Astrophysics (Index)About

HI region

(HI, HI cloud, diffuse HI cloud)
(cloud of neutral atomic hydrogen)

A HI region (read as "H one region") is a cloud of generally neutral atomic hydrogen, typically with on the order of 1 in 10,000 atoms ionized. Their typical temperature range is ~100 K, in which case the material is referred to as a cold neutral medium (CNM). Above that, they are unstable, but are stable again above 10,000 K. Between those temperatures, they generally cool or heat to a stable state. They typically also have some helium and other elements. They are not very luminous, mostly the 21-cm line, which along with the spectral lines of other elements embedded in the cloud, allows them to be detected.

Other kinds of hydrogen clouds include HII regions and molecular clouds. HI regions and HII regions are termed diffuse clouds and molecular clouds (which are even cooler and more dense) are termed dense clouds.

(hydrogen,cloud type,gas)
Further reading:

Referenced by pages:
Dwingeloo Obscured Galaxy Survey (DOGS)
evaporating gas globule (EGG)
galactic electron density
Galactic All-sky Survey (GASS)
Gould's Belt
helium (He)
neutral atomic hydrogen (HI)
HII region (HII)
HI supershell
Parkes HI Zone of Avoidance Survey (HIZOA)
hydrogen (H)
interstellar astrophysics
interstellar medium (ISM)
Lockman hole
Lyman-alpha forest
Lyman-Werner photon
molecular cloud
Orion Nebula (M42)
rare designator prefixes
SN 185
three dimensional model
21-cm line