H I region
interstellar cloud composed of neutral atomic hydrogen
An HI region or H I region (H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI).
These regions do not emit detectable visible light (except in spectral lines from metal elements).
H I regions are clouds of neutral hydrogen which glow faintly with infrared light. Because they are common in our galaxy, they obscure distant views at extreme ultraviolet and soft x-ray wavelengths. An exception is the Lockman Hole, through which we have a fairly clear window to distant objects.
Another window into deep space is the Chandra Deep Field South. This was chosen for the Chandra X-ray Observatory satellite.
