| Emission nebula | |
|---|---|
Sh2–155
| |
| Observation data: J2000.0 epoch | |
| Right ascension | 22h57m 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 | Sh2-155, Caldwell 9, LBN 110.11+02.44 |
| See also: Lists of nebulae | |
Sh2-155 (also designated Caldwell 9, Sharpless 155orS155, 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. Sh2-155 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]
Sh2-155[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 Sh2-155 is relatively faint for amateur observation, some of its structure may be seen visually through a moderately sized telescope under dark skies.[6][7]
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.[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]
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 Sh2-155 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 Sh2-155.)[4][13]
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