The Sharpless Catalogue was intended to be a comprehensive list of HII regions north of declination -27 in the Northern Hemisphere. HII regions are formations of ionized hydrogen. The ones listed in the Sharpless Catalogue are bright enough to be captured by amateurs using conventional means. There are a few objects below declination -27 and a few of the objects are actually planetary nebulas or supernova remnants rather than HII regions.
The list (Sh1) was first compiled by Stewart L. Sharpless (1926–2013) using photographic plates from the Palomar Sky Survey (POSS I) and published in 1953 containing 142 entries. He accomplished this while working at the United States Naval Observatory Flagstaff Station. In 1959 he released an updated version (Sh2) containing 313 entries.
Rigorously examining photographic plates in those days was not a trivial affair, so the amount of work and expertise Sharpless displayed in compiling his catalogue cannot be overstated. It continues to be one of the most important compilations available.
The Sh2 catalogue is also a staple for amateur astrophotographers. Many of its objects also appear in other catalogues such as the Messier and NGC catalogues. I have embarked on a long-term quest to capture all 313 Sh2 objects—a feat rarely accomplished by amateurs. But as of this writing in August 2025, I still have a very long way to go with 142 of the 313 still remaining. And roughly 20 of them are too far south for me to capture at home or from Deep Sky West near Santa Fe. So I may be forced to do some travel or to place remote telescopes at a Southern Hemisphere location.
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