A HI Region (pronounced as "H one region") is a cloud of generally neutral atomic Hydrogen, e.g., with only about 1 in 10,000 atoms ionized. Typically it is about 100K. Above that, they are unstable, but are stable again above 10,000K. They typically also have some Helium and other elements. They are not very luminous, mostly the 21cm Line, which along with the Spectral Lines of other elements embedded in the cloud, allows them to be detected.
Dwingeloo Obscured Galaxy Survey (DOGS)
Galactic Electron Density
Galactic All-Sky Survey (GASS)
HII Region (HII)
Parkes HI Zone Of Avoidance Survey (HIZOA)
Orion Nebula (M42)
Three Dimensional Model