The Cave Nebula is a dim, complex star-forming region in Cepheus, popular with astrophotographers but challenging visually. It combines emission, reflection, and dark nebulosity around a distinctive “cave mouth” arc.
Basic facts
The Cave Nebula is Sharpless 2-155 (Sh 2-155), also known as Caldwell 9, S155 and LBN 529. It lies in the constellation Cepheus at roughly 2,400–3,000 light‑years away, embedded in the larger Cepheus molecular cloud complex (Cepheus B / Cep OB3 region). The nebula is an H II emission region with associated reflection and dark nebulae, giving images a mix of bright red, bluish and dusty structures.
Physical nature
Sh 2-155 sits on the ionized edge of the Cepheus B molecular cloud, where UV radiation from nearby massive OB stars ionizes hydrogen and sculpts the bright rim that looks like a cave mouth. The region hosts ongoing, “triggered” star formation, with studies showing a progression of young stellar objects’ ages from the ionization front back into the cloud, consistent with a compression wave moving through the gas. One of the main ionizing sources is the O‑type star HD 217086 in the Cep OB3b association, with nearby B‑type stars contributing to surrounding reflection nebulosity.
Observational properties
The Cave Nebula has an integrated visual magnitude around 7.7–8 but very low surface brightness, so it is difficult visually and best suited to imaging. Narrowband imaging (especially H‑alpha and SHO palettes) reveals the strong H II emission in the bright arc, extended faint hydrogen around it, and dark dust lanes cutting through the field. In broadband RGB, red dominates due to hydrogen emission, with subtle blue reflection regions and extensive brown dust, often requiring many hours of integration under dark or narrowband‑friendly skies.
Location and framing
On the sky, the Cave Nebula lies near the Cepheus OB3 association, southwest of the star Iota Cephei, in the northern Milky Way band. In typical framing, imagers center on the curved emission rim, with the “cave” opening pointing roughly toward the field’s interior and dust lanes and reflection nebulosity filling the surroundings. Its angular size—on the order of half a degree for the broader complex—makes it a good match for small to medium refractors with modern APS‑C or Four‑Thirds sensors.
Name and confusion
The name “Cave Nebula” was popularized by Patrick Moore, inspired by photographs where the bright arc resembles the mouth of a cave. Historically, the same nickname was also applied to Ced 201 (VdB 152), a different reflection nebula in Cepheus, so some older sources may attach “Cave Nebula” to that object instead of Sh 2‑155. Modern usage overwhelmingly reserves “Cave Nebula” for Sh 2‑155 / Caldwell 9 in the Cepheus B cloud
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