The Cave Nebula – Caldwell 9

The Cave Nebula – Caldwell 9

10/13 -10/26 2025, Belleville, MI

Caldwell 9 (Sh2-155), the Cave Nebula, is a complex emission nebula and star-forming region located in the constellation Cepheus, along the rich northern Milky Way. It lies at an estimated distance of ~2,400 light-years (≈730 pc) and spans roughly ~35 light-years across. The Cave Nebula is part of a larger molecular cloud complex associated with the Cepheus OB3 association, a region known for ongoing massive star formation. The nebula’s appearance is shaped by a combination of ionized hydrogen emission, dust absorption, and reflection, giving rise to the distinctive “cave-like” dark intrusion that gives the object its common name.

The nebula’s ionization is driven primarily by HD 217086, a hot O-type star (spectral type O7 V) embedded near the nebula’s edge. Intense ultraviolet radiation from this star excites surrounding hydrogen gas, producing strong Hα emission, while also eroding the dense molecular cloud through photoevaporation. Infrared and X-ray surveys have revealed hundreds of young stellar objects (YSOs) within the region, spanning a range of evolutionary stages from deeply embedded protostars to pre-main-sequence stars. This makes Sh2-155 an important laboratory for studying triggered star formation, where stellar feedback both initiates and suppresses further collapse in nearby gas.

The Cave Nebula was cataloged in the mid-20th century as part of the Sharpless catalog of H II regions and later included as Caldwell 9 in Patrick Moore’s Caldwell list, which highlights visually and photographically interesting deep-sky objects beyond the Messier catalog. Scientifically, Sh2-155 has contributed to our understanding of feedback-regulated star formation, especially in how massive stars influence their natal environments through radiation and winds. Its combination of bright emission ridges, dark dust lanes, and embedded stellar population makes it a well-studied example of a young, dynamically evolving H II region in the local spiral arm.

Sources & References

• SIMBAD Astronomical Database — Sh2-155 / Caldwell 9
  https://simbad.u-strasbg.fr/simbad/sim-id?Ident=Sh2-155
• NASA/IPAC Extragalactic Database (context & associations)
  https://ned.ipac.caltech.edu
• SEDS — Cave Nebula (Caldwell 9) overview
  https://spider.seds.org/spider/Caldwell/c009.html
• Wikipedia — Cave Nebula / Sh2-155 (distance, ionizing star, YSOs)
  https://en.wikipedia.org/wiki/Cave_Nebula
• Bally et al., Star Formation in Sh2-155 (ApJ; infrared/X-ray YSO studies)
  https://iopscience.iop.org

Astrophotography