Sh2-155, better known as the Cave Nebula, is a diffuse emission nebula located in the constellation Cepheus. It lies about 2,400 light-years from Earth and forms part of a much larger complex of gas and dust that includes emission, reflection, and dark nebulae.

The Cave Nebula is an ionized H II region, meaning its gas is energized by nearby young, hot stars. These stars emit intense radiation that causes the surrounding hydrogen gas to glow and sculpt the nebula into its characteristic cave-like appearance.

Sh2-155 is an active stellar nursery. Radiation and stellar winds from massive stars, including the O-type star HD 217086, compress nearby molecular clouds and trigger the formation of new generations of stars. Observations show a sequence of stellar ages across the region, supporting the idea that star formation here is being actively driven by these processes.

Source: https://en.wikipedia.org/wiki/Sh_2-155