Sh2-155, Cave Nebula

Sh2-155 (also designated Caldwell 9, Sharpless 155 or S155) is a diffuse nebula in the constellation Cepheus, within a larger nebula complex containing emission, reflection, and dark nebulosity. It is widely known as the Cave Nebula.. Sh2-155 is an ionized H II region with ongoing star formation activity, at an estimated distance of 2400 light-years from Earth. Sh2-155 was first noted as a galactic emission nebula in 1959 in the extended second edition of the Sharpless catalogue, being a part of the much larger Cep OB3 Association. Sh2-155 lies at the edge of the Cepheus B cloud (part of the Cepheus molecular cloud), and is ionized by young stars 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. A study of the region's young stellar objects by the Chandra X-ray Observatory and Spitzer Space Telescope shows a progression of stellar ages in front of the cloud, supporting the hypothesis of triggered star-formation. Photographed with APO94 mm refractor telescope and ASI 2600MC color camera and IDAS NB1 narrowband filter in Stuvsta, October 2023. Exposure was a total of 249 min with 3 and 4 min exposures during two nights.