Caldwell 9 — Emission Nebula in Cepheus
C9
About Sh2-155
Description
The Cave Nebula (Sh2-155) is an emission nebula and dim HII region in Cepheus, about 2,400 light-years away. It lies at the edge of the Cepheus B molecular cloud and contains active star formation.
Observing Tips
A challenging visual target requiring dark skies and at least an 8-inch telescope. An OIII or UHC nebula filter is essential. The nebula appears as a faint, irregular glow with a dark intrusion. Best observed in autumn evenings when Cepheus is near the zenith.
History
Cataloged by Stewart Sharpless in his 1959 catalog of HII regions as Sh2-155. The name "Cave Nebula" comes from the dark lane that appears to cut into the bright emission region.
Fun Facts
The Cave Nebula sits at the boundary between a hot ionized bubble blown by massive stars and a dense molecular cloud. This interface triggers new star formation.
Observe
1Properties
Position & Identifiers
2How easy to spot?
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | Hard+ | Hard | V. hard+ |
| 150 mm Newton 150mm Newt. | Medium+ | Medium | Hard+ |
| Celestron C8 (203 mm SCT) C8 203mm | Medium+ | Medium+ | Medium |
Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs
With O-III filter
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | Medium+ | Hard+ | Hard |
| 150 mm Newton 150mm Newt. | Easy | Easy | Medium+ |
| Celestron C8 (203 mm SCT) C8 203mm | Easy | Easy | Medium+ |
3Visibility
Set a location in User Settings to see visibility data.
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Filter Response Guide
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Eyepiece View
Explore
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Surface Brightness
Discover
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Light Travel Time Machine
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Relativistic Travel
Community Photos (1)
Credit: Hewholooks. License: CC BY-SA 3.0. (Wikimedia Commons)
Skybred Mar 2, 2026
Nearby in the Sky
Other targets within a few degrees — pan your scope a little and keep exploring.
Visibility scores assume a 150 mm Newton at Bortle 4.
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