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 (HI), 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 temperature, 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. HI is not very luminous, mostly the 21-cm line, which along with the spectral lines of other elements embedded in the cloud, allows the clouds 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:
https://en.wikipedia.org/wiki/H_I_region
https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095938213
https://www.daviddarling.info/encyclopedia/H/HI.html
https://scienceworld.wolfram.com/astronomy/HIRegion.html
https://www.britannica.com/science/hydrogen-cloud
https://www.astronomy.ohio-state.edu/pogge.1/Ast871/Notes/Intro.pdf

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

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