Cave Nebula

Caldwell 9, known as the Cave Nebula (Sh2-155), is a captivating emission nebula located approximately 2,400 light-years away in the constellation Cepheus. Part of the Cepheus B molecular cloud complex, this active star-forming region features a dense, dark cloud of gas embedded within a giant emission nebula. The distinctive cave appearance is created by the dark dust lane that forms the back wall, while the top edge is illuminated by the intense radiation from hot, massive stars within the Cepheus OB3 association. This wide-field image was captured by the Mosaic camera on the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory, utilizing hydrogen-alpha (red), sulphur [S II] (blue), and I-band (orange) filters to reveal the complex gaseous structures. In this view, north is oriented to the left and east toward the bottom.

Star formation in the Sh2-155 region is actively driven by the Cepheus OB3 association, a group of young, hot O- and B-type stars whose ultraviolet radiation ionizes the surrounding gas and sculpts the cloud's cavern-like profile. Detailed studies have revealed multiple generations of star formation, with the triggering process propagating through the molecular cloud over millions of years as each generation of massive stars compresses adjacent clouds into new bursts of stellar birth. Embedded within the densest portions of the cloud are young stellar objects and protostars still too deeply shrouded in dust to be seen at visible wavelengths but readily detected in infrared surveys.

Despite its modest surface brightness, the Cave Nebula is a rewarding target for long-exposure narrowband imaging, where hydrogen-alpha emission traces the glowing walls of the nebular cavity and the dark molecular cloud forms the dramatic foreground. Visually, it requires a large aperture and very dark skies, appearing as a faint, irregular glow with a notable absence of stars marking the dark lane. Patrick Moore included it as the ninth entry in his Caldwell catalog, recognizing Sh2-155 as a compelling example of active star formation accessible from northern latitudes.