H I region

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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.