HI REGION A cool gaseous nebula containing mostly atomic hydrogen.
HII REGION
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However, radio emission can be detected from neutral hydrogen, or HI regions.
(a) Home of most Solar System comets. [F88]
(b) HI regions extending to more than 100,000 AU from the Sun, barely gravitationally bound, postulated as the birthplace of comets. [H76]
Oort's Constants ...
Tritium, 3H, a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests occurs in Earth's hydrosphere and atmosphere. Interstallar regions of neutral hydrogen are referred to as HI regions.
ionized atom (missing two electrons), and so on. Because emission nebulae are composed mainly of ionized hydrogen, they are often referred to as HII regions. Regions of space containing primarily neutral (atomic) hydrogen are known as HI regions.
 See also: Magnitude, Spectrum, Earth, Spectral type, Rotation
  
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