The Sharpless catalog of emission nebulae contains 313 entries representing H-II emission nebulae with a declination of -27 degrees or above. Compiled and published by American astronomer Stewart Sharpless (USNO) in 1959 (see here), the catalog contains a wide variety of star formation regions across the Milky Way including various Herbig-Haro and Wolf-Rayet objects as well as a number of well-known emission nebulae such as the Trifid (M20), the Eagle (M20), Orion (M42), Lagoon (M8), the Crescent (NGC 6888), and the Crab (M1) nebula.

Note: Sh2-176 is an impressive but dim crescent-shaped emission nebula in Cassiopeia and which also happens to be a planetary nebula with a surface brightness of 15.5 mag/arc-sec² and a hydrogen-rich central star (see Napiwotzki and Schoenberner). Lying between Schedar (α-Cas, mag 2.22) and Caph (β-Cyg, mag 2.26), Sh2-176 was discovered by Sabbadin et al in 1977 to be an ancient planetary nebula approximately 490 light-years away with estimates for its age ranging from 24,000 to over 180,000 years old based on the computed kinematics (expansion rate and nebular diameter). Expanding at 19 km/sec, Sh2-176 has been estimated to span 3.1 to over 22.8 light-years across depending on the source. As indicated by the image below, the nebula is asymmetric in brightness with the more pronounced nebulosity being observed to the southeast. This asymmetricity is perhaps the result of bow shocks involving the interstellar medium and the planetary nebula itself where, for example, the brighter portion of the nebula represents the interaction of the nebula meeting up against the interstellar medium while the former is moving through space at approximately 19 km/sec and with some of the original mass being left behind and "downstream" (opposite side of the planetary nebula's bright crescent). The central star associated with Sh2-176 is a blue subdwarf with a magnitude of 18.74.

Note: The only two images of Sh2-176 which seem to be available on the internet are the results here and here.