211016 - Sh2-155 "Cave" nebula

ASTRO:
     type=Emission nebula
     mag=7.7
     const=Cepheus
     dist=2400 ly; size=35 ly
IMAGE:
     location=EB Driveway
     BrtlCls=5
     exposure=DSLR OSC; 38x360s (3.8 hr), ISO3200
     palette="HOO"
EQUIPMENT:
​     optics=ES102CF APO / Stellarvue 0.8x rdcr, FL(eff.)=571mm, f(eff.)/5.6
     camera=Nikon D90 (mod)
     filter=Radian Triad QNB
     mount=Celestron AVX
     guiding=Orion 60x240mm, ZWO ASi224MC
SOFTWARE:
     acquisition=APT, PHD2, Stellarium
     processing=PixInsight (RCAstro), Photoshop (Lumenzia), LrC

Sharpless 155 (or Sh2-155) and Caldwell 9 (or C9) is a diffuse emission nebula in the constellation Cepheus, within a larger nebula complex containing emission, reflection, and dark nebulosity. It is widely known as the "Cave Nebula".  The Cave is an ionized H-II region with ongoing star formation activity, at an estimated distance of 2400 light-years from Earth.

The Cave nebula 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 The Cave is too faint for naked (unaided) eye observation, some of its structure may be seen visually through a moderately sized telescope under dark skies.

The Cave 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 large and hot type O 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 Space Telescope 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 Sir Patrick Moore, astronomer and former President of the British Astronomical Association, presumably derived from photographic images showing a curved arc of emission nebulosity resembling the mouth of a cave.   The name's application to the nebula has come into vogue through the nebula's inclusion in Moore's Caldwell catalogue as object Caldwell 9.