Astrophysics (Index)About

neutral atomic hydrogen

(HI, H I)
(state of hydrogen when sufficiently cool)

Neutral atomic hydrogen in astrophysics is indicated by the abbreviation HI or H I (pronounced "H one"). The abbreviation HI is used in some indications of neutral hydrogen spectral lines such as the 21-cm line, e.g., [HI]. HI consists of a gas of independent hydrogen atoms, (i.e., not joined as molecules of two hydrogen atoms) that each include a single orbiting electron. In space, with sufficient density and sufficiently low temperature, (generally less than 50 K) they can combine to form such molecules. Regions of neutral hydrogen are generally below 100 K but near early stars, may be thousands. The 21-cm line is used to detect neutral atomic hydrogen at a distance, to detect HI regions and galaxies that include them.

Further reading:

Referenced by pages:
Arecibo Observatory (NAIC)
Australian Square Kilometre Array Pathfinder (ASKAP)
galactic worm (GW)
GALEX Arecibo SDSS Survey (GASS)
gas fraction estimation
Galactic All-sky Survey (GASS)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
halo model
HI 4-Pi Survey (HI4PI)
ionized hydrogen (HII)
HII region (HII)
HI region (HI)
HI supershell
Parkes HI Zone of Avoidance Survey (HIZOA)
Hough transform (HT)
hydrogen (H)
interstellar medium (ISM)
Magellanic Bridge
magnetic field
particle number (N)
spectral line designation
21-cm line