The Cave Nebula, Sh2-155 (Caldwell 9) in Cepheus, about 2,400 light-years away. It is the glowing edge of the Cepheus B molecular cloud: hot young stars of the Cepheus OB3 association are lighting the gas and eating into the cloud, and the dark dust lane along the bright rim is what gives it the look of a cave mouth. Around it, the whole field is laced with faint red hydrogen and dark dust, the part that takes all night to show.
This one was a learning night for the Celestron Origin: the first pass came out streaked with walking noise, traced to a dark frame from June shot 6 degrees too warm. Re-calibrated with fresh darks taken at the night’s own temperature, and the stars come from a separate true-colour stack so they keep their real colours.