Sh2-155 (also catalogued BN 529, LBN 110.11+02.44, Caldwell 9, or Cave Nebula) is a diffuse nebula in the constellation Cepheus, within a larger nebula complex containing emission, reflection, and dark nebulosity. Sh2-155 is an ionized H II region with ongoing star formation activity. The estimated distance of Sh2-155 is about 725 parsecs (2400 light-years) from Earth. The Cave Nebula lies at the edge of the Cepheus B cloud (part of the Cepheus molecular cloud), and is ionized by very young stars (less than 100,000 years old) from the Cep OB3 association. It has been suggested that radiation from the hot O-type star HD 217086 is compressing the region, triggering the formation of a new generation of stars. This complex contains also LDN 1216 - a large dark nebula above Sh2-155 and VdB 155 - the blue reflection nebula just right of LDN 1216 (see mouseover). The illuminating star of VdB 155 is HD 216629 - a B2IV blue giant of mag. 9.37, which gives the bluish colour to the nebula.