The Cave Nebula

A dark arch carved out of a wall of red gas, like the mouth of a cavern hollowed straight into the sky: that silhouette is what earned Sh2-155 its nickname, the Cave Nebula. It lies in the constellation Cepheus, roughly 2,400 light-years away, on the edge of a vast cold molecular cloud that astronomers call Cepheus B.

The red glow comes from ionised hydrogen. The main culprit is HD 217086, a hot O-type star belonging to the Cep OB3 association, a group of young massive suns born from the same cloud. Its ultraviolet radiation strips electrons from hydrogen atoms; as they recombine, they re-emit the deep red light that fills the field. The « cave » itself is not empty: it is the still opaque material of the cloud, seen in silhouette against the glowing gas.

This boundary between star and cloud is a laboratory. By compressing the gas ahead of it, radiation from massive stars can push new stars into collapse, a process known as triggered star formation. X-ray observations by the Chandra telescope and infrared data from Spitzer have revealed a genuine age gradient among the young stars here, the youngest buried inside the cloud, the more evolved ones on the illuminated side. It is one of the clearest pieces of evidence for the mechanism.

This image is the first light of a new instrument, installed remotely in the Nevada desert, and of a monochrome camera: the field is built layer by layer, a luminance exposure for detail and three colour exposures, red, green and blue, recombined afterwards. It totals 13 h 35 of integration. The frame covers only part of the complex, which may later be extended into a mosaic.