The Cave Nebula (Sh2-155) lies at distance of 2,400 light-years in the constellation Cepheus. It is a large star-forming cloud of molecular hydrogen spanning approximately 10 light-years. Hot class O and B blue supergiant stars to the upper right of the image are ionizing the cloud's hydrogen gas, causing it to glow at the hydrogen-alpha wavelength of 656.28 nanometers. As their intense light evaporates gas from the cloud surface, it becomes ionized and the fierce stellar winds emitted by those stars blow it back against the cloud, forming a glowing, compressed rim. This pressure also triggers formation of more stars within the cloud. Residual dust within the cloud reflects and scatters the star light, causing portions to appear blue, just as air molecules in our atmosphere scatter sunlight causing a blue sky. This is one of the rare areas in the sky in which all three types of nebulae, red emission, blue reflection, and dark, can be seen within a small area.

Telescope: Celestron Compustar C14.

Exposures: Ha:R:G:B=900:95:95:95 minutes = 19 hours 45 minutes total exposure at f/7.5.

This image was published in the July, 2019 issue of Astronomy magazine.