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 (M16), Orion (M42), Lagoon (M8), the Crescent (NGC 6888), and the Crab (M1) nebula.

Note: The large and very dim emission nebula Sh2-173 in Cassiopeia lies 8,800 light-years away and spans another 77 light-years across. Commonly referred to as the Phantom of the Opera nebula owing to its resemblance of the leading masked character in Andrew Lloyd Webber's 1986 musical, Sh2-173 lies approximately three degrees north of Caph (β-Cas, mag 2.27) and within the Perseus spiral arm of the Milky Way. Sh2-173 has been estimated by Cichowolski et al to be between 600,000 and 1,000,000 years old. The emission nebulosity hosts seven bright Cas OB5 stars and is believed to be primarily ionized by the single O-type star BD+60 39. Sh2-173 has been confirmed to be expanding (16 km/sec) and to be associated with stellar formation, for Cichowolski et al have confirmed 46 young stellar objects (see Table 6). It is believed that Sh2-173 is the second of three hierarchical generations of emission nebulosity.