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, 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, at an estimated distance of 725 parsecs (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. 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.
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.
Other name designations
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 201 or VdB 152, 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 as object Caldwell 9. (SIMBAD lists the name for Ced 201, but not for Sh2-155.)
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Imaging telescopes or lenses: William Optics 81GT IV
Imaging cameras: ZWO ASI 6200 MM Pro
Mounts: Celestron Advanced VX Goto GEM Celestron AVX
Guiding telescopes or lenses: William Optics RedCat 51 Petzval APO Redcat 51
Guiding cameras: ZWO ASI522mc-pro color ZWO
Software: Adobe Photoshop 2021 · PixInsight 1.8 Ripley PinInsight 1.8.8
Filters: ZWO SII 7nm 2″ · ZWO OIII 7nm 2″ · ZWO H-alpha 7nm 2″
ZWO H-alpha 7nm 2″: 30×300″ (2h 30′) (gain: 135.00) -16C bin 1×1
ZWO OIII 7nm 2″: 37×300″ (3h 5′) (gain: 135.00) -15C bin 1×1
ZWO SII 7nm 2″: 35×300″ (2h 55′) (gain: 135.00) -16C bin 1×1
Integration: 8h 30′
Avg. Moon age: 17.10 days
Avg. Moon phase: 93.76%
RA center: 22h58m15s.98
DEC center: +62°38′22″.2
Pixel scale: 4.040 arcsec/pixel
Orientation: 6.464 degrees
Field radius: 1.344 degrees