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SH2 155

Cep

• Askar D1 Ha-O3
• Askar D2 S2-O3

• 51 x 300 sec - 4h 15m
• 25 x 300 sec - 2h 05m
• Total exposure: 6h 20m

Sh 2-155 (also designated Caldwell 9, Sharpless 155 or S155, or LBN529) 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, though that name was applied earlier to Ced 201, a different nebula in Cepheus. Sh 2-155 is an ionized H II region with ongoing star formation activity, at an estimated distance of 725 parsecs (2400 light-years) from Earth.
Sh 2-155 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. Although Sh 2-155 is relatively faint for amateur observation, some of its structure may be seen visually through a moderately sized telescope under dark skies.
Sh 2-155 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.

The name "Cave Nebula" was coined for this object by Patrick Moore, presumably derived from photographic images showing a curved arc of emission nebulosity corresponding to a cave mouth. The name was also used earlier to refer to another brighter but unrelated reflection nebula in Cepheus known as Ced 20 or VdB 152, positioned at R.A.: 22h 13m 27s Dec.: +70° 15′ 18″ (2000). The name's application to Sh 2-155 has come into vogue through the nebula's inclusion in Moore's Caldwell catalogue[16] as object Caldwell 9. (SIMBAD lists the name for Ced 201, but not for Sh 2-155.)