Sh 2-155

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,^[1] at an estimated distance of 725 parsecs (2400 light-years) from Earth.^[2][3]

Sh 2-155

Emission nebula
Sh 2–155
Observation data: J2000.0 epoch
Right ascension	22h 57m 17.14s
Declination	+62° 28′ 33.4"
Distance	2400 ly
Apparent magnitude (V)	7.7
Apparent dimensions (V)	50.0'x30.0'
Constellation	Cepheus
Physical characteristics
Radius	35 ly
Absolute magnitude (V)	16
Designations	Sh 2-155, Caldwell 9, LBN 110.11+02.44
See also: Lists of nebulae

Sh 2-155^[4] was first noted as a galactic emission nebula in 1959 in the extended second edition of the Sharpless catalogue,^[5] being a part of the much larger Cep OB3 Association.^[5] 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.^[6][7]

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.^[8] 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.^[9] 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.^[10][11]

Other name designations

Amateur photograph of the Cave Nebula in the Hubble palette

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.^[12] The name was also used earlier to refer to another brighter but unrelated reflection nebula in Cepheus known as Ced 201^[13] or VdB 152,^[14][15] 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.)^[4][13]