Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona (http://www.caelumobservatory.com/index.html)
The Cave Nebula (Sharpless 155 or Sh2-155) is a dim and very diffuse bright emission nebula surrounded by dust. It has a radius of about 35 light-years, and is located some 2400 light-years away in the constellation of Cepheus. The nebula has gotten its nickname from the darker area, at the center left, which visually appears as a cave.
The name Sharpless comes from a catalog of 312 emission nebulae (H II regions). The first edition was published by Stewart Sharpless in 1953 with 142 objects (Sh1) and the second and final version was published in 1959 with 312 objects (Sh2).
This crescent-shaped star-forming region (HII region) that consists of double ionized hydrogen, lies within and arose from the large molecular cloud known as Cepheus B, which contains reflection, emission, and dark nebulae.
The stars within the Cave Nebula are very young, mostly less than 100,000 years old, while new stars are born at a high rate within the dust clouds of the nebula. Two bright stars (HD 217061 and HD 217086) illuminate the bright ridges of the Cave Nebula.