SH2 157 - Lobster Claw Nebula

November 2025

Sharpless 2-157 (Sh2-157), sometimes referred to as the “Lobster Claw Nebula,” is a large H II emission complex in the constellation Cassiopeia, approximately 7,200 light-years away in the Perseus Arm of the Milky Way. The region contains a mix of ionized gas, dark molecular structures, and embedded young stellar objects. The nebula’s distinctive curved structure is shaped primarily by ultraviolet radiation and stellar winds from massive hot stars belonging to young OB associations within the Cassiopeia region of the Perseus Arm. According to the Sharpless catalog, Sh2-157 is part of a broader complex of hydrogen-rich material excited by these luminous O- and B-type stars (Sharpless 1959). Modern spectroscopic studies show that the strong Hα and S II emission originates from photoionization, while the intricate blue-toned filaments trace regions dominated by doubly ionized oxygen (O III).