of the thirteen densest areas of the nebula is approximately 9 M☉.