The Cave Nebula, also known as Sh2-155 (Sharpless Catalog) is a large, faint emission nebula in Cepheus. Like all emission nebulae, it is caused by extremely hot O and B class stars ionizing the gas around them and causing it to glow like a neon sign. Each gas has a specific color frequency that it glows when ionized, and with hydrogen it is a deep red at 656.3 nm. Dark ribbons of dust also trace through Sh2-155, blocking the light of hundreds of forming stars inside the nebula as well as many times more beyond the gas cloud.
Takahashi EM-200 Temma Jr., Takahashi TSA-102 at f/6, FLI ML8300. 180 minutes total exposure time.