Sh2-155, also known as the Cave Nebula, is a bright emission nebula located in the constellation Cepheus, about 2,400 light-years away. It is a region of active star formation, where intense ultraviolet radiation from nearby young, hot stars ionizes the surrounding hydrogen gas, causing it to glow. The nebula gets its nickname from the dark, cave-like structure formed by dense clouds of gas and dust, which obscure parts of the glowing nebula. Sh2-155 is a complex region with a mix of emission, reflection, and dark nebulae, making it a favorite for astrophotographers and a rich area of study for astronomers exploring stellar birth and evolution.

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