Sivan 2 is a very large and extremely faint emission nebula located in the constellation Cassiopeia. Due to its exceptionally low surface brightness, it remains a rarely imaged and poorly studied object, even among deep-sky astrophotographers.
The nebula is situated roughly between M31 (Andromeda Galaxy) and the open cluster NGC 7789 (Caroline’s Rose). It spans a wide area of the sky and is primarily detectable in H-alpha, with weaker contributions in OIII, making narrowband imaging essential for revealing its structure.
Sivan 2 belongs to the group of diffuse Galactic emission nebulae catalogued by Sivan in the 1970s. Its ionization sources are not clearly identified, and the object does not exhibit a bright central star, adding to its elusive nature. The faint, filamentary structures visible in deep integrations suggest a complex and extended cloud of ionized gas within the Milky Way.
This object is a challenging but rewarding target, showcasing the beauty of ultra-low-contrast Galactic nebulosity that only becomes visible with long integration times and careful processing.
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