Formally SH2-155, the Cave Nebula lies about 2,400 light-years away within the constellation Cepheus. It’s part of a much larger region of gas and dust (called a molecular cloud). This region is mostly ionized Hydrogen, but there’s just enough Oxygen – blue in this image – to make for a pretty picture.
Capturing that Oxygen is challenging, though, especially in light-polluted skies. It took a few nights to produce this image, and it would benefit from even more time.
Research shows there is some active star formation happening inside there.