The Heart and Soul Nebula (also known as the W3/W4/W5 Complex) is a large nebula complex, visible in the direction of the constellation Cassiopeia, in which intense star formation takes place. Its physical location within the Milky Way falls in the Perseus Arm, one of the main arms of our Galaxy, about 6,800 light-years from Earth. Despite its great distance, its observation and study are facilitated by the fact that there is no obscuration due to dark nebulae in the line of sight, thus making it particularly simple and effective to determine a large number of structures, from clusters of high-mass stars to those of low mass and sources well enveloped in nebulosity. Thanks to this ease of observation it is also possible to study the relationships between high-mass stars and the surrounding interstellar medium, the present star formation processes and even the traces of past phenomena, thus allowing us to trace evolutionary models for large molecular nebular complexes. Furthermore, thanks to its overall vision it is possible to study large structures such as superbubbles and their dynamics, as well as easily determine the thickness of the Perseus Arm at the point where the complex is located. The most studied region of the complex is the one called W3, where the most intense formation processes take place. After the Orion Nebula, W3 is the most studied nebula in the celestial vault, in particular since the 1980s.
Observation
The region of the Heart and Soul Nebula is located in the direction of one of the northernmost reaches of the Milky Way, very deep in the Northern Hemisphere, in the constellation of Cassiopeia. The two main clouds are not observable either with the naked eye or with the aid of binoculars. To be able to identify them optically, instruments with magnifications greater than 100 and a very clear and dark sky are needed. A UHC filter allows to obtain a greater contrast, with which it is also possible to see the filamentary structures hosted in both nebulae. Their position is about 5° north of the famous Double Cluster in Perseus.
Structures
Westerhout 3
Region W3, also catalogued as NGC 896, is the smallest and westernmost region of the system; it is a nebulous complex made up of several sections, catalogued as W3 North, W3 Main and W3(OH), whose combined mass is equivalent to about 70,000 M ☉. W3 North comprises the northernmost section of the cloud, W3 Main coincides with the brightest part located to the west and W3(OH) is the dark region located slightly southeast of W3 Main. The entire complex has a shell shape, inside which are the clouds NGC 896 and IC 1795, which emit visible light.
NGC 896 is one of the brightest nebular clumps, located in the western sector of the cloud, but it does not show any well-defined concentrations of stars. IC 1795, on the contrary, is surrounded on its northern and southern sides by associations of young stars, belonging to the W3 Main and W3(OH) sectors respectively. The dominant central star of IC 1795 is a blue dwarf on the main sequence, catalogued as BD +61°411, with an apparent magnitude of 10.28 and a spectral type of O6.5V. In addition to this there are three other stars of class O and B. Their age, calculated through stellar evolutionary models after having determined their position in the HR diagram, is between 3 and 5 million years, an intermediate age between the 6-20 million years of the shell structure that encloses the complex and 1 million years, which is the age of the youngest stars still shrouded in the nebulosity of the compact regions W3 North, Main and (OH). The W3 Main region, in particular, is one of the most studied sites of massive star formation after Orion; it is one of the densest and richest areas of H II regions within a radius of 2000 parsecs from the Sun, which host and hide a young association of blue stars, which ionize the residual gas clouds with their radiation. Several distinct regions can be distinguished, all catalogued with uppercase Latin letters: W3H, W3J and W3K are dispersed regions, W3A, W3B and W3D are compact regions and W3C, W3E, W3F and W3G are ultracompact regions, in addition to these, there are eight hypercompact regions, with an actual diameter of about 0.01 pc, visually equivalent to less than 1 arcsecond. Images taken at various wavelengths, such as visible, Hα and other wavelengths, show the presence of an emission jet coming from W3 Main which appears to emerge from the molecular cloud surrounding it. The exciting stars of these minor regions have been largely identified through observations carried out at various wavelengths, including radio waves: they are class O stars.
Westerhout 4 (Heart Nebula)
W4, also known as the Heart Nebula due to its two-lobed shape, also has a shell-like structure and is the largest of the three main nebulae of the complex; at its center is the open cluster IC 1805, a designation sometimes also used for the entire nebula associated with it. In fact, IC 1805 forms the central part of the large Cassiopeia OB6 association; the brightest stars in the cluster are blue dwarfs, 24 of which have a mass greater than 10 M ☉, with an age between 1 and 3 million years. Among these blue stars there are 9 of spectral class O, enclosed within a radius of about 10 pc, while the total radius of the open cluster is about 18 pc. Of these O-class stars, studies of the radial velocity and the Gaussian curve have revealed that 6 are double stars, Among these there is one, HD 15558, which appears to be a triple star.
Multiple episodes of star formation are active in the vicinity of the nebula, through images obtained in Hα, an expanding superbubble of ionized hydrogen of 1200 pc in size and emerging from the galactic plane has been discovered, whose age, about 10-20 million years, is an indicator of the fact that it originated following the action of the stellar wind of a first generation of massive stars. From this region emerge over 30 molecular clouds and globules dispersed in the surrounding space, as seen above, whose presence suggests that a giant molecular cloud once existed in the region, which later dissolved due to an intense first episode of star formation.
Westerhout 5 (Soul Nebula)
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