Grasslands Observatory
Sharpless and Other Miscellanedous Objects
SH2-112
Stewart Sharpless (1926-2013) at the U.S. Naval Observatory published a catalog of 313 HII regions north of declination -27 degrees (Sharpless, 1959). This was an update to his Part I publication in 1953 of HII regions based on a series of 48-inch Palomar Schmidt telescope plates between galactic longitudes 315 degrees and 105 degrees and extending on either side of the galactic circle. He updated and extended his work to the entire sky north of declination -27 degrees based on prints in the Palomar Sky Survey after they became available. He attempted to exclude reflection nebulae based on a comparison of the red and blue photographs in the Palomar atlas (from Hunter, 2023).
Sharpless objects are some of the finest HII regions in the sky with wonderful emission nebulae. They are sometimes labeled Sh-1 for objects described in his first catalog. It is best to ignore his first catalog and use the information from his 1959 publication. These are the Sharpless-2 objects, usually listed as Sh2 or SH2, though as with many other designations in astronomy, there is considerable variation in how observers and authors attach designations for common objects (from Hunter, 2023).
While the Sharpless objects are gorgeous imaging subjects, they are very difficult or impossible to observe visually. Some are quite faint and require very long exposures to properly show their extent and structure. The best and most extensive images of the Sharpless objects are those of Dean Salman at his site The Best of the Sharpless Catalog. Another good site to learn about Sharpless objects is Galaxy Link. The Sharpless objects are confined to the northern sky above declination -27 degrees corresponding to the southern limit of the Palomar Sky Surveys.
Gum Nebulae; RCW Catalog (from Hunter, 2023)
Colin S Gum (1924-1960), an Australian astronomer, conducted a photographic survey for diffuse emission nebulosities (HII regions) in the Southern Milky Way and published his survey in 1955 (Gum, 1955). His catalog lists positions and dimensions of 85 physically separate HII regions with details of the stars exciting the nebulae and their distance moduli when available. A major part of his paper was contained in a thesis he presented as a requirement of the degree of Doctor of Philosophy in the Australian National University, Canberra. The survey had been started by CW Allen for CSIRO (Commonwealth Scientific and Industrial Research Organization) at the Mount Stromlo Observatory in 1950 and extended by Gum starting in late 1951.
For most of the program an f/1 100 mm Schmidt camera was used with 22 mm diameter disks of film punched from 35 mm roll film. The field of view was 11 degrees in diameter with the focal plane scale 35 arcminutes per mm. A combination of absorption filters was used with Kodak 103a-E or Super-XX emulsions. A detailed and interesting discussion of his photographic techniques is provided by Gum in his 1955 paper. Gum’s work is most remembered for the large HII regions he described, the most famous of which is Gum 12 (sometimes called the Gum Nebula) an emission region extending across 36 degrees in Vela and Puppis. In 1959 Gum was appointed Head of the Observational Optical Astronomy program at the University of Sydney. Tragically, he died in a skiing accident at Zermatt, Switzerland in 1960.
In 1960 AW Rodgers, CT Campbell, and JB Whiteoak published the Atlas of Ha Emission in the Southern Milky Way (Rodgers, 1960). This resulted from a survey program started in 1957 by Bart Bok (1906-1983) for detection of southern sky HII regions using a “Meinel-Pearson 8-inch f/1 flat field Schmidt [telescope].” They felt “…that in this survey we have reached fainter limiting emission measures in a given region of the sky than did Gum, primarily because of the increase in resolution of our camera.” The subsequent catalog published after their 1960 article lists 182 HII regions broken up into two groups, those greater than 4 arcminutes in diameter and those less than 4 arcminutes in diameter. This catalog includes many regions listed by Gum and is considered an extension of Gum’s work. There is moderate overlap with Sharpless Catalogue-2, though Sharpless covered the northern sky, and Gum and RCW covered mainly the southern sky.
Sharpless Catalogue of HII Regions (SH2 or Sh2)
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References and Links
ESO online Digitized Sky Survey. https://archive.eso.org/dss/dss
Galaxy Link
Hunter T, Dobek G, McGaha J. The Barnard Objects-Then and Now, Springer-Nature, New York, 2023.
Rodgers AW, Campbell CT, Whiteoak JB. A Catalogue of Ha-Emission Regions in the Southern Milky Way. MNRAS 1960; 121: 103-110.
Sharpless S. A Catalogue of HII Regions. ApJS 1959; 4: 257-279.
The Best of the Sharpless Catalog
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