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. Further details at http://www.perseus.gr/Astro-DSO-Nebulae-Sh2-176.htm .